Method, use of adapted optotypes and device for determining visual acuity characteristics of a subject

Abstract

In a method for determining visual acuity characteristics of a subject having at least one astigmatic ametropia, ametropia data of the subject is provided, wherein the ametropia data include at least one axial position of a required optical cylinder correction. A preferred direction (V1; V2) is selected in such a way that this preferred direction (V1; V2) either corresponds to the axis position assigned to the optical cylinder correction or is rotated by 90° to this axis position. Alternatively, the preferred direction can be derived from wavefront data using a point spread function. An optical effect is applied at least in the selected preferred direction. At least one adapted optotype is displayed, which has a directional feature, wherein the adapted optotype is displayed orientated such that its directed feature is arranged parallel to the preferred direction (V1; V2). Visual acuity characteristics of the subject are determined for the selected preferred direction (V1; V2), taking into account at least one dimension of the directed feature of the adapted optotype and the applied optical effect.

Description

[0001]Applicant: Rodenstock GmbH "Method, use of adapted optotypes and device for determining visual acuity characteristics of a test subject" Our reference: R 3436WO - hy / mu Method, use of adapted optotypes and device for determining visual acuity characteristics of a test subject The invention relates to a method, a use of adapted optotypes and a device for determining visual acuity characteristics of a test subject. Determining the visual acuity as visual acuity characteristics of a test subject with a visual impairment, in particular an astigmatic visual impairment, is a central task in optometry. An astigmatic visual impairment of the test subject can be compensated for by providing and / or applying an optical cylinder correction to the test subject in addition to any required optical sphere correction.Known methods for determining the visual acuity of a subject with astigmatic refractive error are usually lengthy, complex and / or error-prone, as they often rely on active feedback from the subject. Furthermore, to determine visual acuity, it is necessary to compensate for the subject's astigmatic refractive error by holding an optical cylinder correction and an optical sphere correction, for example using a phoropter or trial frames. Holding the optical cylinder correction in front of the trial frames or phoropter is cumbersome and requires space, which is why the trial frames or phoropter can only be combined with other devices such as an eye tracker with difficulty. Without holding the required cylinder correction in the optical unit used, the astigmatic refractive error cannot be compensated for.As a result, often no visual acuity or only a distorted visual acuity can be determined in the relevant range for the test subject, especially if the test subject has a significant cylinder error. The invention is based on the object of simply and reliably determining visual acuity characteristics of test subjects with astigmatic refractive error. This object is achieved by the subject matters of the independent claims. Preferred embodiments are the subject matters of the dependent claims. One aspect relates to a method for determining visual acuity characteristics of a test subject who has at least one astigmatic refractive error. This involves providing refractive error data of the test subject, wherein the refractive error data include at least one axial position of a required optical cylinder correction.A preferred direction is selected such that this preferred direction either corresponds to the axial position assigned to the optical cylindrical correction or is rotated by 90° to this axial position. Alternatively, the preferred direction can be derived from wavefront data using a point spread function. An optical power is applied at least in the selected preferred direction. At least one adapted optotype is displayed which has a directional feature, wherein the adapted optotype is displayed aligned such that the directional feature is arranged parallel to the preferred direction. Finally, the subject's visual acuity characteristics are determined for the selected preferred direction, taking into account at least one dimension of the directional feature of the adapted optotype and the applied optical power.The subject's visual acuity characteristics can be determined for one eye, for both eyes individually (monocularly), or for both eyes together (binocularly). Preferably, the visual acuity characteristics are determined monocularly for each eye of the subject individually. The subject has astigmatic refractive error and therefore requires optical cylinder correction, which corrects and / or reduces his or her refractive error. People with astigmatic refractive error usually also require optical sphere correction, which is combined with the optical cylinder correction. These optical corrections can, for example, be integrated into a spectacle lens and / or a contact lens and / or an intraocular lens for the subject. The applied optical effect can be achieved by holding an optical correction (such as a lens) in front of the subject's respective eye.With the optical correction, the light entering the subject's eye is manipulated. This allows the optical corrections to correspond to a predetermined optical power, in particular an optical power with a spherical and / or cylindrical optical power. In order to select the preferred direction, the subject's refractive error data is first required. The refractive error data can, for example, have been determined as part of a subjective and / or objective refraction. The refractive error data can be available as prescription data for the subject. The method for determining visual acuity can be integrated into an objective and / or subjective refraction determination and / or carried out subsequently to it. The refractive error data include at least the axial position of the required optical cylindrical correction. In addition, the refractive error data can also include the strength of the required optical cylindrical correction.The refractive error data can also include a required optical sphere correction, the required optical cylinder correction and / or the axis position of the cylinder assigned to the cylinder correction. Alternatively, the refractive error data can be based on a wavefront analysis and on wavefront data determined in this way. The preferred direction is selected based on the refractive error data, in particular based on the assigned axis position. The preferred direction can either be the axis position directly or a direction rotated by 90° to this axis position. If the refractive error data is based on wavefront data, the preferred direction can also be derived from this wavefront data using a point spread function. The preferred direction can be derived from a point spread function calculated based on the wavefront data. Here, the preferred direction can be, for example,the direction and / or axis of the smallest extent of the point spread function can be selected. The direction of the least confusion can be selected here, e.g. as the direction of the smallest standard deviation of the point spread function. The axis position is usually arranged in a plane that is approximately perpendicular to a selected direction of gaze of the test subject. The direction of gaze can be selected, for example, as a direction of gaze of the test subject in the position of use, which is defined in the relevant standards. The axis position is arranged and / or defined in a plane approximately perpendicular to the position of use. The axis position can in particular be arranged in a plane in which a spectacle lens and / or a contact lens of the test subject is to be positioned.This means that the axial position can coincide in particular with an axis of an optical cylindrical correction which is to be integrated into a spectacle lens and / or a contact lens for the test subject. If the axial position is selected directly as the preferred direction, the preferred direction is arranged approximately in the first principal section of the required optical cylindrical correction and is also arranged approximately perpendicular to the test subject's line of sight. If a direction rotated by 90° to the axial position is selected as the preferred direction, the rotation by 90° occurs within a plane which is approximately perpendicular to the test subject's line of sight. In this case, the selected preferred direction can be arranged approximately in the second principal section of the required optical cylindrical correction and can also be approximately perpendicular to the test subject's line of sight. Once a preferred direction has been selected as described, the optical effect is applied at least in the selected preferred direction.In this case, for example, a rotationally symmetrical optical lens can be provided, i.e., an optical sphere correction. In particular, the optical sphere correction can be applied which, according to the refractive error data, at least partially corrects the subject's refractive error in the selected preferred direction. If the axial position of the required optical cylindrical correction is selected as the preferred direction, i.e., the first principal section of the cylindrical correction, then the optical effect can be, for example, an optical sphere correction of the strength that exactly corresponds to the required sphere correction contained in the refractive error data, without taking the subject's cylindrical error into account. If the direction rotated by 90° to the axial position is selected as the preferred direction, i.e., the second principal section through the cylindrical correction, then the optical effect can be, for example,An optical sphere correction with a strength is applied which results from the sum of the sphere correction stored in the refractive error data plus the cylinder correction stored there (e.g., both specified in diopters). When calculating the sum, the signs of the required sphere correction and the required cylinder correction must be taken into account. If the refractive error data contain a required sphere correction of s and a required cylinder correction of z (sometimes abbreviated as c), a correction with the value s can be applied for the first principal section as the optical power, and one with the value s+z for the second principal section. The optical power can be applied by holding the specified optical power in front of at least one eye of the subject. This can be done, for example, by physically holding the respective optical lens in front of the subject, e.g.using trial glasses and / or a refraction unit. In an alternative approach, the optical effect is not applied physically, but can be simulated using a wavefront simulation. The exact type of application can therefore depend on the refraction unit used. In order to apply the optical effect at least in the selected preferred direction, an optical spherical effect is preferably used, e.g. a rotationally symmetrical lens. This produces the desired optical effect not only in the selected preferred direction, but also across the entire sphere. An optical spherical effect can also be simulated virtually. With the optical effect applied in this way, the test subject's visual impairment can be at least partially corrected, at least for the selected preferred direction.It is noteworthy that no optical cylindrical correction is required to determine the visual acuity characteristics. To determine the visual acuity characteristics according to the method, it is sufficient that the visual acuity characteristics are determined by applying and / or maintaining, for example, a purely optical sphere correction, without the need for an optical cylindrical correction. Applying an optical sphere correction as the optical power is usually easier to implement than applying an optical cylindrical correction, for example because an axial position does not have to be taken into account for the sphere correction. The method is thus simplified by dispensing with optical cylindrical corrections. The visual acuity characteristics can be determined, for example, at least a visual acuity with the applied optical power, and / or a sensitivity of the test subject, and / or a visual acuity-correction value pair, and / or at least one refraction value.A visual acuity-correction value pair contains information about the subject's visual acuity if the corresponding correction is applied as an optical effect for at least the selected preferred direction. In this respect, the visual acuity-correction value pair can also contain the corresponding preferred direction. Determining visual acuity based on optotypes, such as optotypes, is generally known. According to the invention, however, not (only) normal and unadapted standard optotypes are used, but adapted optotypes that are adapted to the selected preferred direction and thus to the subject's visual impairment. Suitable for this purpose are optotypes that have a directional feature. The adapted optotype has a feature with an orientation as the directional feature that the subject is to recognize in the context of a visual task. Optotypes with directional features are generally known, such as Landolt rings or the Snellen E. However, for exampleLandolt rings are aligned by default so that the gap of the respective Landolt ring is located either exactly at 0°, 90°, 180°, etc. In contrast to this generally accepted arrangement of the gap of the Landolt rings as optotypes, adapted optotypes are used whose directional feature is arranged exactly and / or as exactly parallel as possible to the selected preferred direction. If the axial position assigned to the required optical cylinder correction is, for example, exactly 12°, or generally exactly the angle α, and this axial position is selected as the preferred direction, the adapted optotype is positioned so that its directional feature is displayed exactly at the angle 12°, generally at the angle α. The optotype is thus adjusted exactly to the selected preferred direction for which the applied optical sphere correction well corrects the subject's refractive error.The arrangement of the directional feature of the adjusted optotype in the preferred direction enables the test subject to recognize this feature of the adjusted optotype even if it is not fully corrected, i.e., not also cylindrically corrected. Even if the test subject cannot see the adjusted optotype completely clearly because their astigmatism is not corrected by optical cylindrical correction, they can still at least recognize the directional feature if their visual acuity is sufficiently good. Thus, the adjusted optotype enables the test subject to recognize at least the directional feature of the adjusted optotype if they are optimally and / or at least sufficiently corrected in the preferred direction by the applied optical power.Finally, the subject's visual acuity and / or visual acuity characteristics can be determined for the selected preferred direction, taking into account at least one dimension of the directional feature of the adapted optotype. The determination of the visual acuity can be carried out in the usual way, i.e. depending on the dimension of the directional feature that the subject can just recognize and / or identify with the applied optical power. In order to determine the dimension required for the visual acuity calculation, the subject can be shown several adapted optotypes one after the other and / or simultaneously, which differ with regard to one dimension of the directional feature. As part of at least one visual task, the subject can be asked to recognize at least one adapted optotype. As part of the visual task and / or a sequence of visual tasks, for example,that at least one adjusted optotype is displayed in an increasingly smaller size so that the visual tasks become gradually more difficult. Alternatively, the differently sized adjusted optotypes can also be displayed simultaneously. This makes it possible to determine up to which dimension of the directional feature the test subject can still recognize the adjusted optotype. The test subject's ametropia data required for the procedure can, for example, correspond to the best correction and / or the best refraction that the test subject needs to correct his or her ametropia. Additionally or alternatively, the ametropia data can also deviate slightly from the best required optical corrections. For example, the ametropia data can correspond to the data determined for the test subject on the basis of an objective refraction measurement.The objectively determined refraction data usually correspond very precisely to the axial position of the cylinder correction actually required by the test subject, at least in the axial position determined here. As an optical power, in addition to the best correction, for example, an optical correction that is slightly "blurred", i.e. modified, compared to the best correction can be applied, for example in the context of determining the test subject's sensitivity. Intentionally "worsened" refractive error data can therefore also be used. In one variant, the applied optical power can be completely independent of the test subject's refractive error data. In this case, the dimension of the directional feature of the displayed adjusted optotype can be kept constant, and instead the applied optical power can be varied for this constant dimension until the test subject can recognize the directional feature of the adjusted optotype (orcan no longer recognize). In this way, a visual acuity-correction value pair can be determined, which can be independent of the (subjectively and / or objectively determined) optimal correction. In principle, however, a visual acuity determination as visual acuity characteristics for the selected preferred direction can be determined most accurately if the optimal optical correction required by the test subject in the selected preferred direction is used as the optical effect. The method makes it possible to determine the test subject's visual acuity characteristics, such as visual acuity, without having to provide and / or apply optical cylinder corrections to the test subject. Instead, the adjusted optotypes are used, which are precisely adjusted to the axial position of the required cylinder correction and can therefore make the application of optical cylinder corrections superfluous. This makes it possible to determine the visual acuity characteristics with acheaper device which cannot apply optical cylindrical correction itself. Furthermore, the method makes it possible to combine the refraction unit used with additional devices, as there is more space available for additional devices if the option of applying the required cylindrical correction can be dispensed with. For example, a refractometer, in particular an autorefractometer, can be used as the refraction unit to apply the optical effect. When measuring visual acuity, the subject's refractive error can only be corrected for the selected preferred direction, but not for the perpendicular principal section of the required cylindrical correction. Optotypes can be used as adapted optotypes whose lowest spatial frequencies lie in the direction of the most uncorrected principal section, i.e. are aligned parallel to it.Alternatively or additionally, the highest spatial frequencies of the optotypes used can lie in the direction of the best-corrected principal section, i.e., be aligned parallel to it. When using objective measurements and / or combined objective and subjective measurements to provide the refractive error data, the axial position, i.e., the orientation, can be taken directly from the objective data, as objective measurements of the axial position are normally very reliable. The objectively measured cylinder, i.e., the objectively measured required optical cylinder correction, can be somewhat reduced compared to the measured refractive error data, as the objectively measured cylinder is often not accepted by the subject at its full strength. If necessary, the spherical equivalent must be calculated before adjusting the cylinder power, and this must be used instead of the sphere when calculating the principal sections.According to one embodiment, the directional feature of the adjusted optotype has a sequence of light and dark areas that follow one another along the preferred direction. The sequence of light and dark areas can, for example, be aligned perpendicular to the lines of a hatching, or perpendicular to the arrangement of the gap of a Landolt ring. Thus, perpendicular to the gap of a Landolt ring, the dark edge of the circle is first followed by the light gap and then again by the dark edge of the circle. Thus, the directional feature of a Landolt ring is arranged perpendicular to the gap. In a Snellen E, the directional feature is arranged perpendicular to the three parallel E lines. In general, the directional feature can correspond to a sequence of at least one light area following a dark area, preferably at least the interruption of a dark area by a light area or, conversely, at least the interruption of a light area by a dark area.The light and / or dark areas can be designed as lines, for example, and / or have edges aligned perpendicular to the preferred direction. According to one embodiment, an unadapted standard optotype with a directional feature is provided. The standard optotype is rotated in a display plane such that its directional feature is arranged parallel to the preferred direction. Finally, the standard optotype rotated in this way is displayed as the adapted optotype. The display can in particular take place on a screen within a display plane. In this case, the unadapted standard optotype can initially be used as a starting point, which is rotated (e.g. purely mathematically, without being displayed) so that it is correctly aligned with the selected preferred direction. The unadapted standard optotype is thereby transformed into the adapted optotype.Only after this internal calculation is the adjusted optotype displayed. The display plane can be the plane within which the screen can display the optotypes, for example. The display plane is preferably arranged approximately perpendicular to the subject's line of sight and / or approximately parallel to the selected preferred direction. By using an initially unadjusted standard optotype with a directional feature, the adjusted optotype can be provided from this one unadjusted standard optotype in a simple manner by rotating it, e.g. around the center point of the standard optotype and / or around another point in the display plane. This adjusted optotype can then be adjusted to any axial position of the required optical cylinder correction.In addition, the adjusted optotype can be scaled as required on the display, so that the adjusted optotype can be represented and / or displayed larger or smaller, depending on the visual task currently assigned to the subject. According to one embodiment, the axis position assigned to the optical cylinder correction and located in the first principal section of the required optical cylinder correction is selected as a first preferred direction. An optical sphere correction is applied as the optical effect, which corrects the subject's ametropia in the first principal section according to the ametropia data. The visual acuity of the subject for this first principal section is determined as the visual acuity characteristics.Alternatively or additionally, a direction rotated by 90° to the axial position is selected as a second preferred direction, which is located in the second principal section of the required optical cylindrical correction. An optical sphere correction is applied as the optical effect, which corrects the subject's ametropia in the second principal section according to the refractive error data, and the subject's visual acuity for this second principal section is determined as the visual acuity characteristics. If the first preferred direction is selected, the optical sphere correction can be precisely the optical sphere correction that is stored in the refractive error data as the required optical sphere correction.If the second preferred direction is selected, i.e. the second principal section, the sum of the optical sphere correction stored in the refractive error data plus the stored optical cylindrical correction can be selected as the optical sphere correction. This sum corresponds to the correction required by the test subject in the second principal section. Thus, with the optical sphere correction selected in this way, the test subject's refractive error is corrected relatively well and / or as best as possible, at least in the selected preferred direction, i.e., in the selected principal section. In a further development, the test subject's visual acuity is determined for both the first and second principal section of the required optical cylindrical correction, and from this, a direction-independent visual acuity is derived.In other words, the first preferred direction is selected and the subject's visual acuity for the first principal section is determined, and the second preferred direction is selected and the subject's visual acuity for the second principal section is determined. As a result of the visual acuity determination, the result for each principal section can initially be given. In addition, a direction-independent visual acuity can be derived from these two visual acuity values. The direction-independent visual acuity can, for example, be given as the highest value determined during the measurements, as the lowest value, as the arithmetic mean, as the geometric mean, as the harmonic mean, as the logarithmic mean, as the quadratic mean, as the cubic mean, or as a combination of selected ones of the aforementioned values. According to one embodiment, the visual acuity is determined for only one of the two principal sections. In this case, an excellent principal section is selected.The principal section can be selected as the excellent principal section for which the required optical correction is more pronounced in the plus direction, or more pronounced in the minus direction, or for which a stronger correction is required in terms of magnitude, or for which a weaker correction is required in terms of magnitude. The principal section can also be selected whose axis position is closer to the vertical, or the principal section whose axis position is closer to the horizontal. In general, visual acuity can be understood as detectability as a function of size, but also as detectability as a function of other parameters that influence the image, such as contrast. A combination of detectability as a function of size and detectability of contrast and / or other of these parameters can also be used as visual acuity.According to one embodiment, a Landolt ring is used as the adapted optotype, the gap of which is displayed rotated by 90° to the selected preferred direction. The orientation of the gap rotated by 90° to the selected preferred direction causes the sequence dark-light-dark across the gap of the Landolt ring to be arranged exactly in the selected preferred direction. Thus, the directional feature of the Landolt ring is arranged exactly parallel to the selected preferred direction, and the visual acuity for the selected preferred direction can be easily determined. According to one embodiment, a Snellen E is used as the adapted optotype, in which the connecting line connecting the three parallel E lines is arranged parallel to the selected preferred direction. The directional feature of this Snellen E is the sequence of the three parallel E lines, e.g.the sequence light (background), dark of the upper line, light of the space in between, dark of the middle line, light of the space in between, dark of the lower E line, and finally light of the background. This also enables the Snellen-E to be easily adapted as a standard optotype to the preferred direction and thus used as an adapted optotype. In a further development, the adapted optotype is displayed at least once rotated 90° clockwise to the preferred direction and at least once rotated 90° counterclockwise to the preferred direction. The test subject is asked to differentiate between these two different rotated adapted optotypes as part of a visual task. For example, the test subject can be asked to differentiate whether the gap in the Langolt ring is directed to the left or to the right, provided the preferred direction is approximately vertically upwards.The same applies to the use of the Snellen E as an adapted optotype. According to one embodiment, a hatched area is used as the adapted optotype, in which the hatching lines are arranged perpendicular to the selected preferred direction. A figure, for example, a circle, a rectangle, a symbol, an animal, letters, or the like, can be used as the hatched area. The figure is filled with hatching. Preferably, the figure does not have any border lines that could disrupt the hatching, but is simply designed as a hatched, borderless figure. The hatching lines are displayed perpendicular to the preferred direction, since the directional feature, as the relevant feature of the optotype, is the sequence of alternating light and dark areas of the hatching. It can be advantageous if the figure itself is as uniformly formed as possible and has few details, for exampleis designed as a circle or a square. Preference is given to figures that are as simple and as detailed as possible. To adjust the orientation of the directed feature, either the entire optotype including the hatching can be twisted and / or rotated, or just the hatching within the constant area and / or figure. The hatching can be binary, i.e. have hard black and / or white edges, or it can be designed with a continuous gradient. For example, the hatching with a continuous gradient can be designed with a sinusoidal intensity gradient or a similar intensity gradient.According to one embodiment, in addition to the adjusted optotype, at least one further optotype is displayed, the gray value of which approximately corresponds to an averaged gray value of the adjusted optotype, and the test subject is asked to differentiate the displayed optotypes from one another as part of a visual task. Instead of an optotype with an averaged gray value, an optotype can also be used whose hatching is not perpendicular to the selected preferred direction like the adjusted optotype, but approximately parallel to the preferred direction. Such an optotype appears to the test subject as an essentially gray optotype perpendicular to the preferred direction due to their astigmatic refractive error, which is incorrectly corrected for this hatching orientation.For example, several such gray optotypes and one adapted optotype can be displayed on a display plane as part of a visual task, or conversely several adapted optotypes and one such gray optotype. The test subject can be asked to identify the one of the displayed optotypes that differs from the other optotypes. According to one embodiment, the applied optical power is varied at least up to a limiting refraction for the selected preferred direction, from which the test subject recognizes the directional feature of the adapted optotype. In this case, the dimension of the displayed adapted optotype can be kept constant. In this case, an extreme value of the refraction unit used can be used as the starting value of the applied optical power, for example, ±20 diopters. Alternatively, a diopter value can also be used as the starting value, which diopter value can be adjusted by a predetermined deviation of, for example,±5 diopters from the optical sphere correction actually required according to the refractive error data. After applying the starting value, the applied optical power is varied, e.g., continuously or in fixed steps, until the subject can (or can no longer) recognize the directional feature of the adjusted optotype. The optical power currently applied when recognizing the directional feature of the adjusted optotype corresponds, as the limiting refraction, to an optical correction at which the subject has a visual acuity dependent on the dimension of the directional feature of the displayed adjusted optotype. This determines a visual acuity-refraction value pair for the preferred direction as visual acuity characteristics.If this method is repeated with at least a second adapted optotype in which the directional feature has different dimensions, a second visual acuity-refraction value pair can be determined which differs from the first determined visual acuity-refraction value pair. From these two different visual acuity-refraction value pairs, the sensitivity of the subject can be determined, for example. In an alternative embodiment, the dimension of the directional feature of the adapted optotype is varied at least up to a limit dimension up to which the subject recognizes the directional feature of the adapted optotype. In this case, the applied optical power can be kept constant. As an optical power, for example, an optical correction can be applied which corrects the subject's ametropia in the selected preferred direction according to the ametropia data. E.g.An optimal optical correction can be used here, which has been determined as part of an objective and / or subjective refraction. With this alternative, the dimension of the directed feature of the adapted optotype can be varied and it can be checked up to which limit dimension the test subject can still recognize the directed feature. From this limit dimension, the visual acuity can be determined in the classic way. The variation in the dimension of the directed feature of the adapted optotype can be achieved by displaying adapted optotypes of different dimensions and / or by varying the size of the displayed adapted optotype(s). This can be done as part of at least one visual task and / or a sequence of different visual tasks, with at least one adapted optotype being displayed as part of each visual task.Thus, the visual acuity with this applied optical power can be determined as visual acuity characteristics for the selected preferred direction. Here, too, a visual acuity-refraction value pair with the associated preferred direction can be determined. If this method is repeated with at least a second optical power applied in the selected preferred direction, a second visual acuity-refraction value pair can be determined which differs from the first determined visual acuity-refraction value pair. From these two different visual acuity-refraction value pairs, the subject's sensitivity, for example, can be determined. In general, according to one embodiment, at least one visual acuity and / or at least one sensitivity and / or at least one visual acuity-refraction value pair and / or at least one refraction value can be determined as visual acuity characteristics.In particular, a value related to the visual acuity can be determined, i.e. at least one visual acuity and / or at least one sensitivity and / or at least one visual acuity-refraction value pair. The sensitivity can be determined as a function of a sensitivity metric, e.g., specifically for at least the selected preferred direction. In addition, a sensitivity for the second preferred direction can also be determined, i.e., the direction rotated by 90° to the first selected preferred direction. However, a direction-independent sensitivity can also be determined (alternatively or additionally). The direction-independent sensitivity can, for example, be determined from the two sensitivities for the first and second preferred direction, or on the basis of two direction-independent visual acuity-refraction value pairs (where, for example,the associated direction-independent visual acuity has been determined as an average of the visual acuity values ​​for the two preferred directions), or on the basis of a capable sensitivity metric which can determine a direction-independent sensitivity from at least two direction-dependent visual acuity-refraction value pairs. According to one embodiment, the test subject is presented with at least one visual task dependent on the displayed adjusted optotype, which the test subject answers by providing active and / or passive feedback. One embodiment of active feedback can, for example, be that the test subject verbally answers a question from an optician and / or another examiner regarding a visual task. Active feedback can also be given, for example, by pressing a button and / or a mouse, with a gesture and / or with a gaze. The test subject's gaze can be recorded, for example, using an eye tracking unit.Using such an eye tracking unit, a passive response, i.e. passive feedback, can also be given. In this way, the eye tracking unit can detect which optotype the test subject is currently fixating. This can determine whether the test subject is, for example, subconsciously fixating on an optotype that differs from the others because they have recognized it, or whether the test subject is unable to recognize the different optotype. In principle, visual tasks with passive and active response can be combined. Preferably, the response, i.e. the feedback, from the test subject is recorded without any intervention from an examiner. In this way, the test subject can either actively enter the feedback themselves, for example using a button and / or a mouse-like control, or it can be recorded passively.The omission of an examiner as a necessary recipient of the visual task eliminates a possible source of error in the determination of visual acuity, namely the human examiner. Furthermore, costs and / or time can be saved by omitting the human examiner. According to one embodiment, the visual acuity of the test subject is determined in the selected preferred direction with two different optical powers applied, and from this, the sensitivity of the test subject is determined. For example, the visual acuity can be determined once in the selected preferred direction with the optimal and / or best optical power for this selected preferred direction, and again with a different optical power applied. This additional optical power can, for example, be shifted by ±0.5 dpt compared to the best power. From the two visual acuity values ​​that result for the test subject with the two different optical powers (ieCorrections) can be used to determine the sensitivity of the subject. In principle, the sensitivity can also be determined on the basis of two visual acuity values, neither of which is determined at the optimal correction. Using a mathematical model, the sensitivity of the eye and / or the subject can be calculated from the two determined visual acuity values. Thus, it is not absolutely necessary for the best correction to already be known at the time of the visual acuity determination. Not all correction values ​​and / or visual acuity values ​​used for sensitivity measurement have to be recorded with the method according to the invention. Thus, e.g., within the framework of an autorefractometric and / or aberrometric measurement, a first visual acuity can be determined, e.g., at a given distance from the objectively determined refraction, and a second visual acuity can be determined, e.g.,as part of a subsequent subjective refraction, at the best optical correction resulting from the subjective refraction. However, two or more visual acuity values ​​can also be recorded using the method according to the invention. Thus, for example, as part of an autorefractometric and / or aberrometric measurement, a first visual acuity value can be determined, e.g. at the objectively determined best correction, and a second visual acuity value at a given distance from this. To calculate the subject's actual visual acuity, the objective refraction determined in this way can be used as the best optical correction. According to one embodiment, a subjective and / or objective refraction is carried out and the subject's visual acuity data is derived from the subject's refractive error determined in this way.For example, the refractive error data can be determined from an objective refraction, whereby the determined optimal optical corrections and the determined optimal axial position are used as refractive error data. Alternatively or additionally, a subjective refraction can be performed. In this case, the refractive error data can be based on the result of the subjective refraction. Finally, the two results can also be combined and an average of the objectively determined best correction and the subjectively determined best correction can be used as refractive error data. Particularly within the scope of a sensitivity determination, optical corrections that deviate from the determined best correction can also be used as refractive error data. In one embodiment, for example, an objective refraction measurement is first carried out on the test subject and the best optical correction determined in this way is used as the refractive error data.A subjective refraction is then performed, wherein two visual acuity values ​​are determined using the method according to the invention during the subjective refraction. The sensitivity is determined from these two visual acuity values. In particular, a visual acuity value is determined after completion of the subjective refraction based on refractive error data resulting from the subjectively determined, best optical correction. In this embodiment, both an objective refraction and a subjective refraction are performed, the subject's visual acuity is determined, and its sensitivity is determined. To determine visual acuity, only a spherical optical correction is applied. An optical cylinder correction is not required. According to one embodiment, the subject's visual acuity is determined using the method as a visual acuity characteristic and converted into a different visual acuity type.This conversion can be performed retrospectively. For example, visual acuity usually depends on the optotype used. Since there are different methods for determining visual acuity, e.g. based on numbers or using a grating, e.g. FrACT, the visual acuity values ​​can depend on the determination method used. The visual acuity values ​​​​depending on the measurement method can be converted into one another. The conversion can be performed using a calibration function which performs the desired conversion. The calibration function can be determined using regression from a data set which contains a large number of visual acuity values ​​and thus visual acuity types for the same person which were determined using different measurement methods (e.g. based on numbers and FrACT). In this way, a correlation between the two different visual acuity types and / or visual values ​​can be established using the data set.In the simplest case, the calibration function can be a function of the visual acuity determined using the method and calculate the visual acuity value that would have resulted based on the other desired determination method as the function value. In order to improve the conversion accuracy, the calibration function can depend on further parameters, e.g. the person's pupil diameter prior to the visual acuity measurement, an orientation of the selected preferred direction, the adjusted optotypes used, the uncorrected optical power in one or both principal sections (e.g. in the best corrected or the most uncorrected principal section), further parameters of the adjusted optotypes used (e.g. their contrast), or a combination of some or all of these parameters. The use of the calibration function makes it possible to convert the visual acuity value determined using the method into a visual acuity value that can be compared with other, e.g.optotypes not provided with a directional feature are calculated using a method. According to one embodiment, the at least one adapted optotype is displayed without correction and / or without complete correction of the optical cylinder correction required by the test subject, but nevertheless sharply represented for the test subject. In this case, image elements that appear sharp can be displayed to the test subject despite a lack of or incomplete correction of the astigmatic refractive error. For this purpose, the at least one image element is displayed as an adapted optotype, which is aligned to the preferred direction in such a way that it is perceived as sharp by the test subject. This takes place without and / or without complete correction of the optical cylinder correction required by the test subject. Because the at least one directional feature of the adapted optotype is displayed aligned parallel to the preferred direction, the test subject can perceive it sharply.In this way, the adjusted optotype can still be perceived sharply despite the lack of (complete) cylindrical correction and can be fixated better and / or more easily by the test subject than an optotype that is displayed out of focus. This can be advantageous, for example, when measuring accommodative ability, since test subjects with uncorrected astigmatism perceive an out-of-focus object in the viewer more quickly. If, for example, an image of a hot air balloon is used as the target of a visual task, the direction in which the stripes run on the hot air balloon (see also Figures 6 and 7) can be displayed arranged parallel or perpendicular to the selected preferred direction. The test subject can perceive the image of the hot air balloon with the stripes aligned in this way as a directional feature sharply even without cylindrical correction and can therefore perceive it more quickly.The hot-air balloon's striped pattern can be displayed like a hatch, adjusted to the selected preferred direction. This makes it possible to perform a visual task with sufficient accuracy, for example, when determining a refraction value and / or a visual acuity value without correction of the subject's astigmatism (or with incompletely corrected astigmatism).One aspect relates to the use of adapted optotypes, each of which has a directed feature arranged parallel to a preferred direction, which either corresponds to an axial position associated with an optical cylinder correction required by a subject, or which is rotated by 90° to this axial position, or the preferred direction can be derived from wavefront data using a point spread function in order to determine the subject's visual acuity characteristics for the selected preferred direction, taking into account at least one dimension of the directed feature of the adapted optotype. The use of the adapted optotype can occur in particular within the framework of the method described above. Therefore, all statements regarding the method can also relate to the use, and vice versa.One aspect relates to a device for determining visual acuity characteristics of a subject who has at least one astigmatic refractive error. The device has a selection module that selects a preferred direction, wherein this preferred direction either corresponds to an axial position assigned to an optical cylinder correction required by the subject, or is rotated by 90° to this axial position. Alternatively, the preferred direction can be derived from wavefront data using a point spread function. A refraction unit is configured to apply an optical power to the subject at least in the selected preferred direction. The refraction unit can, for example, be designed as an aberometer and / or as a refractometer and / or apply a rotationally symmetric lens as a sphere correction as the optical power.A display module has a display and displays at least one adapted optotype with a directional feature on the display such that the directional feature of the adapted optotype is arranged parallel to the preferred direction. A visual acuity characteristic determination module determines the visual acuity characteristics of the subject for the selected preferred direction, taking into account at least one dimension of the directional feature of the adapted optotype and the applied optical power. The device can, for example, be used to carry out the method described above and / or to use the adapted optotypes described above. Therefore, all statements regarding the device also relate to the method and the use, and vice versa. As a dimension of the directional feature, for example, a distance between hatching lines, a contrast intensity, a thickness of lines and / or a width of gaps can be used.According to one embodiment, the device has an eye-tracking unit which tracks at least one eye of the subject when displaying the at least one adapted optotype. By means of the eye-tracking unit, on the one hand, a direction of gaze of the subject can be determined and, on the other hand, active and / or passive feedback from the subject can be registered as a response to a visual task. In the context of this invention, the terms "substantially" and / or "approximately" can be used to include a deviation of up to 5% from a numerical value following the term, a deviation of up to 5° from a direction following the term and / or from an angle following the term. Terms such as above, below, above, below, lateral, etc. refer - unless otherwise specified - to the reference system of the earth in an operating position of the subject matter of the invention.The invention is described in more detail below with reference to exemplary embodiments shown in the figures. Identical or similar reference numerals can identify identical or similar features of the embodiments. Individual features shown in the figures can be implemented in other exemplary embodiments. These show: Figure 1A shows exemplary embodiments of displayed optotypes for determining a visual acuity; Figure 1B shows the visual impression of a test subject resulting from the optotypes shown in Fig. 1A, which has been corrected for the second principal section of their cylindrical refractive error with an optical sphere correction; Figure 1C shows the visual impression resulting from the optotypes shown in Fig.1A shows the visual impression of a test subject which is corrected for the first principal section of their cylindrical visual impairment with an optical sphere correction; Figure 2A shows exemplary embodiments of adapted visual symbols for a visual task for a test subject which is corrected for the second principal section of their cylindrical visual impairment with an optical sphere correction; Figure 2B shows the visual impression of a test subject which is corrected for the second principal section of their cylindrical visual impairment with an optical sphere correction for the optotypes shown in Fig. 2A; Figure 3A shows exemplary embodiments of adapted visual symbols for a visual task for a test subject which is corrected for the second principal section of their cylindrical visual impairment with an optical sphere correction; Figure 3B shows the visual impression of a test subject which is corrected for the second principal section of their cylindrical visual impairment with an optical sphere correction3A shows the visual impression of a test subject resulting from the optotype shown, which is corrected for the second principal section of his cylindrical visual impairment with an optical sphere correction; Figure 4A shows the visual impression of a test subject resulting from differently sized optotypes of adapted optotypes, wherein the test subject is corrected for the second principal section of his cylindrical visual impairment with an optical sphere correction; Figure 4B shows the visual impression of a test subject resulting from differently sized optotypes of adapted optotypes, wherein the test subject is corrected for the second principal section of his cylindrical visual impairment with an optical sphere correction; Figure 5A shows embodiments of adapted optotypes for a visual task for a test subject who is corrected for the second principal section of his cylindrical visual impairment with an optical sphere correction; Figure 5B shows the visual impression of a test subject resulting from the optotype shown in Fig.5A shows the visual impression of a subject corrected for the second principal section of their cylindrical refractive error with an optical sphere correction; Figure 6 shows an exemplary image or photo that gives the viewer a sense of distance; Figure 7 shows the image or photo from Figure 6 with exemplary adjusted optotypes integrated into or superimposed on the image; and Figure 8 shows a diagram of the amplitude of accommodation as a function of age (Duane curve). The figures show embodiments of optotypes and the resulting visual impression for a subject with astigmatic refractive error. The assumption is made for a subject who has a sphere refractive error of +2.75 dpt and an astigmatism of -3.0 dpt with an axial position of 12°.This refractive error data is to be understood as an example, and the following exemplary embodiments are generally applicable to test subjects who have a refractive error in the sphere of s and an astigmatism of z with an axial position of α. The test subject's refractive error can be recorded as part of a subjective and / or objective refraction. This results in refractive error data that contain the spherical and astigmatic refractive error including the axial position, for example at least the value set {s; z; α}, in the example the value set {+2.75 dpt; -3.0 dpt; 12°}. Fig. 1A shows exemplary embodiments of optotypes that can be displayed to the test subject to determine their visual acuity. Landolt rings are used as optotypes, the gap of which is displayed from left to right at the angles 180°, 135°, 90°, 45°, and 0°.These five left Landolt rings are common standard optotypes that can also be used in a conventional visual acuity determination. On the right in Fig. 1A, two special and adapted optotypes are shown in which the gap is shown aligned at the angles 168° and 78°. When determining the visual acuity as a visual acuity characteristic, an optical correction is applied and / or held in front of the test subject as an optical effect, through which the test subject can view the optotypes and attempt to recognize them. For this purpose, a refraction unit can be used, which is arranged, for example, in front of the eye or eyes of the test subject. For this purpose, a refraction unit can be used, for example, by means of which only optical sphere corrections can be applied to the test subject, but not necessarily optical cylinder corrections.Thus, a refraction unit that only corrects spherical vision can be used, or a refraction unit that can only create a limited selection of axial positions and / or optical cylindrical corrections. To determine the subject's visual acuity characteristics, a preferred direction is first selected from the refractive error data. The preferred direction is a direction in a plane that is approximately perpendicular to the subject's line of sight. The preferred direction can be located in the same plane in which the axial position of the subject's cylindrical refractive error is defined. The preferred direction can now be either the axial position α, and thus the first principal section of the subject's cylindrical refractive error, or a direction perpendicular to this in the same plane, α +90°, and thus the second principal section of the subject's cylindrical refractive error.In the example, the first preferred direction for the first principal section would be a direction of 12° in a plane approximately perpendicular to the subject's line of sight, and the second preferred direction for the second principal section would be a direction of 102° in a plane approximately perpendicular to the subject's line of sight. If the first preferred direction is selected, the subject can be given an optical sphere correction of s using the refraction unit, in this example +2.75 dpt. This means that their refractive error is correctly corrected in the first preferred direction, but not in the other directions, particularly not perpendicular to the first preferred direction. If the second preferred direction is selected, the subject can be given an optical sphere correction of s+z using the refraction unit, in this example -0.25 dpt (calculated from: +2.75 dpt - 3.0 dps).As a result, their refractive error is correctly corrected in the second preferred direction, but not in the other directions, especially not perpendicular to the second preferred direction. If, for example, the test subject is given an optical sphere correction of -0.25 dpt, their refractive error is corrected relatively accurately in the second preferred direction V2 at 102°. This correction is shown schematically on the far left in Fig. 1B. Since the axial length of the refractive error is normally determined by looking at the test subject's eyes, and the optotypes are usually displayed from the test subject's perspective, the measurement angle of the axial position is exactly a mirror image of the display angle of the optotypes. This means that the second preferred direction V2 is aligned at the display angle of 78° on a display on which the optotypes are displayed, which corresponds to an axial position at the measurement angle of 102° as viewed from the test subject's eyes. e.g.a display angle directed vertically upwards (corresponding to “12 o’clock”) corresponds to the 90° position. Likewise, a measurement angle directed vertically upwards (corresponding to “12 o’clock”) on the display plane corresponds to 90°. A display angle directed to the right at “3 o’clock” corresponds to 0°. However, this display angle directed to the right corresponds to a measurement angle rotated to the left (since it is mirror-inverted) when viewed on the display plane, i.e. a measurement angle of 180°. This results in different angle values ​​between the display angles defined on the display plane of the display and the measurement angles measured with a view of the test subject’s eyes. Fig. 1B shows the test subject’s visual impression when viewing the optotypes shown in Fig. 1A. The optotypes appear blurred, particularly in the direction perpendicular to the second preferred direction V2. The angle shown in Fig.The visual impression shown in Fig. 1B was calculated for the test subject from the example with the refractive error data {s=+2.75 dpt; z=-3.0 dpt; α=12°}. As shown in Fig. 1B, the optotypes in which the gap is displayed at the display angles of 90° and 45° appear particularly blurred to the test subject. Specially adapted optotypes are now used for the test subject in which the gap is aligned perpendicular to the second preferred direction V2, i.e., at the display angles of 168° and 348°. The adapted Landolt ring at the display angle of 168° is also shown above in Fig. 1A as the actually displayed adapted optotype. In these two adapted optotypes, a directional feature of the Landolt ring is aligned exactly parallel to the second preferred direction V2, namely the transition from the black circle edge to the white gap and back to the black circle edge.Therefore, at least the gap between the two adjusted Landolt rings "rotated" to the display angles of 168° and 348° appears relatively sharp to the test subject, see the two right-hand visual impressions in Fig. 1B. This is because the test subject's ametropia in his second principal section, i.e. along the second preferred direction V2, is fairly well and / or optimally corrected by the optical sphere correction of -0.25 dpt. The directional feature of the Landolt rings, whose gap is displayed at the display angles of 90° and 45°, is relatively steep (i.e., almost perpendicular) to the corrected second preferred direction V2, which is why a very blurred visual impression results, especially for these two optotypes. But even for the other standard optotypes, there is blurring in the visual impression, which makes an accurate determination of visual acuity impossible.If the subject is given an optical sphere correction of +2.75 dpt, their refractive error is relatively accurately or optimally corrected in the first preferred direction V1 at a measurement angle of 12°. This correction is indicated on the far left in Fig. 1C. This axial length at the measurement angle of 12° appears on the display in the display plane at a display angle of 168°. The first preferred direction V1 is thus aligned on the display at a display angle of 168°. Fig. 1C shows the subject's visual impression when viewing the optotypes shown in Fig. 1A. The optotypes appear blurred, particularly in the direction perpendicular to the first preferred direction V1. The visual impression shown in Fig. 1C is again calculated for the subject from the example with the refractive error data {s=+2.75 dpt; z=-3.0 dpt; α=12°}. As shown in Fig.1C, the standard optotypes in particular, in which the gap is displayed at the display angles of 180°, 135°, and 0°, appear very blurry to the test subject. Specially adapted optotypes can now be used for the test subject, in which the gap is aligned perpendicular to the first preferred direction V1, i.e., at the display angles of 258° and 78°. The adapted Landolt ring at the display angle of 78° is also shown above in Fig. 1A as the actually displayed adapted optotype. In both adapted optotypes, the directional feature of the Landolt rings is aligned exactly parallel to the first preferred direction V1, namely the transition from the black edge of the circle to the white gap and back to the black edge of the circle. Therefore, at least the gap of the two adapted Landolt rings rotated to the display angles of 258° and 78° appears relatively sharp to the test subject, cf. the two right-hand visual impressions in Fig. 1C.This is again because the subject's refractive error is fairly well and / or optimally corrected in their first principal section, i.e. along the first preferred direction V1, by providing the optical sphere correction of +2.75 dpt. The directional feature of the Landolt rings, the gap between which is displayed at the display angles of 180°, 135°, and 0°, is relatively steep (i.e., almost perpendicular) to the corrected first preferred direction V1, which is why these three optotypes in particular appear very blurred. As part of a visual task to determine visual acuity, the subject, corrected in their second preferred direction V2 with their optical sphere correction of -0.25 dpt, can now be asked where the gaps in the two optotypes displayed at the display angles of 168° and 348° point, e.g., whether they point more to the left or right.Alternatively or additionally, the test subject, corrected in their first preferred direction V1 with their optical sphere correction of +2.75 dpt, can be asked, as part of a different visual task to determine their visual acuity, where the gaps between the two optotypes displayed at display angles of 258° and 78° point, e.g. whether they point more upwards or downwards. This makes it possible to check whether or not the test subject can still recognize the directed feature as a detail of the adjusted optotype. Depending on the size of the detail that the test subject can just barely recognize, the visual acuity for the selected preferred direction V1 and / or V2 can be determined. To calculate the resulting visual impression, which is shown in Fig. 1B and 1C and in the following figures, it was assumed that the test subject's pupil diameter is 3.0 mm, the wavelength is 550 nm, and the distance to the display is 5 m.Furthermore, it was assumed that the optotypes are displayed as a rendered image with 1024x1024 pixels, with the image having a side length of 204.8 mm and the side length of a pixel corresponding to 40µrad = 0.1375 arc minutes. The Snellen E code belongs to the same category of adapted optotypes as the Landolt rings. The relevant feature, i.e. the directional feature of the Snellen E code, is the sequence: - dark area of ​​an outer cross line, - bright area of ​​the background, - dark area of ​​the middle cross line, - bright area of ​​the background, and - dark area of ​​the other outer cross line. If necessary, the bright background above and / or below can also form part of the sequence. Thus, when using Snellen E codes, the longitudinal line connecting the cross lines must be oriented parallel to the selected and corrected preferred direction V1 or V2, which, as with the Landolt ring, only allows two different orientations. Fig.Figure 2A shows additional adjusted optotypes for the subject with the exemplary refractive error. Borderless squares hatched with solid lines are used as adjusted optotypes and displayed on the screen. The hatching lines of the first, third, and fourth optotypes from the left are aligned parallel to a display angle of 78°, while the hatching lines of the second optotype from the left are aligned parallel to a display angle of 168°. The hatching lines of each optotype all have the same thickness and the same orientation. Any two adjacent hatching lines are always the same and constant distance from each other. The hatching lines provide a directional feature of the adjusted optotypes. The direction of the directional feature is the direction of the alternation of the light and dark areas, i.e., the direction perpendicular to the hatching lines.If the test subject is again corrected by applying a purely spherical optical correction of -0.25 dpt for his second preferred direction V2, the visual impression shown in Fig. 2B results for the test subject. The first, third and fourth optotypes from the left appear as grey spots, while the hatching of the second optotype can be recognised by the test subject. The test subject can thus recognise the adjusted optotype that differs from the others in a visual task for determining visual acuity. In the example, it is the second optotype from the left, whose directional feature is aligned parallel to the selected and corrected second preferred direction V2, i.e. the display angle of 78° corresponding to the measurement angle of 102°, i.e. the second principal section of the test subject's cylindrical refractive error.The distance between two adjacent hatching lines and / or the thickness of the black hatching lines can be used as the size of the detected detail to determine visual acuity. Fig. 3A shows further optotypes for the test subject with the example visual impairment. The optotypes used are again borderless squares hatched with solid lines and shown on the display. The hatching lines of the first, third and fourth optotypes from the left are aligned parallel to a display angle of 315°, while the hatching lines of the second optotype from the left are aligned parallel to a display angle of 135°. The size of the squares as well as the spacing and thickness of the hatching lines can correspond to the optotypes shown in Fig. 2A. Again, the second optotype from the left is different from the others.If the test subject is again corrected by applying a purely spherical optical correction of -0.25 dpt for his second preferred direction V2, the test subject will experience the visual impression shown in Fig. 3B. Since none of the optotypes is well adapted to the corrected second preferred direction V2 at the display angle of 78°, the test subject cannot recognize a single hatching of the optotypes, because for each of the four optotypes he will receive the visual impression of a gray spot or blurry square. The visual impression thus differs significantly from the visual impression resulting from optotypes optimized for the test subject, see Figs. 2A and 2B. Fig. 4A shows the test subject's visual impression when corrected by applying a purely spherical optical correction of -0.25 dpt for his second preferred direction V2 at the display angle of 78°.Four borderless hatched squares are displayed as optotypes, with their hatching lines parallel to a display angle of 168°. Thus, the directional feature of these optotypes is aligned parallel to the corrected second preferred direction V2, and the subject can at least identify the hatching of some of the adjusted optotypes, e.g., the two adjusted optotypes on the right. However, the hatching lines of the optotypes have different widths and different spacings. The left optotype has a distance between two black hatching lines of logMAR-0.66, the second optotype from the left has a distance of -0.26, the third optotype from the left has a distance of 0.14, and the fourth optotype from the left has a distance of 0.54. This distance can be used as a measure of visual acuity.If the test subject only recognizes the two right-hand optotypes, of which the more finely hatched optotype (i.e., the third from the left) has a distance of logMAR 0.14, this smallest still recognized detail can be used to determine visual acuity. Fig. 4B shows the test subject's visual impression when it is again corrected by applying a purely spherical optical correction of -0.25 dpt for his second preferred direction V2 at a display angle of 78°. Four borderless hatched squares are again displayed as optotypes, whose hatching lines have different widths and different spacings, as in Fig. 4A. From left to right, the optotypes have a distance between two adjacent black hatching lines of logMAR -0.66; -0.26; 0.14, and 0.54, just as in Fig. 4A.The hatching lines are displayed parallel to the display angle of 78°, which is why they appear maximally blurred to the test subject; this is because the directional features of the optotypes are aligned perpendicular to the corrected second preferred direction V2. Thus, the optotypes used in Fig. 4B can at best be used as optotypes that appear gray, but not as adjusted optotypes recognizable by the test subject corrected in this way. Fig. 5A shows further optotypes for the test subject with the ametropia used as an example. The optotypes used are borderless squares hatched with continuous lines and shown on the display. The hatching lines of all optotypes are aligned parallel to a display angle of 168°. However, only half of the squares are provided with the aforementioned hatching lines.For the first and third optotypes from the left, the upper halves are each provided with hatching, while for the second and fourth optotypes from the left, the lower halves are each provided with hatching. The other halves are filled in gray for the first and second optotypes from the left, while for the third and fourth optotypes from the left they have perpendicular (shortened) hatching, i.e., parallel to a display angle of 78°. The hatching lines provide a directional feature of the adjusted optotypes. The direction of the directional feature is the direction of the alternation of the light and dark areas, i.e., the direction perpendicular to the hatching lines. If the test subject is again corrected for his second preferred direction V2 by applying a purely spherical optical correction of -0.25 dpt, the test subject will have the visual impression shown in Fig. 5B.For the first and third optotypes from the left, the hatching appears in the upper half, and for the other two, in the lower half. The other half of each optotype appears as a gray patch. It makes little difference to the test subject whether the other half is actually filled with a medium gray value or with the hatching lines parallel to the selected second preferred direction. As part of a visual task for determining visual acuity, the test subject can thus be asked to differentiate between the displayed adjusted optotypes depending on their visual acuity. This means that optotypes with differently filled areas can also be used, particularly hatching that only fills part of the adjusted optotype. The distance between two adjacent hatching lines and / or the thickness of the black hatching lines can again be used as the size of the recognized detail for determining visual acuity.As an alternative to the hatched squares shown in the figures, all types of figures such as circles, rectangles, symbols, animals, letters, etc. that are filled with hatching can be used as adapted optotypes. They should preferably not have any edge lines that could influence the visual impression of the hatching. The hatching lines can be perpendicular to the selected and corrected preferred direction V1 or V2, since the directional and relevant feature is the alternation of light and dark areas of the hatching lines. It is advantageous for the method if the figure itself has as few details as possible, such as a circle or a square. To adjust the orientation, the entire optotype including hatching or just the hatching within the optotype can be rotated. The hatching can be binary, i.e. with hard black edges, or continuous, e.g. with a sinusoidal intensity curve.Visual tasks To determine visual acuity, the test subject can be given visual tasks in which adapted optotypes are displayed. With the visual tasks, a distinction can be made between visual tasks with active and passive feedback from the test subject. Active feedback can be understood as a statement from the test subject, which can be given either verbally or by consciously looking at an optotype and recording the direction of gaze using eye tracking. Passive feedback can be understood as following a presented optotype that is moving. Based on a gaze movement recorded by an eye tracking unit, it can be concluded whether the optotype is still reliably recognized. The optotypes are displayed with a defined presentation type and thus presented to the test subject. A presentation type refers to properties such as contrast, size or frequency of the hatching.The size is a particularly important property for optotypes of the type that can be displayed in two ways that are mirror-inverted with respect to the selected preferred direction, for example Landolt rings and Snellen rings. The frequency of hatching is a particularly important property for optotypes of the type that have hatched areas and / or consist of hatched areas. The presentation method can be worsened by changing the presentation method to make it less recognizable, for example by making it smaller (particularly in the case of adapted optotypes of the type that can be displayed in two ways that are mirror-inverted with respect to the selected preferred direction), reducing the contrast and / or increasing the hatching frequency (particularly in the case of adapted optotypes of the type that have hatched areas and / or consist of hatched areas).In one embodiment of a visual task with active feedback, one or more adapted optotypes of the type are presented which can be displayed in two ways that are mirror-inverted with respect to the selected preferred direction. The test subject is asked to recognize the orientation of the optotypes displayed on these optotypes. Displaying multiple optotypes of the same presentation type allows for a more reliable assessment of the response. A deterioration in the presentation type to the point where the optotypes can no longer be recognized allows the determination of visual acuity. In one embodiment of a visual task with active feedback, one or more adapted optotypes of the type are presented which have hatched areas and / or consist of hatched areas. The test subject is asked to recognize the presence of hatching in the optotype.Showing several optotypes of the same presentation type allows a more reliable assessment of the response. A deterioration in the presentation type to the point where the optotypes or the presence of hatching can no longer be recognized allows the determination of visual acuity. Furthermore, one or more adapted optotypes with an adapted orientation and one or more adapted optotypes with a different orientation can be presented, e.g. with an orthogonal orientation. For optotypes of the type that have hatched areas and / or consist of hatched areas, the optotypes can be presented with even filling and the test subject can be asked whether they can see any differences and / or which one or more optotypes differ from the others. This type of visual task is also referred to as a "forced choice".In one embodiment of a visual task with passive feedback, one or more adapted optotypes of one of the aforementioned types are presented and move. The presentation type can be deteriorated continuously and / or gradually. Based on the eye movement recorded by an eye tracking unit, conclusions can then be drawn about the presentation conditions under which the optotype and / or visual object can still be reliably recognized. From this, the subject's visual acuity can be derived and / or determined. The optotypes can be shown using a light field display. The applied optical sphere corrections do not have to be physically applied, but can be simulated as wavefronts. Combination with eye tracking For visual tasks with active feedback, eye tracking allows the process to be automated; for visual tasks with passive feedback, eye tracking may be mandatory.In one embodiment, the subject's gaze direction when completing the visual task can be determined using at least one image and / or video of the pupil and / or one or more Purkinje reflexes. The gaze direction can also be determined from a combination, e.g., using Purkinje reflexes determined in a video of the pupil. For this purpose, the device for determining visual acuity can have at least one calibrated image recording device, e.g., a digital camera. In one embodiment, an eye tracking unit is used to position an optical unit, i.e., for centering and / or focusing the optical unit. In this case, for example, a measuring head of the device can be centered and / or focused. This positioning can then be kept constant, and this eye tracking unit can be used to determine the subject's gaze direction, e.g., for passively and / or actively responding to a visual task.This allows different tasks to be solved with just a single eye tracking unit. Integration into an optometric measuring device The determination of visual acuity can be combined with a determination of refractive error, in particular for determining refractive error according to sphere, cylinder and axis. This can also determine lower and possibly higher order aberrations. For this purpose, a device for determining visual acuity is connected and / or combined with an autorefraction or aberrometry unit. In one embodiment, an autorefractometer and / or an aberrometer is used as the refraction unit, which has a display unit and an optical unit used to display the adjusted optotype and thus the target. If necessary, this unit can also be used for fogging for the determination of visual acuity without the need for additional optical components.Ideally, the display unit is designed as a programmable display in order to be able to show images adapted to the various tasks. Alternatively, the optotype display can also be used in a fogged form for the autorefractometric and / or aberrometric measurement. The displays can be switched between using a beam splitter and / or mechanical means. Such a device allows the refractive error to be determined first using the autorefractometer and / or aberrometer, and from the result of this measurement, the refractive error data can be derived, along with the preferred direction and the power to be applied for this preferred direction. With such a device, an existing eye tracking unit, which is used to center and / or focus the autorefractometric and / or aberrometric measurement, can be used for eye tracking the subject's gaze direction.The device can use the same unit to present at least one optotype and thus the target and, if applicable, a fogging during the autorefractometric and / or aberrometric measurement. For autorefractometric and / or aberrometric measurements, the use of a comparatively small optotype as a target can be sufficient or even useful, as it pre-adjusts the subject's gaze before fine adjustment is effected by looking at a distinguished optotype. In the fogged state, a small optotype can be used as a target and / or be advantageous, since - if the subject cannot perceive any details - the small bright spot of a small target guides the gaze better than a larger one.For visual acuity determination, however, a larger optotype is often helpful as a target, as it allows the presentation of several different optotypes and, particularly for visual tasks that require eye movement, a larger eye movement. This can be achieved with a dividing device that allows both the vergence and / or divergence, i.e. the "optical distance", of the light from the presented target as well as the size of the image of the presented target. This can be achieved, for example, by combining a display with two axially movable spherical lenses, where the axial distances between the two lenses can be adjusted independently of the display. One or more other measuring units can be integrated in addition to or instead of the display, such as an opacity unit, a topography and / or topometry unit, a Scheimpflug camera and / or a tonometry unit.Here, too, individual components from multiple units can be used. Influence of accommodation and other principal sections If the precise design of the optotypes and / or the visual task could lead to an influence of accommodation and / or the principal section not being viewed, the procedure can be varied as follows. Accommodation pulls the planes in which both principal sections produce a sharp image forward (i.e. in the direction from the retina to the lens). In this embodiment, the posterior principal section (i.e. the one refracted less by the eye) is therefore sharply imaged on the retina with the spherical correction and / or if fogging is desired (e.g. as part of a sensitivity determination), it is imaged in front of the retina.In order to avoid the influence of accommodation, the direction in which the principal section refracted less sharply by the eye produces a sharp image is selected as the preferred direction for the presentation of the optotypes. According to one embodiment, the principal section for which refraction is weaker in the subject's eye is thus selected as the preferred direction. This can reduce the effect of accommodation on the determination of visual acuity. In order to avoid the influence of the other principal section when determining visual acuity for an optical sphere correction that deviates from the best correction in the principal section in question, e.g. in the case of applied blur, e.g. to determine a sensitivity, the correction can deviate from the direction that would be required for the correction in the other principal section.Here, particularly with regard to the aforementioned consideration of accommodation, an examination of the less refractive principal section and a blur in the positive direction are used, which is also referred to as fogging in the narrower sense. Thus, in one embodiment, the principal section for which weaker refraction occurs in the subject's eye is selected as the preferred direction. To determine sensitivity, a visual acuity value is determined by applying an optical sphere correction that deviates in the positive direction from the determined optimal optical sphere correction. In addition, a visual acuity value can be determined by applying the determined optimal optical sphere correction. From these two visual acuity values, both the subject's visual acuity and their sensitivity can be determined as accurately as possible, whereby the influence of both accommodation and the other principal section can be reduced.Consideration of HOA If a wavefront measurement of the eye is performed, possibly also taking higher order aberrations (“Higher Order Aberration”, abbreviated as HOA) into account, this wavefront measurement can be used instead of the objectively and / or subjectively determined refraction values ​​to provide the refractive error data. This allows the preferred direction to be selected based on the wavefront measurement, to which the adjusted optotypes are then aligned. Furthermore, the strength of the optical sphere correction to be applied for this selected preferred direction can also be derived from the wavefront measurement. A point spread function can be determined from the wavefront data, the optical sphere correction (and / or sphere corrections) applied when determining visual acuity, and a pupil size using methods known from the literature.Instead of determining the preferred direction of the adjusted optotypes based on the subjective and / or objective refraction in second (Zernike) order, the preferred direction can be derived from the point spread function, e.g. from the direction and / or axis of the smallest extent of the point spread function. In this case, the direction of the smallest confusion can be selected, e.g. as the direction of the smallest standard deviation of the point spread function. Determining the sensitivity In some embodiments, the sensitivity of at least one eye of a test subject or spectacle wearer is determined. This allows the calculation, optimization or evaluation of a spectacle lens for the at least one eye of the test subject to take into account the determined sensitivity of the at least one eye of the test subject. This can be used in the manufacture of a spectacle lens.In methods for optimizing a spectacle lens according to the state of the art, a spectacle lens is optimized by minimizing or maximizing an objective function that incorporates actual values ​​and corresponding target values ​​of at least one imaging property or aberration of the spectacle lens. The at least one imaging property or aberration can represent a direct quantification of a wavefront deviation from a reference wavefront. An example objective function is the function: where: i (i = 1 to N) denotes an evaluation point of the spectacle lens; R Ist (i ) the actual spherical power or refractive error at the i-th evaluation point; R Ist (i ) denotes the spherical target power or the target refractive error at the i-th evaluation point; A st Ist (i )denotes the astigmatism or astigmatic error at the i-th evaluation point; A st Soll (i ) the target astigmatism or the target astigmatic error at the i-th evaluation point. The quantities G R,i , G A , i ,...are weights of the respective imaging property or aberration that are used in the optimization. A direct quantification of a wavefront deviation in diopters without taking into account the effective pupil size is not the best possible criterion for describing and assessing the perception of a spectacle wearer through a spectacle lens due to the dependent depth of field. On the basis of this knowledge, DE 10 2017 007 663 A1 proposes to directly consider the visual acuity (visual acuity) in the target or quality function. The visual acuity included in the target or quality function depends on an assignment of at least one imaging property or aberration of a spectacle lens system, whereby the at least one imaging property or aberration can be evaluated on a suitable evaluation surface (e.g. on the vertex sphere or in the eye). The spectacle lens system can consist of at least one spectacle lens (e.g.a spectacle lens of refractive spectacles). However, the spectacle lens system preferably comprises further components such as a model eye or eye model, which can be based on average values ​​of spectacle wearers or on at least one individual parameter of the wearer's eye. In other words, the spectacle lens system on which the assignment of at least one imaging property or aberration to the wearer's visual acuity is based can be a spectacle lens-eye system. As described in DE 102017007663 A1, an exemplary target or quality function that is derived from the visual acuity V via the assignment of the at least one imaging property or aberration. DU s , j to the visual acuity of the wearer or an average wearer, e.g. have the following structure: In the above formula, a function that describes the dependence of visual acuity on at least one imaging property or aberration of a spectacle lens system at the i-th evaluation point (i = 1, 2, 3, ..., N) on an evaluation surface. In other words, an exemplary assignment of at least one imaging property or aberration of a spectacle lens system to the visual acuity of the subject or spectacle wearer or of an average spectacle wearer when viewing an object through the spectacle lens system. The argument DU s , jis generic and can refer to any imaging property or aberration of a spectacle lens system, which describes the effect of the spectacle lens system on a light beam emanating from an object or the difference between the effects of the spectacle lens system on a light beam emanating from an object and on a reference light beam converging on the retina of the eye. One or more imaging properties or aberrations can be included in the objective or quality function and evaluated, where the subscript , ≥ 1 denotes the j-th imaging property or aberration. V Ist ( DU s, j ( i) ) denotes the visual acuity, which is determined based on the assignment and the actual value of at least one imaging property of the spectacle lens to be calculated (e.g., to be optimized) or evaluated at the i-th evaluation point, and V Soll ( DU s, j ( i)) denotes the corresponding target value of the visual acuity. The at least one imaging property or aberration can be calculated or evaluated on a suitable evaluation surface. The subscript "s" accordingly stands for any evaluation surface of the at least one imaging property or aberration. DU s , j . The evaluation surface can be, for example, a plane (evaluation plane) or a curved (e.g., spherical) surface. The evaluation surface can be, for example, the vertex sphere or a surface in the eye, e.g., one of the following planes or surfaces: a plane or a (e.g., spherical) surface behind the cornea, the anterior surface of the eye lens or a plane tangential to the anterior surface of the eye lens, the posterior surface of the eye lens or a plane tangential to the posterior surface of the eye lens, the plane of the exit pupil (AP); or the plane of the posterior surface of the lens (L2). GV The size s,iso , irefers to the weighting of the image property DU s , jspecified visual acuity at the i-th assessment point. For example, one of the visual acuity models described in DE 102017007663 A1 or any other suitable visual acuity model (which in particular describes the visual acuity as a function of refraction or mal-refraction) can be used, preferably in combination with a rule as to how the visual acuity model is to be incorporated into the objective function of an optimization in conjunction with a transformation of the target specifications and weights. It should be noted at this point that, within the scope of this description, a sensitivity metric (as described further below) can preferably be used based on such a visual acuity model (as a functional dependence of a visual acuity value on the refraction / mal-refraction). In particular, a preferred sensitivity metric could be used as the derivation of a visual acuity model (i.e. the function of the visual acuity value on the refraction / mal-refraction) after the refraction / mal-refraction.The objective function can also be used to evaluate a spectacle lens, whereby the actual value of at least one imaging property of the spectacle lens to be evaluated is calculated at at least one evaluation point on the spectacle lens to be evaluated and compared with the corresponding target value. As is also clear from DE 10 2017 007 663 A1, knowledge of the so-called sensitivity, i.e. the change in visual acuity with incorrect refraction, is particularly helpful for calculating, optimizing and / or manufacturing highly individual and high-quality spectacle lenses. In this way, the assignment of at least one imaging property or aberration of a spectacle lens system to the visual acuity of the wearer or the function can be determined. V ( DU s, j ( i)) depend parametrically on the measured initial visual acuity and / or the determined sensitivity of the spectacle wearer. Sensitivity is a (particularly phenomenological) quantity or parameter used in ophthalmology and ophthalmology with which the dependence of visual acuity on a refractive error can be described or specified. The sensitivity of an eye is understood in particular to be the change in the visual acuity of the eye when there is a change in a refractive error. In particular, sensitivity can be defined as the derivative of the visual acuity after the refractive error or as the local derivative of the visual acuity after the refractive error for a specific refractive error. The refractive error is a deviation of an effect or refraction presented to at least one eye of the test subject when determining the visual acuity from an ideal refraction determined or known for at least one eye.The ideal refraction (hereinafter also referred to as optimal refraction or target refraction) can be determined, for example, from a conventional objective and / or subjective refraction measurement. In particular, the sensitivity describes how much the visual acuity changes when an optical effect or correction in front of the eye changes. The sensitivity can be quantitatively described, in particular, with the help of a sensitivity metric and / or with the help of a visual acuity model. The sensitivity of at least one eye of a test subject can thus be taken into account when calculating and / or creating individual spectacle lenses, in particular when creating multifocal spectacle lenses such as ophthalmic spectacle lenses. Spectacle lenses can have transitions between areas with different optical corrections, for example transitions between a visual point for distance and a visual point for near.These transitions between lens areas with different optical corrections can be designed differently. For example, they are referred to as hard transitions or soft transitions, depending on how strong or gentle the change in refraction along the transition is. With highly customized and high-quality lenses, such a transition (but also other areas of the lens) can be adjusted to the sensitivity of at least one eye of the subject or wearer. To determine the sensitivity of at least one eye of a subject with regard to blur, the presence of at least two applied powers and the resulting visual acuity is required. Within the scope of the invention, these can be determined as visual acuity-refraction value pairs of the visual acuity characteristics.Relevant models and corresponding formulas for calculating sensitivity are described below. According to the state of the art, to determine sensitivity, quantized powers are presented to the subject or to at least one of the subject's eyes (e.g., in steps of 0.25 D using conventional trial lens sets). Using a visual acuity chart with optotypes in quantized sizes or quantized visual acuity steps, the corresponding visual acuity is determined for each of the presented powers. Furthermore, the optimal correction (or the optimal refraction or target refraction) must be determined for the subject in order to convert the presented powers into an incorrect refraction. The double quantization associated with the conventional method leads to high measurement uncertainty.Conventional methods are not only complex but can also be psychologically disadvantageous, since after the optimal refraction has been determined, at least one of the test subject's eyes is provided with a poorer correction, and the test subject must then perform visual tasks with this poorer correction to determine the sensitivity. This sequence is necessary with the conventional procedure, since a defined fogging for visual acuity measurement can only be set once the optimal refraction is known. According to one aspect, it is therefore an object of the present invention to determine the sensitivity of at least one of the test subject's eyes, which is required in particular for calculating, optimizing, evaluating and / or manufacturing highly individual and high-quality spectacle lenses, in an improved manner, in particular simply and quickly.Furthermore, it may be an object of the present invention to provide a method and a device for calculating, optimizing, evaluating, and producing spectacle lenses that are highly individualized and of high quality due to the consideration of the sensitivity of at least one eye of the subject. It may also be an object of the present invention to provide such improved spectacle lenses. Determining the sensitivity as visual acuity characteristics by varying the applied optical power. In some embodiments, the sensitivity of at least one eye of a subject is determined as visual acuity characteristics based on at least two provided visual acuity-refraction value pairs.While in a first alternative, the applied optical power is kept constant and varied, in a second alternative, the dimension of the directional feature of the adjusted optotype is kept constant and the applied optical power is varied. This second alternative is explained in more detail below.In this second alternative, the visual acuity-refraction value pairs can be provided by the following steps: - projecting a target, which can contain at least one adapted optotype, with an adjustable target refraction corresponding to the applied optical power, into the at least one eye of the subject, wherein the target is designed to verify a predetermined visual acuity; and - determining a visual acuity limit refraction of the at least one eye of the subject associated with the predetermined visual acuity by varying the target refraction of the target projected into the at least one eye of the subject and detecting a subject action with which it is determined that the identifiability of the target for the subject has changed at the time of the subject action.As already mentioned at the beginning, the "sensitivity" (with regard to blurriness) of at least one eye of the subject is understood to mean the dependence of the visual acuity of at least one of the subject's eyes on a refractive error, where the "refractive error" is a deviation of an effect or refraction applied to at least one of the subject's eyes during visual acuity determination from an ideal or optimal refraction (target refraction) determined or known for at least one eye. "Visual acuity" is a measure of the (central) visual acuity of at least one of a subject's eyes. Visual acuity is usually determined in bright conditions. In particular, visual acuity can be defined as the inverse of the smallest detectable gap in the standard optotype, the Landolt ring. In humans, visual acuity can be determined using a vision test. For this purpose, the subject is presented with optotypes, and the subject's answers determine whether the subject has correctly identified them.Visual acuity depends on which optotypes the test subject can recognize with the set and / or applied refraction. The optotypes usually have a defined size, brightness, shape, and contrast. The optotypes can be displayed or projected on a chart. In the method according to the invention, the target comprises at least one adapted optotype for each visual task, in which the directional feature is arranged parallel to the selected preferred direction. The use of a projector instead of a chart has the advantage of being independent of the test distance. DIN standards exist for reproducible visual acuity testing. According to these, the standard optotype is the so-called Landolt ring, a ring of defined width with a gap of the same width that can be arranged in eight different directions.By recognizing the direction of the gap, the test subject demonstrates that their resolving power corresponds at least to the width of the gap. In practice, however, standardized images of numbers are usually used as optotypes to facilitate communication. There are also other standardized optotypes, such as the "Snellen-E", the "Pflüger-E-hook", in which the middle line is shorter, and others that are suitable for testing the visual acuity of illiterate people and preschool children, as well as for non-verbal communication. When determining visual acuity, a distinction is made between that with correction, such as glasses or contact lenses, and that without correction. Visual acuity without correction is also referred to as raw visual acuity. The abbreviations "sc" ("sine correctione", Latin for "without correction") and "cc" ("cum correctione", Latin for "with correction") are also frequently used.The sensitivity of at least one eye can be determined in particular based on a sensitivity metric. By using a sensitivity metric, a sensitivity can be calculated even if the applied refraction values ​​do not have a predetermined distance from each other. The sensitivity metric represents the dependence of visual acuity on a (mis)refraction. The distance between two refraction values ​​can be part of a sensitivity metric. The sensitivity metric can be defined in the metric space of refraction values. Each refraction value of the sensitivity metric can be assigned a visual acuity value, or vice versa. The refraction can, for example, be defined in an at least three-dimensional space. For example, a refraction value can usually be described with the coordinates s, c, and α.Here, s can depend on the strength of an optical correction of the sphere, c on the strength of an optical correction for a cylinder, and α on the axial position of this cylinder. The strength of the optical correction for the cylinder is sometimes denoted by z as an alternative to c. In this metric space of refraction values, at least the refraction values ​​for a given first and a given second visual acuity can be determined and are therefore known when calculating the sensitivity. The sensitivity metric can be used to determine the sensitivity as a function of basically any two different refraction values. By using such a sensitivity metric, the determination of the sensitivity is independent of visual acuity measurements at given refraction values, as is common with conventional methods.In this way, the determination of sensitivity can be made independent, on the one hand, of visual acuity measurements at at least one predetermined and / or fixed refraction distance from the refraction result (or from the optimal refraction or target refraction), and, on the other hand, of visual acuity measurements at at least one predetermined and / or fixed relative refraction distance between the two applied refractions. This can make it easier for both the refractionist and the test subject to determine the measurement data required to determine sensitivity. Examples of a sensitivity metric The sensitivity can be calculated using a metric space in which different refraction values ​​represent individual points. A refraction value can, for example, be represented three-dimensionally, e.g., with the coordinates s, c, and α. Here, s can depend on the strength of a spherical correction and, for example,in diopters (which can also be abbreviated to dpt). c can depend on the strength of a cylindrical correction and can be specified, for example, in dpt. α can depend on the axial position of the cylindrical correction and can be specified, for example, in degrees, e.g. from 0 to 180°. Alternatively, other coordinates can be used. In the following, it is assumed, by way of example, that the best refraction (also referred to as optimal or ideal refraction in this description), i.e. in particular a specific objective, subjective refraction result, is referred to in this sensitivity metric as s0, c0, and α0 and the corresponding visual acuity as v0. When carrying out the method, at least two visual acuity-refraction value pairs are provided. In general, n refractions s1, c. i , α iwith the corresponding visual acuity vi with i є [1, …, n] and n ³ 2. At least one visual acuity-refraction value pair of at least one eye of the subject may already be known and provided as a known value pair. The provision includes, in particular, determining and / or measuring. In a possible sensitivity metric, the distance of a refraction i from the best refraction in the middle sphere di and in the cylinder ai is calculated using equation (1): Simple bilinear model of a sensitivity metric with knowledge of a target refraction. In one embodiment of a bilinear model of a sensitivity metric, the following relationship shown in equation (2) applies to the dependence of visual acuity for each individual measurement for a refraction i. In a simplified case, it can be assumed that the subject cannot compensate for fogging through accommodation. lg 7 # = 89∙ | " #| + 8 : ∙ / # + lg 7 . (2) Here, m d for the sensitivity at a spherical distance and m a for the sensitivity at a cylindrical distance. Such a separation between a spherical and a cylindrical refraction error can be used to account for the fact that subjects can react very differently to these two components of a refraction error. For example, from data from D. Methling: Bestimmung von Sehhilfen, 2nd ed. Ferdinand Enke Verlag, Stuttgart 1996, it can be determined that, empirically determined for the population average, equations (3) approximately apply: In general, the above equation (2) has the independent parameters a , m d , v0. Therefore, the system of equations (2) can be solved with three measurements i=1,2,3 of refractions (s1, c1, α1; s2, c2, α2; s3, c3, α3) at three (in particular given) different visual acuity values , v2, v3can be uniquely solved to the system of equations (2a): For example, a visual acuity measurement can be performed under optimal correction conditions, i.e., at the target refraction (in particular, determined from an objective and / or subjective refraction measurement). Then, for example, at i=3: (s3, c3, α3) = (s0, c0, α0). Under this optimal correction condition, a3 = a0 = 0 and d3 = d0 = 0. Thus, the third of equations (2a) is automatically fulfilled. The other equations then take the following form of the system of equations (4): The system of equations (4) thus provides an exemplary embodiment of a simplified bilinear model of a sensitivity metric. The system of equations (4) can be solved for two additional visual acuity values ​​(for i=1,2) with knowledge of the target refraction and two additional refraction values. The visual acuity-refraction value pairs used here can be determined using the method according to the invention. Thus, the sensitivity can be determined from the system of equations (4). The sensitivity describes the dependence of visual acuity on the (mis)refraction. This can be expressed, for example, by the values ​​8 :and 89. If, in addition to the visual acuity v0, more than two additional refractions are measured at given visual acuity values ​​during the target refraction, the sensitivity can be determined more accurately by determining md and ma from all data using a fitting procedure, e.g., the least squares method. Furthermore, outliers can be excluded from the measured data to increase the quality of the sensitivity determination. Simplified linear model of a sensitivity metric with knowledge of a target refraction. In a further simplified, less individual model of the sensitivity metric, e.g., if only one measurement is available for an incorrect refraction i=1, a relationship between the spherical and cylindrical refraction distance can be assumed according to equation (5): Here, the parameter f can be derived from empirical values ​​and, for example, be a scalar. With the assumption according to equation (5), the system of equations (2) simplifies to the following equation (6): lg 7 # = 8 ∙ ( " # + F ∙ / # ) + lg 7 . lg v G = m ∙ ( a G + f ∙ d G ) + lg v . (6) This allows the sensitivity m to be determined from a measurement at a misrefraction i from equation (7): A value for f can be derived from relevant specialist literature, e.g., f = 1 / 2 can be set, derived from Applegate, RA, Sarver, EJ, Khemsara: “Are all aberrations equal?”, J Refract Surg. 2002, 18: pages 556–562. Or f = 1 can be set, derived from Atchison et al.: “Blur limits for defocus, astigmatism and trefoil”, VisionResearch, 2009. A linear relationship does not necessarily have to be assumed for equation (5). Alternatively, more complex relationships can be set up and the sensitivity derived from them, e.g. as a function of a number of independent parameters and / or refraction measurements, by inserting them into appropriately resolved relationships, cf. equations (4) and (7). The sensitivity can also be derived from a fitting method, such as least squares.Further models of a sensitivity metric with knowledge of the subjective refraction The sensitivity can also be calculated on the basis of a different model. For example, models from R. Blendowske, Unaided Visual Acuity and Blur: “A Simple Model”, Optometry and Vision Science, Vol. 92, No. 6, 2015, are known which are characterized by their particular simplicity and which are based on only a few parameters. Such simple models are particularly suitable for calculating sensitivity and for adaptation when there is little data, for example because they can easily avoid overfitting. If a larger number of parameters are individually available, a model with many different parameters is more suitable, as is described in the document DE 102017007663 A1. In principle, a variety of different models can be used. The model used in an individual case can depend on the number of parameters provided ordetermined visual acuity-refraction value pairs. With a sufficiently large number of visual acuity-refraction value pairs, relatively complex, not necessarily linear models can be set up whose parameters can be adapted to the measurements. The models listed above as examples can be generalized, e.g. by having a function describing visual acuity in the power vector space have contours of constant visual acuity, which correspond to ellipsoids or ovoids containing the point of maximum visual acuity. This can be done analogously to a method presented in A. Rubin and WF Harris: "Closed Surfaces of Constant Visual Acuity in Symmetric Dioptric Power Space", Optometry and Vision Science, Vol. 78, No. 10, 2001. Axial ratios can vary individually within a range of 0.25 to 4.Instead of individually measured values, means, medians, or other estimates of the corresponding model parameters of the population can also be used to calculate visual acuity. In one embodiment, a generalization of the above equation (6) leads to different factors f, e.g., equation (8):. Where ai ort and ai ort the astigmatism of the refraction error with orthogonal (J0) or oblique (J45) axis positions and are defined as: / PST \H # = − 2 sin(25#) + \N2 sin(25.). U represents a rotation matrix, which represents the orientation of an ellipsoid of constant visual acuity in the power vector space of the vectors PQR PST / # , / # * determined. The eigenvalues ​​8 D , 82, 8 Wdenote the sensitivities to fogging in the direction of the first, second, and third column vectors, respectively, of the rotation matrix U in the power vector space. Embodiments of models of a sensitivity metric without knowledge of the target refraction In some embodiments, the sensitivity can be determined without knowledge or determination of the target refraction. This can be done if an associated refraction or visual acuity limit refraction is determined for several predetermined different visual acuity values. In this case, the best refraction or target refraction can be determined from the resulting measurement data. Furthermore, an actually determined best refraction can be checked from the measurement data using a model of a sensitivity metric. It can be assumed that fogging, i.e., an intentional incorrect refraction, can be compensated for by the test subject through accommodation of at least one eye.In this case, a point can be selected in the linear model according to the above equations (2) and (6) at which the visual acuity curve bends. In non-linear models where saturation occurs, the best refraction can be calculated directly as a parameter of the system of equations. To do this, the misrefraction, i.e. the distances di and ai, must be replaced in the corresponding formulas, in particular already in equation (1), by the difference between the best refraction and the set or applied correction. The embodiments of models of a sensitivity metric explained above represent examples to clarify how the sensitivity can be determined within the scope of the present invention. The target can in particular be a real target (or real object) or a virtual target (or virtual object). In particular, the target can be a real object or a virtually projected object (ora projected virtual object). A target can be realized, for example, by a display (e.g. with one or more lenses and / or with one or more mirrors), by a light field display, and / or by a badaloptometer (which enables constant magnification despite changes in the effect) and projected into at least one eye of the test subject. A "virtual object" or "virtual target" is understood in particular to be an optical imaging system which generates wavefronts emanating from virtual object points so that these wavefronts hit at least one eye of the test subject.The wavefronts generated by the virtual target (each corresponding to a virtual object point) and impinging on at least one eye of the subject can have an adjustable spherical curvature and / or an adjustable cylindrical curvature component, wherein the cylindrical curvature component is preferably adjustable both with regard to the amount of curvature and with regard to the axial position. Preferably, the virtual position of the virtual object (target) can be changed so that different accommodation states of the at least one eye can be stimulated in this way. In particular, the position of the virtual object can preferably be changed between a position for stimulating distance accommodation and a position for stimulating near accommodation.In addition, the position of the virtual object can preferably be adjusted in such a way that at least one eye of the test subject is no longer able to accommodate the virtual object. In this case, the virtual object (target) can only be perceived by the test subject as blurred in all directions. This causes the ciliary muscles to relax. Such a state is referred to as a "foggy" state. A target is projected into at least one eye of the test subject with an adjustable or variable target refraction (or target effect). This projection can be carried out with the aid of an optical system with which the effect or refraction of the target, i.e. the target refraction, can also be adjusted and / or varied. In the context of this invention, "target refraction" therefore means the refraction (applied or applied by the optical system).induced) refraction (in particular spherical and / or astigmatic refraction) with which the target is projected into at least one eye of the test subject or with which the target is presented to at least one eye of the test subject. A target is regarded in particular as an optical projection into or onto the eye of the test subject such that this projection creates an image on the retina of the eye which corresponds to the image of a real object at a specific distance from the eye. This specific distance is also referred to here as the virtual position for the virtual target. In other words, a target within the meaning of this description is in particular an image of an object in at least one eye of the test subject. A backlit slide, for example, can be used as an object.Since in the case of a virtual target the target is not (directly) a real object at the virtual position, a virtual position beyond infinity can also be simulated by appropriately designing the optical system for projection. This then corresponds to wavefronts that converge towards the eye (i.e. in the propagation direction). The projection of a target (in particular a virtual target) into at least one eye of the subject using an optical system is generally known, so it will not be discussed in more detail within the scope of the present invention. For example, the projection of a target into at least one eye of the subject is described in K. Nicke and S. Trumm: "Brillengläser der Zukunft – Schritt 3 Der DNEye Scanner", Der Augenoptiker, June 2012, or also in the publication DE 102013000 295 A1.The target projected into at least one eye of the test subject is designed to verify a predefined, in particular predetermined and / or known, visual acuity (or a predefined visual acuity level). “Verifying a predefined visual acuity” is understood here in particular to mean that with the aid of the target it can be determined or ascertained (in particular based on a test subject action) whether the at least one eye of the test subject achieves the predefined visual acuity or the predefined visual acuity level. In other words, the target specifies a specific visual acuity or a specific visual acuity level, the attainability of which for the at least one eye of the test subject can be determined (in particular based on a test subject action). In particular, the target is designed (in particular dimensioned) in such a way that a predefined visual acuity or a predefined visual acuity level can be or is assigned to the virtual target.In other words, the target is a target with a predetermined visual acuity or a predetermined visual acuity level. This means that the test subject, in particular with ideal refraction or with a correction of any visual impairment of at least one eye of the test subject, can recognize or identify the target, provided that at least one eye of the test subject reaches or has at least the visual acuity or the visual acuity level predetermined by the target. This visual acuity level is linked to the dimension of the directional feature of the adapted optotype and / or depends on it. In particular, the target can comprise or be an adapted optotype suitable for determining the visual acuity. The dimension or size of the directional feature of the optotype depends on the predetermined visual acuity or the predetermined visual acuity level. In particular, the dimension orThe size of the directional feature of the optotype is chosen such that only a test subject with a visual acuity that corresponds at least to the specified visual acuity or the specified visual acuity level can recognize and / or identify the directional feature of the optotype. The target can also be an image or photograph containing two or more details, the recognition of which can each be assigned to a specified visual acuity or a specified visual acuity level. The image can in particular depict objects (such as a road leading to infinity, a sky, a distant balloon, etc.) that can evoke a feeling of vastness or distance in the viewer. The above-mentioned details contained in the image (such as symbols or strips of fabric on a hot air balloon or the basket of a hot air balloon, clouds or symbols on clouds, lines on a road, symbols on signs at the roadside, etc.) are expressly included in the term optotype in this description. A particularly suitable symbol as an optotype comprises, for example, one or more concentric rings which merge into a circle when the degree of blur is given. The determination of the visual acuity or the visual acuity levels of a target, target or optotype can, as is well known, be carried out, for example, by calculating the angle of view of details or by recognizing test subjects with known visual acuity characteristics. After projecting the target into at least one eye of the test subject, a visual acuity limiting refraction of at least one eye of the test subject is determined which corresponds to the given visual acuity or the given visual acuity level. The “visual acuity limiting refraction” or “visual acuity level limiting refraction” is understood to mean the refraction or limiting refraction at or above which the identifiability of the target for the test subject changes. In particular, the “visual acuity limiting refraction” or“Limiting visual acuity refraction” is understood to mean the refraction or limiting refraction at which the test subject can a) first recognize and / or identify the target held up to them or the virtual target projected into their at least one eye, which is characterized by a given visual acuity or a given visual acuity level, starting from a blurred state by varying the target refraction (created or brought about by the optical system), or b) just no longer recognize and / or identify it starting from an unblurred state by varying the target refraction (created or brought about by the optical system). The limiting visual acuity refraction is determined by varying the target refraction of the target projected into the at least one eye of the test subject and by detecting a test subject action (e.g. a message or input from the test subject, in particular pressing a button or joystick).The target refraction can be varied step by step or, preferably, continuously. The target refraction is preferably varied monotonously and / or continuously. The test subject’s action signals or establishes that the test subject’s identifiability of the target has changed at the time of the test subject’s action. In other words, the test subject signals by means of the test subject’s action that they can recognize or identify the target for the first time with the target refraction present or applied at the time of the test subject’s action, or that they can no longer recognize or identify it for the first time or at the moment. In particular, the visual acuity limit refraction corresponds to the target refraction or target effect present or applied by the optical system at the time of the test subject’s action. The sensitivity of at least one eye of the test subject is thus determined taking into account the specified visual acuity or target effect.the specified visual acuity level and the determined corresponding visual limit refraction. For this purpose, adapted optotypes can be used, the dimension of the directed feature of which is assigned to the specified visual acuity values ​​or the specified visual acuity levels. The method can be carried out in particular within the framework of autorefractometric or aberrometric measurements. For this purpose, at least one pair of visual acuity level and the corresponding applied power is recorded. This is done by a signal from the subject during the change of the applied power for a target with a defined visual acuity level (i.e., defined size of an optotype). As already mentioned, at least two pairs of visual acuity level and the corresponding applied power are required to determine the sensitivity. With conventional methods, for defined applied powers, it is determined which visual acuity level the subject achieves with these powers (i.e.at which size the test subject can still recognize optotypes). In this variant of the method, however, the dimension of the directional feature of the adjusted optotype (and thus the visual acuity level) remains constant for at least one of these pairs and the applied power is changed. The test subject signals when they can just about recognize or can no longer recognize an adjusted optotype with a defined size. In contrast to the state of the art, in this alternative, the visual acuity level for a specific applied power - with a priori known or a priori unknown refraction error - is not required to determine the sensitivity, but rather the applied power that is necessary to achieve a given visual acuity. This procedure allows the sensitivity to be determined quickly and easily.In particular, the procedure allows for the determination of sensitivity (as a subjective measurement) during a normal objective refraction measurement simply and without significant additional effort. In particular, it avoids complex measurements during a subjective refraction, and eliminates the psychologically disadvantageous step of providing the subject with a worse correction after determining the best refraction and thus requiring them to perform visual tasks. Furthermore, the procedure can be advantageously combined with other measurements to determine individual parameters for advanced spectacle lenses (e.g., near vision measurement, pupillometry, keratography) and for optometric or ophthalmological screening, or with measurements for diagnostic purposes (such as keratography, opacity, pachymetry, tomography, tonometry, or retinal imaging).In one embodiment, prior to the step of projecting a target designed to verify (or determine) a predetermined visual acuity into at least one eye of the subject, an objective and / or subjective refraction result (in particular a combined refraction result based on an objective and subjective measurement, which also includes further data such as lower and / or higher order aberrations from aberrometry or further biometric data such as corneal shape, lens-retina distance, anterior chamber depth, etc.) of at least one eye of the subject is determined. A "refraction result" is understood in particular to mean a determined refraction value. In this way, in contrast to the previous procedure, the determination of the sensitivity can be combined with one or more aberrometric or autorefractometric measurements.In particular, the determination of visual acuity can be combined with the measurement of autorefractometric or aberrometric data in the non-accommodated and accommodated states. Preferably, the objective refraction value or the objective refraction result is determined in a fogged state. For this purpose, a target (e.g. an image or photo) can be presented to the test subject or a corresponding virtual target can be projected into at least one eye of the test subject (with the aid of the optical system), which has an effect that leads to the test subject only being able to see the target blurred (or not completely sharply), thereby achieving relaxation of the ciliary muscles of at least one of the test subject's eyes. Such fogging can, for example, be carried out with an additional effect of approximately 1.25 dpt to 1.5 dpt compared to the optimal refraction of at least one of the test subject's eyes.In a special embodiment, the accommodative state of the eye can also be tracked in order to obtain even more reliable values ​​for the sensitivity. Preferably, before the step of varying the target refraction, the target is projected into at least one eye of the test subject with such a starting target refraction, i.e. the initially applied optical power at least in the selected preferred direction, that the test subject can only perceive the target in a blurred manner (or not completely sharply) and / or cannot identify it. In other words, a starting target refraction is preferably selected such that the test subject cannot focus the target or adjusted optotypes through accommodation. This is achieved in particular by shifting the starting target refraction in the plus direction compared to the optimal refraction of at least one eye of the test subject.Only by changing the target refraction towards minus can a state be achieved in which the test subject can recognize and / or identify the target or optotype. This has the additional advantage that the test subject does not initially know the target or optotype and is therefore more likely to carry out the test subject's action at the right time, namely only when they can actually identify the target or optotype. If, on the other hand, the test subject already knows the target or optotype beforehand or at the start of the measurement (due to a corresponding starting target refraction with which they can see the target or optotype clearly), it has been recognized within the scope of the present invention that although such an approach is a possible alternative, it may be inferior to the above-mentioned preferred embodiment in terms of accuracy and reliability. This is because a test subject who has already identified the target or optotypeA person who already knows the optotype in advance often tends to signal the point in time at which, after varying the target refraction in the plus direction, they no longer recognize and / or can no longer identify the target or optotype somewhat too late. In a further embodiment, the method comprises determining an optimal refraction (target refraction) of the at least one eye of the subject, either before or after the steps of projecting a target, which is designed to verify a predetermined visual acuity, into the at least one eye of the subject and determining a visual acuity limit refraction associated with the predetermined visual acuity of the target. In particular, the method can comprise determining an objective and / or subjective refraction or an objective and / or subjective refraction result. Determining an optimal refraction can also comprise determining a combined refraction ora combined refraction result based on an objective and / or subjective refraction measurement, which also takes into account, in particular, other data such as low and / or higher order aberrations from aberrometry or other biometric data such as corneal shape, lens-retina distance, anterior chamber depth, etc., of at least one eye of the test subject. In this sense, the terms "refraction" and "target refraction" (or "refraction result") in connection with "optimal refraction" should not be limited to corrections of low-order aberrations (e.g. sphere and astigmatism), but can also include higher-order aberrations. Therefore, the term "refraction" could also be understood generally as "correction".Preferably, the optimal refraction of the at least one eye of the subject is determined in a fogged state, which can be achieved by holding a corresponding target or projecting a corresponding target into the at least one eye of the subject (see above). Furthermore, according to this preferred embodiment, the visual acuity achieved by the at least one eye of the subject when compensating for any ametropia of the at least one eye of the subject (e.g., based on a determined optimal refraction) is determined. In other words, the visual acuity is determined after the ametropia determined by the refraction measurement has been substantially corrected using an optical system or lenses whose effect corresponds to the determined refraction result, i.e., the visual acuity cum correctione (VCC). The visual acuity can be determined using known methods.In particular, the determined optimal refraction and the measured associated visual acuity represent one of the at least two provided visual acuity-refraction value pairs that are used or taken into account when determining the sensitivity. In this way, it is possible to combine the determination of the sensitivity with measurements of the objective and / or subjective refraction or to integrate it into such measurements. The sensitivity can thus be determined quickly and easily, particularly in conjunction with other measurements. In a further embodiment, the method further comprises the steps of: - determining a subjective refraction result ora subjective refraction for the at least one eye of the subject; - determining the visual acuity achieved by the at least one eye of the subject when compensating for any visual impairment of the at least one eye of the subject on the basis of the determined subjective refraction result. The determined subjective refraction and the visual acuity of the at least one eye of the subject determined in this subjective refraction preferably represent one (or a further, in particular a second, third, fourth, etc.) of the visual acuity-refraction value pairs provided by the method for determining the sensitivity. Furthermore, the method preferably comprises determining an optimal refraction of the at least one eye of the subject on the basis of the subjective refraction result and an objective refraction result.The optimal refraction is, in particular, a combined refraction from the subjective and objective refraction results. The determination of a combined refraction result from an objective and subjective refraction measurement is generally known and is therefore not explained in more detail within the scope of this description. For example, a combined refraction can be determined by first performing an objective refraction measurement and then adjusting the objective refraction result using a subsequently performed subjective refraction. In particular, it is also possible to determine a combined refraction by calculating an average of the objective and subjective refraction. In a further embodiment, the sensitivity is determined based on at least one calculated misrefraction, wherein the at least one calculated misrefraction is calculated on the basis of a determined optimal refraction.The optimal refraction can be a determined objective and / or subjective refraction. In particular, the optimal refraction can represent a combined refraction from an objective and subjective refraction. Preferably, the misrefraction is determined "ex post," i.e., only after projecting a target designed to verify a predetermined visual acuity into at least one eye of the subject, and after determining a visual acuity limit refraction associated with the predetermined visual acuity of the target. Preferably, the misrefraction is only determined after determining at least one visual acuity-refraction value pair. Preferably, the misrefraction is determined after performing an objective and / or subjective refraction measurement, and in particular after determining an ideal refraction or an ideal refraction result from an objective and subjective refraction measurement.For example, in a preferred embodiment, the following steps can be carried out, in particular in the specified order: 1) Carrying out an objective refraction measurement (within the scope of the inventive procedure); 2) Determining at least one visual acuity-refraction value pair (within the scope of the inventive procedure); 3) Carrying out a subjective refraction measurement; 4) Determining an ideal refraction or an ideal refraction result from the objective and subjective refraction measurement; and 5) Calculating the incorrect refractions and the sensitivity based on the result from step 4, i.e. on the basis of the determined ideal refraction or the determined ideal refraction result. In a further embodiment, varying the target refraction comprises a monotonous decrease in the target refraction and / or a monotonous increase in the target refraction.In a further embodiment, a visual acuity limit refraction of at least one eye of the subject associated with the predetermined visual acuity is determined by decreasing the target refraction and detecting a subject action while decreasing the target refraction, and / or by increasing the target refraction and detecting a subject action while increasing the target refraction, wherein with each subject action it is determined that the identifiability of the target for the subject has changed at the time of the respective subject action. In this way, the "blur point" is approached from different directions. In other words, one blur point can be determined when increasing the target refraction and another blur point when decreasing it. These blur points can be different from one another and can subsequently be averaged.In particular, the sensitivity can be determined within the framework of minimizing the least squares using known metrics from both blur points. In a further embodiment, at least two of the provided visual acuity-refraction value pairs are provided by the following steps: - Projecting a first target with a first adjustable and / or variable target refraction into the at least one eye of the subject, wherein the first target is designed to verify a given (predetermined and / or known) first visual acuity (or a given first visual acuity level); - Determining a value corresponding to the given first visual acuity (orthe predetermined first visual acuity level) of the at least one eye of the test subject by varying (in particular continuously, monotonously and / or constantly varying) the first target refraction of the first target projected into the at least one eye of the test subject and detecting a first test subject action with which it is signaled or determined that the identifiability of the first target for the test subject has changed at the time of the first test subject action; - projecting a second target with a second adjustable and / or variable target refraction into the at least one eye of the test subject, wherein the second target is designed to verify a predetermined (predetermined and / or known) second visual acuity (or a predetermined second visual acuity level) which differs from the predetermined first visual acuity (or the predetermined first visual acuity level); - determining a value corresponding to the predetermined second visual acuity (orthe predetermined second visual acuity level) associated second visual acuity limit refraction of the at least one eye of the test subject by varying (in particular continuously, monotonously and / or constantly varying) the second target refraction of the second target projected into the at least one eye of the test subject and detecting a second test subject action with which it is signaled or determined that the identifiability of the second target for the test subject has changed at the time of the second test subject action. In particular, the determination of the sensitivity of the at least one eye of the test subject takes place using or taking into account the predetermined first visual acuity and the determined associated first visual acuity limit refraction, as well as under further use or taking into account the predetermined second visual acuity and the determined associated second visual acuity limit refraction. Preferably, the first predetermined visual acuity orthe first predetermined visual acuity level of the first target is smaller than the second predetermined visual acuity or the second predetermined visual acuity level of the second target. For example, the first predetermined visual acuity or the first predetermined visual acuity level can have the value 0.8 logMar, while the second predetermined visual acuity or the second predetermined visual acuity level has the value 1.0 logMar. Or, for example, the first predetermined visual acuity or the first predetermined visual acuity level can have the value 0.4 logMar, while the second predetermined visual acuity or the second predetermined visual acuity level has the value 0.8 logMar or 1.0 logMar. It is understood that other values ​​can also be selected. The change in the predetermined visual acuity or the predetermined visual acuity level from one virtual target to the next target is preferably in the range from 0.2 logMar to 0.7 logMar, preferably in the range from 0.2 logMar to 0.5 logMar, and particularly preferably in the range from 0.2 logMar to 0.3 logMar.In a further embodiment, determining a visual acuity limit refraction comprises measuring and / or monitoring an accommodation state of at least one eye of the subject, wherein the accommodation state is measured in particular at least at the time of or immediately after the subject's action. The results of such a measurement or monitoring can be used to control the process (e.g., aborting or repeating individual steps in the event of unwanted accommodation (e.g., exceeding a certain threshold). The measurement can be performed continuously or only during or immediately after the subject's action. Furthermore, an accommodation state (sphere, cylinder, low- or higher-order aberrations), ideally measured at the time of the subject's action, can be included in the calculation of the sensitivity or the refraction error.For example, the amount of accommodation can be subtracted from the distance refraction value by the amount of the applied power. Expressed in formulas, the simplest case is as follows: Sensitivity represents visual acuity V as a function f of the refractive error F, i.e. V = f(F). Here, the refractive error F is: - without accommodation, the difference between the actually applied power T and the ideal power I, i.e. F = T – I; and - with accommodation, the difference between the actually applied power T and the currently measured power Ga, i.e. F = T – Ga. If there is a deviation D between the ideal power I and the measured power with a relaxed eye (power G0), the following applies: I = G0 + D. Accordingly, in this situation, F = T - (G0+D) or F = T - (Ga+D). For spherical values, this formula can be used as described. For cylindrical values, the cross-cylinder formula should be used accordingly.Zernike coefficients (also for higher-order aberrations) or power vectors can be used analogously. Alternatively or additionally, determining a visual acuity limit refraction can comprise measuring and / or monitoring a pupil size (e.g. pupil radius) of at least one eye of the subject, wherein the measurement of the pupil size takes place in particular at least at the time of or immediately after the subject's action. The pupil size can be measured, for example, using a camera which is part of a refraction unit, e.g. an autorefractometer or aberrometer, or using a separate camera. The pupil size measured at the time of the subject's action (i.e. at the point of blur) or shortly before or after (e.g. up to 2 seconds before the point of blur is reached) can be used to determine the sensitivity of at least one eye of the subject to blur.In particular, the measured pupil size can be used to quantify the blur of the image on the retina, preferably with the help of a suitably parameterized eye model and a known additional fog. Instead of a complete eye model, a simpler description can also be used. For example, the angle at which the disc of confusion of a blurred point can be observed for a given pupil and a given additional fog can be calculated (see, for example, WO 2019034525 A1). Within the framework of such a visual acuity model, the sensitivity can be determined as the deterioration in visual acuity per angle of the disc of confusion.In a further embodiment, in order to determine a visual refraction, the test subject is given a visual task with at least two, preferably at least three, particularly preferably at least four, in particular four or eight, possible different answers, wherein the test subject can answer the visual task based on the test subject’s action. A “visual task” is understood here in particular to be a task which has a predetermined and thus verifiable solution. In particular, the visual task is therefore a verifiable task (i.e. a visual task whose solution is known and thus verifiable). In other words, the test subject’s action goes beyond simply communicating the recognizability or identifiability of the target. The visual task is preferably based on a forced choice, i.e. the test subject is “forced” to make a selection from several or more possible answers.to make at least two or a multitude of possible answers, with the correct answer preferably being predetermined or known. In this case, such a visual task is referred to as a “forced choice” visual task. The visual task can be solved or a selection can be made, for example, with the help of a joystick with which the test subject can operate in different directions. For example, the visual task can consist of the test subject having to identify the position or direction of the gap in an adapted optotype with the help of a joystick. If the adapted optotype is a Landolt ring, for example, there are two possible positions and therefore two possible answers for the test subject as to how the directed feature can be arranged parallel to the selected preferred direction. It goes without saying that, in principle, other optotypes can also be used, so that the test subject’s answers depend on the adapted optotype.In this way, the method becomes more precise and reliable than if the test subject only had to provide unverified feedback (e.g. "yes" or "no" or "recognizable" or "not recognizable"). In a further embodiment, before the step of determining a visual acuity limit refraction, first aberrometric data of the at least one eye of the test subject are acquired, preferably for a distance accommodation state and / or a foggy state of the at least one eye of the test subject and in particular at a first brightness. Furthermore, the method preferably comprises acquiring second aberrometric data of the at least one eye of the test subject for a near accommodation state of the at least one eye of the test subject, in particular at a second brightness whose value is lower than the first brightness. The acquisition of second aberrometric data preferably takes place before the step of determining a visual acuity limit refraction.In the context of this description, “aberrometric data” (or “aberrometric measurements”) is understood to mean data describing the imaging errors of an eye (measurements to obtain this data), the information content of which corresponds at least to the term of the order “defocus” when represented with Zernike coefficients, but ideally includes higher orders (e.g. coma and spherical aberrations). In particular, the “aberrometric data” can also comprise or be (purely) autorefractometric data. In particular, the acquisition of aberrometric data also comprises the acquisition of (purely) autorefractometric data (i.e. sphere and / or cylinder and / or axis). The first and second brightnesses are preferably each a brightness in the regime of mesopic vision (preferred luminance in the range of approximately 0.003 cd / m²). 2 up to about 30 cd / m 2 , particularly preferably in the range of about 0.003 cd / m 2 up to about 3 cd / m 2, more preferably in the range of about 0.003 cd / m 2 up to about 0.3 cd / m 2 , most preferably in the range of about 0.003 cd / m 2 up to about 0.03 cd / m 2). Brightness is always understood to mean, in particular, the brightness at the location of the eye or the brightness to be detected by the eye. Together with the acquisition of first aberrometric data and / or the acquisition of second aberrometric data (i.e., in particular, at the first or second brightness and at the first or second accommodation state), first or second pupillometric data for at least one eye of the subject can also be acquired. The term “pupillometric data” (or pupillometric measurements) refers to information on the size of the pupil (or measurements for obtaining this data), which includes at least one size specification (for example, in the form of a radius), but can also represent the shape of the pupil in a more complex form. In addition, the pupillometric data can contain information on the position of the pupil (for example, relative to the corneal vertex or to the optical axis of the eye). A further aspect relates to aMethod for calculating, optimizing, or evaluating a spectacle lens for at least one eye of a test subject or spectacle wearer, taking into account the sensitivity of the at least one eye of the test subject, wherein the sensitivity of the at least one eye of the test subject is determined by one of the methods according to the invention. In particular, the method for calculating, optimizing, or evaluating a spectacle lens for at least one eye of a test subject can comprise the following steps: a) Providing an assignment of at least one imaging property or aberration of a spectacle lens system to the visual acuity of the spectacle wearer or of an average spectacle wearer when viewing an object through the spectacle lens system; b) Determining or specifying a target function for the spectacle lens to be calculated or evaluated, in which the assignment from step (a) is to be evaluated; c) Calculating or evaluating the spectacle lens to be calculated or evaluated by evaluating theObjective function, wherein the objective function is evaluated at least once. The assignment of the at least one imaging property or aberration of a spectacle lens system to the visual acuity of the wearer can depend parametrically on the measured initial visual acuity and / or the measured sensitivity of the wearer. The calculation and / or optimization of the spectacle lens can in particular comprise minimizing or maximizing the objective function. The method for calculating, optimizing or evaluating a spectacle lens can further comprise calculating at least one light beam emanating from the object for at least one viewing direction with the aid of wavefront calculation, ray calculation or wavefield calculation through the spectacle lens system and / or through the spectacle lens to be calculated or evaluated up to an evaluation surface in the spectacle lens system. Furthermore, the method for calculating, optimizing or evaluating a spectacle lens can comprise calculating theThe method may comprise calculating the difference between the light beam emanating from the object and a reference light beam converging on the retina of a model eye, and determining the at least one imaging property or aberration based on the calculated difference. The calculation of at least one light beam emanating from the object is preferably carried out by means of wavefront calculation, wherein the calculation of the difference present at the evaluation surface comprises calculating the wavefront difference between the wavefront of the light beam emanating from the object and the wavefront of the reference light beam converging on the retina, wherein the wavefront difference is calculated at the evaluation surface. Furthermore, the method for calculating, optimizing, or evaluating a spectacle lens may comprise assigning a geometric-optical angle and / or a quadratic shape in the space of geometric-optical angles to the calculatedWavefront difference, wherein the at least one imaging property or aberration depends on at least one component of the geometric-optical angle and / or the quadratic shape. Alternatively or additionally, the method for calculating, optimizing or evaluating a spectacle lens can comprise the following steps: - specifying a first surface and a second surface for the spectacle lens to be calculated or optimized; - determining the path of a chief ray through at least one viewing point of at least one surface of the spectacle lens to be calculated or optimized into a model eye; - evaluating an aberration of a wavefront resulting along the chief ray from a spherical wavefront impinging on the first surface of the spectacle lens at an evaluation surface in comparison to a wavefront converging at a point on the retina of the eye model; - iteratively varying the at least one surface to be calculated or optimizedoptimizing surface of the spectacle lens until the evaluated aberration corresponds to a predetermined target aberration. A further aspect relates to a method for producing a spectacle lens, comprising: - calculating or optimizing a spectacle lens according to the inventive method for calculating or optimizing a spectacle lens; and - manufacturing the spectacle lens thus calculated or optimized. Furthermore, the invention provides a computer program product or a computer program product, in particular in the form of a storage medium or a data stream, which contains a program code which, when loaded and executed on a computer, is designed to carry out a method according to the invention, in particular for determining the sensitivity of at least one eye of a subject and / or for calculating, optimizing or evaluating a spectacle lens and / or for producing a spectacle lens. In other words, the invention provides a computer program product whichmachine-readable program code which, when loaded onto a computer, is suitable for carrying out the above-described method according to the invention. In particular, a computer program product is understood to mean a program stored on a data carrier. In particular, the program code is stored on a data carrier. In other words, the computer program product comprises computer-readable instructions which, when loaded into a computer memory and executed by the computer, cause the computer to carry out a method according to the invention. In particular, the invention provides a computer program product which contains program code which is designed and configured, when loaded and executed on a computer, to carry out a method according to the invention for determining the sensitivity of at least one eye of a subject and / or a method according to the invention for calculating, optimizing or evaluating a spectacle lens and / or ato carry out a method according to the invention for producing a spectacle lens. A further aspect relates to a device for determining the sensitivity of at least one eye of a subject, comprising: - a target provision device for providing a target which is designed to verify a predetermined visual acuity and which is configured to display at least one adapted optotype; - an optical system for projecting the target with a target refraction into the at least one eye of the subject, wherein the optical system is designed to adjust and vary the target refraction; - a feedback unit for detecting a subject's action in order to determine that the identifiability of the target for the subject has changed at the time of the subject's action, in particular as a result of varying the target refraction of the target projected into the at least one eye of the subject with the aid of the optical system; and - aVisual acuity limit refraction determination unit for determining a visual acuity limit refraction of at least one eye of the subject associated with the predetermined visual acuity, wherein the visual acuity limit refraction determination unit is designed to detect (in particular to determine and store) the target refraction effected by the optical system at the time of the subject's action. The target provision device can comprise, for example, an electronic display or a digital screen. In particular, the display can be designed such that individual pixels of the display, different areas or different components of the display can be individually controlled, in particular to display composite optotypes. For example, partial segments of a ring can be displayed, with which Landolt-C optotypes with differently directed openings can be generated or displayed. Alternatively or additionally, complete optotypes such as letters or numbers can also be displayed.be designed as complete and in particular switchable LCD elements. Generally, the display is configured to show adapted optotypes. The target provision device can, for example, comprise a folding, sliding, or rotating mechanism, for example magnetic or motorized, with which different targets or images can be displayed and / or exchanged. The targets or images can also be partially transparent and only contain areas that are to be displayed in addition to another image. Transparent, backlit images can also be designed in such a way that certain parts of the image are only visible when one or more specific light sources (e.g. in otherwise shaded areas or with specific wavelengths) are switched on or off. The optical system is, in particular, between at least one eye of the subject and the target provision device or the provided targetarranged. The optical system can be designed as a refraction unit. The optical system is designed to apply or cause different target effects as optical effects at least in the selected preferred direction and thus influence the detectability of the target for at least one eye of the subject. The optical system can be designed to provide different spherical effects as optical effects. This can be done, for example, by arranging one or more spherical lenses, for example in the form of a Badal system. Alternatively or additionally, one or more adaptive lenses, optionally in combination with conventional lenses, can be used or arranged. In more complex cases, the optical system can be designed to apply or cause different cylindrical effects or higher-order effects in addition to or instead of spherical effects. The optical system can have at least oneLens with a spherical power and / or at least one lens with a cylindrical power. For example, the optical system can comprise a magazine with a plurality of spherical lenses and / or cylindrical lenses, each having different spherical or cylindrical powers, and wherein the magazine is designed and arranged such that individual spherical lenses or individual cylindrical lenses and / or a combination of several spherical lenses or cylindrical lenses of the magazine can be selected and used to project the target. The optical system can, for example, also comprise an Alvarez lens system. In other words, the subject is presented with a target (or a projected or virtual target) through which the subject sees the target or virtual target. The optical system can, for example, also comprise two lenses that can be rotated relative to one another, each having at least one cylindrical component in its powers. In particular, the opticalSystem two cylindrical lenses with interlocking, mutually facing, rotationally symmetrical surfaces, preferably flat surfaces. The optical system can also have a positive and a negative cylindrical lens with opposite, equal power, which are mounted so as to rotate relative to each other and preferably displaceable relative to each other. Furthermore, it is possible that when different powers are applied through the optical system, the angle of view of the target changes. This can either be prevented by a corresponding design of the optical system or calculated and compensated for in the display. For this purpose, the angle of view must be determined as a function of the applied power and, based on this actual angle of view, a visual acuity value must be assigned, which can be achieved, for example, by determining the magnification of the optical system and a correspondingly reduced display of the target. Alternatively, the optical system can be equipped withThe target can be calibrated using a camera by directly measuring the size of the target with a camera positioned in place of at least one of the test subject's eyes (and looking into the optical system). The test subject's feedback or the test subject's action can, in principle, be provided verbally. In this case, a user can memorize the state of the optical system during the feedback or test subject's action and / or forward the feedback directly to the feedback system. However, this variant is prone to errors and causes delays. Therefore, direct feedback from the test subject to the feedback system is preferred. For this purpose, the feedback system can, in the simplest case, comprise a button. In a further preferred embodiment, the feedback system can also comprise two buttons ("+" and "-"), three buttons ("+", "-" and "OK"), four buttons (e.g., "+", "-", "OK" and "Cancel"), etc., and / or a joystick. Alternatively or additionally, the feedback system cana microphone for capturing verbal utterances of the subject. In one embodiment, the device comprises an evaluation unit for determining the sensitivity of at least one eye of the subject based on at least two provided visual acuity-refraction value pairs. In this case, the visual acuity limit refraction determination unit can be a component of the evaluation unit. In other words, the evaluation unit can comprise the visual acuity limit refraction determination unit. In a further embodiment, the device comprises an autorefractometric or aberrometric measuring unit for determining one or more objective refractions of the at least one eye of the subject, wherein the autorefractometric or aberrometric measuring unit is preferably designed to measure and / or monitor an accommodation state of the at least one eye of the subject. Furthermore, the autorefractometric or aberrometric measuring unit can comprise aCamera for determining a pupil size (in particular a pupil radius) of at least one eye of the subject. Alternatively or additionally, the autorefractometric or aberrometric measuring unit can comprise a calibration camera for calibrating the optical system. The camera for determining a pupil size and the calibration camera can also be implemented in a single camera, which combines both functions (determining the pupil size and calibrating the optical system). In a further preferred embodiment, the device comprises a pupil size measuring unit (in particular a camera) for determining a pupil size (in particular a pupil radius) of at least one eye of the subject. Alternatively or additionally, the device can comprise an illumination device for generating at least two brightness levels. Alternatively or additionally, the device can comprise a pupillometer device, which is designed toto acquire pupillometric data of at least one eye at a first brightness and to acquire secondary pupillometric data of at least one eye at a second brightness. A further aspect relates to a device for calculating, optimizing, or evaluating a spectacle lens for at least one eye of a subject, taking into account the sensitivity of the at least one eye of the subject, comprising a device according to the invention for determining the sensitivity of at least one eye of the spectacle wearer. The device for calculating, optimizing, or evaluating a spectacle lens can, in particular, comprise the following components: - a surface model database for specifying a first surface and a second surface for the spectacle lens to be calculated or optimized; - a principal ray determination module for determining the course of a principal ray through at least one viewing point of at least one surface of thespectacle lens into a model eye; - an evaluation module for evaluating an aberration of a wavefront resulting along the chief ray from a spherical wavefront impinging on the first surface of the spectacle lens at an evaluation surface in comparison to a wavefront converging at a point on the retina of the eye model; and - an optimization module for iteratively varying the at least one surface of the spectacle lens to be calculated or optimized until the evaluated aberration corresponds to a predetermined target aberration. A further aspect relates to a device for producing a spectacle lens, comprising: - calculation or optimization means designed to calculate or optimize the spectacle lens according to a method according to the invention for calculating or optimizing a spectacle lens; and - processing means designed to process the spectacle lens according to the result of the calculation or optimization. AA further aspect relates to a spectacle lens which has been produced by a method according to the invention for producing a spectacle lens and / or by means of a device according to the invention for producing a spectacle lens. Furthermore, the invention provides for the use of a spectacle lens produced by the manufacturing method according to the present invention, in particular in a preferred embodiment, in a predetermined average or individual wearing position of the spectacle lens in front of the eyes of a specific spectacle wearer for correcting a visual impairment of the spectacle wearer. In particular, a computer-implemented method according to the invention can be provided in the form of ordering and / or industry software. In particular, in such a method, the data required for the calculation and / or optimization and / or production of a spectacle lens can be recorded and / or transmitted. A device according to the invention and / orA system according to the invention, e.g., for ordering a spectacle lens, can in particular comprise a computer and / or data server designed to communicate via a network (e.g., Internet). The computer is in particular designed to execute a computer-implemented method, e.g., ordering software for ordering at least one spectacle lens, and / or transmission software for transmitting relevant data, and / or determination software for determining relevant data, and / or calculation or optimization software for calculating and / or optimizing a spectacle lens to be manufactured, according to the present invention. It is understood that the features mentioned above and those to be explained below can be used not only in the respective combination specified, but also individually or in other combinations, without departing from the scope of the present invention. IndividualEmbodiments for achieving the object are described by way of example with reference to Figures 6-8. The individual embodiments described partly have features that are not absolutely necessary to carry out the claimed subject matter, but which provide desired properties in certain applications. Thus, embodiments that do not have all the features of the embodiments described below are also to be considered as falling within the scope of the described technical teaching. Furthermore, in order to avoid unnecessary repetition, certain features are only mentioned in relation to individual embodiments described below. It is pointed out that the individual embodiments should therefore not only be considered in isolation, but also in a synopsis. Based on this synopsis, the person skilled in the art will recognize that individual embodiments can also be achieved by incorporating one or moreFeatures of other embodiments may be modified. It is pointed out that a systematic combination of the individual embodiments with individual or multiple features described in relation to other embodiments may be desirable and useful and should therefore be considered and also considered as encompassed by the description. Figure 6 shows an exemplary image or photo which includes a hot air balloon and a road and conveys a feeling of distance to the viewer. Such an image can, for example, be projected as a target (in particular as a virtual target) into at least one eye of a subject within the scope of the present invention in order to carry out an objective refraction measurement, for example in a foggy state in which the subject only perceives the image or details of the image in a blurred manner. Figure 7 shows the image or photo of Figure 6 with features integrated into the image or the imagesuperimposed exemplary adapted optotypes for a selected preferred direction. Each of these adapted optotypes has a predetermined visual acuity or a predetermined visual acuity level. Within the scope of the method according to the invention, the image with the adapted optotypes is provided with an adjustable target refraction using an optical system. This target refraction is varied by means of the optical system, and the test subject signals, by means of a test subject action, that the identifiability of the target or the adapted optotypes has changed for him or her at the time of the test subject action. In this way, visual acuity-refraction value pairs can be provided to determine the sensitivity of at least one of the test subject's eyes. One or more targets can be presented to the test subject or projected as virtual targets into the test subject's at least one eye. Depending on the embodiment, two or more targets can be used.which can also be identical in content. For example, a first target can be an image that conveys a sense of distance (see, for example, Figure 6), a second target can be one or more adapted optotypes in a certain size, and a third target can be one or more adapted optotypes in a different size. Alternatively, the first target can be an image that conveys a sense of distance, while the second and third targets can be identical in content and can contain one or more adapted optotypes in one of two sizes each. Alternatively, all three targets can be identical and represent an image that conveys a sense of distance, but contain one or more details, the recognition of which can each be assigned to a visual acuity level. These details are explicitly included in this description by the term adapted optotype. Examples of such details are in an image that, for example,Hot air balloon and a road contains: - Symbols or fabric strips on the hot air balloon and the basket of a hot air balloon, - Clouds or symbols on clouds, - Lines on a road, and / or - Symbols on signs at the roadside. A particularly suitable symbol has, for example, one or more concentric rings that merge into a circle at a given blur. In contrast to the prior art, in these embodiments, the visual acuity level for a specific applied power is not determined for determining the sensitivity, but rather the applied power required to achieve a given visual acuity. Furthermore, the determination of the visual acuity can be combined with the measurement of autorefractometric or aberrometric data in the unaccommodated and accommodated state. In a special embodiment, the accommodation state of the eye can also be tracked in order to obtain even more reliable values ​​for the sensitivity.A. Procedure according to an exemplary embodiment without subjective refraction An examination of the test subject can, for example, take place as follows: 1) Using an autorefractometric or aberrometric measurement, the objective refraction value of the test subject is determined. For this purpose, a first target is presented to the test subject. A suitable optical system presents the test subject with a first power which does not allow him to see the target completely sharply in order to achieve relaxation of the ciliary muscle. 2) A second target is then presented to the test subject and, using the optical system, a second power is applied, with which a test subject with high visual acuity cannot recognize at least one adjusted optotype. This is achieved in particular by selecting a spherical power as the applied optical power which corresponds to the mean sphere or one of the two principal sections of the objective refraction value plus aadditional positive optical power. The latter effect – often called "foggy" – is chosen because the test subject cannot compensate for such an effect through accommodation. To determine the speed at which the applied optical power is changed, standard values ​​based on averages for a large number of test subjects can be used. For example, it is known that visual acuity is approximately halved with a fogging of 0.5 dpt spherical or 1 dpt cylindrical. Preferably, the target refraction as the applied optical power is varied at a speed between 1 / 16 dpt per second and 1 / 2 dpt per second. The additional optical power can also depend on the pupil measured with the aberrometry unit. It can, for example, be inversely proportional to the pupil radius, so that test subjects with smaller pupils are preferentially fogged with a stronger power than those with larger pupils, to ensurethat the blur perceived by all subjects is similar. 3) Alternatively, a sphero-cylindrical power can be applied as the optical power. For example, a cylindrical power can be adopted for the optical system from the objective refraction, and an average objective refraction value can be subjected to an additional positive spherical power. Alternatively or additionally, the objective refraction value can be subjected to an astigmatic offset (so-called astigmatic fogging). The optical power is then slowly changed (e.g., between 1 / 16 dpt per second and 1 / 2 dpt per second) towards optimal or objective refraction (by varying the spherical and / or astigmatic power). 4) As soon as the subject can recognize the directional feature of the adjusted optotype of the second target by changing the applied optical power, he or she reports this (e.g., by pressing the "OK" button). If necessary, he or she can(e.g. using the “+” and “-” buttons) to set the limiting power yourself and confirm it (e.g. also using the “OK” button). The power set is saved as the “limiting visual acuity” or “limiting visual acuity refraction” when the second target is recognized. 5) The third target is presented to the test subject. 6) The optical power is now slowly changed further (e.g. between 1 / 16 dpt per second and 1 / 2 dpt per second) towards the optimal or objective refraction value (by varying the applied optical power). 7) As soon as the test subject can recognize the directional feature of the adjusted optotype of the third target by changing the applied optical power, they report this (e.g. using the “OK” button). If necessary, they can set the limiting power (e.g. using the “+” and “-” buttons) and confirm it (e.g. also using the “OK” button). The effect set is saved as “visual acuity limit effect” or “visual acuity limit refraction” when the third target is detected.The sensitivity can be determined from the visual acuity levels of the two targets, more precisely the two respective dimensions of the directional features of the two adjusted optotypes, the objective refraction value, the applied optical power upon detection of the second target and the applied optical power upon detection of the third target. For this purpose, a sensitivity metric, as described above in exemplary embodiments, can be used. The misrefractions result from the (e.g. spherical and / or astigmatic) distance of the applied optical power upon detection of the respective target from the objective refraction value. B. Procedure according to an exemplary embodiment with subjective refraction In this variant, the above-mentioned steps 5) to 7) from the procedure under Section A can be omitted. Therefore, only the visual acuity for one target and the applied optical power uponRecognition (e.g., of the directional features) of a target. Subsequently, a subjective refraction determination is carried out, and the subjective refraction value and the visual acuity (visus cum correctione, VCC) achieved by the subject are determined. The objective refraction value can be used as the starting value for the subjective refraction determination. Alternatively, the subjective refraction determination with visual acuity determination can be carried out before the steps in Section A. In this case, the performance of autorefraction or aberrometry and the determination of the objective refraction value (step 1) can be omitted and the subjective refraction value can be used instead. The refraction error can be calculated as the spherical or astigmatic distance of the effect upon detection of the target from the subjective refraction value. Instead of the subjective refraction value, aA combined refraction value can be used. This can be calculated based on the subjective refraction value and the objective refraction value or other data (e.g., lower or higher order aberrations from aberrometry or other biometric data such as corneal shape, lens-retina distance, anterior chamber depth). C. Adjustment of the visual acuity level of a target Furthermore, at least one visual acuity level of the adjusted optotype(s) of a target can be adjusted to the subject. This is useful, for example, if the subject's astigmatism cannot be corrected. The visual acuity level(s) of the (virtual) target can then be selected such that the target can still be recognized despite the remaining refraction error due to the astigmatism. Information about the visual acuity (e.g., visual acuity cum correctionem or visual acuity sine correctionem, for example from the subjectiveRefraction determination) are used to determine the target size, i.e. the dimension of the directional feature of the adjusted optotype of the target. If the test subject does not recognize the directional feature of the adjusted optotype despite a slight deviation of the applied optical power from the objective, subjective or combined refraction value, a change to a lower visual acuity level can be made and the corresponding step repeated with a lower visual acuity level. In addition to or instead of this, the findings from step 4 can be used to determine the visual acuity level in step 6. To avoid the test subject already knowing the adjusted optotype in the case of multiple measurements or when changing between eyes, at least one adjusted optotype, symbol or detail in the image can be changed between different measurements or when changing eyes. In particular, the selected preferred direction can be adapted to the astigmatism of theother eye, or switching from an adapted Landolt ring to an adapted Snellen E. Electronic displays are naturally particularly well suited as a target provision device for this purpose. D. Finding the blur point and setting the effect by the subject Finding the blur point As an alternative to the procedure in the previous sections, the optical power presented at the beginning (i.e. in step 2) according to section A or B) can also be a power that allows the recognition of the directional feature of the adapted optotype of the target. This can be an objective, subjective, or combined refraction value. In steps 5) and 6), the presented optical power is then moved away from this optical power in the plus direction. This direction is chosen to prevent accommodation. In steps 4) and 7), the subject then signals the time at which he no longer sees the directional feature of the optotype.can be recognized. If the applied optical powers are determined for two visual acuity levels analogous to the procedure in Section A, in this case the applied optical powers can be determined first (steps 2-4) for the higher visual acuity level and then (steps 5-7) for the lower visual acuity level. This can increase the misrefraction over the course of the procedure, whereby first the directional feature of the adjusted optotype with the more difficult recognition (higher visual acuity level) and then the one with easier recognition (lower visual acuity level) becomes unrecognizable. Correcting the (Un)focus Point Optionally, the test subject can correct the applied (i.e., applied) optical power in steps 4) and 7) of the above embodiments if they are unsure of having signaled the correct time or the correct applied optical power. This can be done, for example, using the "+" and "-" buttons on the feedback unit. Setting the (Un)focus Pointby the test subject The test subject can also be asked directly to set the optical power at which recognition of the directed feature of the adjusted optotype is just possible or no longer possible. This can be done, for example, using the "+" and "-" buttons on the feedback unit. Approaching the (un)focus point from different directions Furthermore, one blur point can be determined when increasing and another blur point when decreasing. These points can be different from each other and subsequently averaged. Alternatively, the sensitivity can be determined from both blur points using known metrics as part of a minimum square error. Repeating the measurement Of course, the blur can be determined several times to increase the measurement accuracy of the method. Monitoring the accommodation state During steps 3), 4), 6) and 7), the method according toSection A or during step 3) or 4) in the method according to Section B, the accommodation state of at least one eye of the test subject is monitored using the autorefractometry or aberrometry unit. The results obtained can be used to control the process (e.g., termination or repetition of individual steps in the event of unwanted accommodation (e.g., exceeding a certain threshold). The measurement can be carried out continuously or only when detectability is signaled. Furthermore, an accommodation state (sphere, cylinder, low- or higher-order aberrations) measured - ideally when detectability is signaled - can be included in the calculation of the sensitivity or the misrefraction. E. Blurring towards minus and integration of a near measurement Blurring towards minus In the above embodiments, the applied optical power corresponds to a misrefraction in the plus direction, since thissubjects cannot be compensated for by accommodation. However, the opposite approach can also be used, ie with an applied optical power which corresponds to a misrefraction in the minus direction. Any accommodation which may occur can be dealt with as follows: - ignoring the accommodation; - measuring subjects who can accommodate, e.g. physiologically (e.g. age-related) or pharmacologically induced (e.g. drops)), or only weakly; - measuring or monitoring the state of accommodation; - using assumptions on the ability to accommodate (e.g. as a function of age according to Duane's curve, see Figure 3). The curve according to Duane shown in Figure 8 is taken from B. Lachenmayr, D. Friedburg, E. Hartmann, A. Buser: “Eye – Glasses – Refraction: Schober Course: understand – learn – apply”, 2005, Fig. 1.29, and was originally published in Alexander Duane: “Studies in monocular and binocolar accommodation with their clinicalapplications“, Transactions of the American Ophthalmological Society, Volume 20, 1922, pp. 132–157, PMID 16692582, PMC 1318318. Duane's curve shows that the capacity of the human eye for accommodation (width of accommodation) decreases continuously from an average of 14 to one diopter from the age of eight to shortly after the age of fifty. The influence of accommodation on the sphere can be taken into account, for example, in the following ways: - The amount of accommodation is subtracted from the amount of the distance of the applied power from the refraction value for distance vision; - The misrefraction is calculated directly from the applied power and the measured or assumed refraction value. In an analogous manner, the astigmatic deviation can also be calculated using the known formalisms (e.g., cross-cylinder formula, power vector notation) using the measured cylinder in order to determine a change in astigmatism due to the Accommodationto be taken into account. Furthermore, measured higher-order aberrations can be taken into account using known metrics. Incorporating a near measurement The procedure described above can be combined with a determination of the objective near refraction values, the maximum accommodation, and / or the aberrations (of lower or higher orders). This can be done as follows: The accommodative state of the eye is monitored using (ideally concurrent and as frequent as possible) autorefractometric or aberrometric measurements. The process begins with an applied optical power that allows the recognition of the directional feature of the adjusted optotype of the target. This can be an objective, subjective, or combined refraction value. In step 5) and, if necessary, in step 6), the applied optical power is then removed from this optical power in the plus direction. In step 4) and, if necessary, in step 7), the subject then signals theTime at which the directional feature of the adjusted optotype can no longer be recognized. If the applied powers are determined for two visual acuity levels, in this case the applied powers for the higher visual acuity level can be determined first (steps 2-4) and then (steps 5-7) for the lower visual acuity level. This allows the misrefraction to be increased over the course of the procedure, whereby first the adjusted optotype with the more difficult recognition (higher visual acuity level) and then the one with easier recognition (lower visual acuity level) becomes unrecognizable. The autorefractometric or aberrometric value measured at the (respective) signaling of the loss of recognition is used to calculate the sensitivity or visual acuity. The value of the autorefractometric or aberrometric measurement corresponding to the greatest accommodation is then used as the value (sphere, cylinder, low- or higher-order aberrations) for the near refraction or for themaximum accommodation capacity is used. F. Monitoring pupil size Furthermore, the pupil size (e.g., as pupil radius) can be monitored, e.g., using a camera arranged in the autorefractometer or aberrometer, or using a separate camera. The pupil size measured at the point of blur or shortly before (e.g., up to 2 seconds before reaching the point of blur) can be used to determine the sensitivity to blur. The measured pupil size can then be used to quantify the blur of the image on the retina with the help of a suitably parameterized eye model and the known additional fogging. For example, the angle at which the scattering disk of a blurred point can be observed for a given pupil and given additional fogging can be calculated (cf. WO 2019034525 A1). Within the framework of such a visual acuity model, the sensitivity can be defined as a deteriorationof visual acuity per angle of the dispersive disc can be determined. G. More complex models for sensitivity In more complex models, a distinction can be made between the influence of spherical fogging or misrefractions and astigmatic fogging or misrefractions. For this purpose, spherical fogging and astigmatic fogging can be determined for the same visual acuity level. I. Combination with other measurements The present invention can be very well combined with other measurements or embedded in them. In a preferred embodiment, the procedure according to section A or B is carried out after an autorefractometric or aberrometric measurement for distance. This autorefractometric or aberrometric distance measurement already represents the first step according to section A and does not need to be carried out again. The procedure according to one of the above sections can be carried out either before or after any measurement for near vision.The former has the advantage that the (virtual) target is initially unknown to the subject and the subject has already familiarized himself with the target for the near measurement. List of reference symbols V1 first preferred direction V2 second preferred direction

Claims

September 12, 2022 Applicant: Rodenstock GmbH "Method, use of adapted optotypes and device for determining visual acuity characteristics of a test subject" Our reference: R 3436WO - hy / mu Patent claims 1. Method for determining visual acuity characteristics of a test subject who has at least one astigmatic refractive error, comprising the steps of: - providing refractive error data of the test subject, wherein the refractive error data contain at least one axial position of a required optical cylindrical correction; - selecting a preferred direction (V1; V2) such that this preferred direction (V1; V2) either corresponds to the axial position assigned to the optical cylindrical correction, or is rotated by 90° to this axial position, or the preferred direction (V1; V2) is derived from wavefront data by means of a point spread function; - applying an optical power at least in the selected preferred direction;- Displaying at least one adapted optotype which has a directional feature, wherein the adapted optotype is displayed oriented such that its directional feature is arranged parallel to the preferred direction (V1; V2); and - Determining the visual acuity characteristics of the subject for the selected preferred direction (V1; V2) taking into account at least one dimension of the directional feature of the adapted optotype and the applied optical power.

2. Method according to claim 1, wherein the directional feature of the adapted optotype has a sequence of light and dark areas which follow one another along the preferred direction (V1; V2).

3. Method according to claim 1 or 2, comprising the steps of: - Providing an unadapted standard optotype with a; 2 directed feature; - rotating the standard optotype in a display plane such that its directed feature is arranged parallel to the preferred direction (V1; V2); and - displaying the thus rotated standard optotype as the adapted optotype.

4. Method according to one of the preceding claims, wherein: - the axial position assigned to the optical cylinder correction and located in the first principal section of the required optical cylinder correction is selected as a first preferred direction (V1), wherein an optical sphere correction is applied as the optical effect, which corrects the subject's ametropia in the first principal section according to the refractive error data, and wherein the subject's visual acuity for this first principal section is determined as the visual acuity characteristics, and / or - a direction rotated by 90° to the axial position is selected as a second preferred direction (V2),which is arranged in the second principal section of the required optical cylinder correction, wherein an optical sphere correction is applied as the optical effect, which corrects the subject's refractive error in the second principal section according to the refractive error data, and wherein the subject's visual acuity for this second principal section is determined as the visual acuity characteristics.

5. The method according to claim 4, wherein the subject's visual acuity is determined for both the first and second principal sections of the required optical cylinder correction, and a direction-independent visual acuity is derived therefrom.

6. The method according to one of the preceding claims, wherein a Landolt ring is used as the adapted optotype, the gap of which is displayed rotated by 90° to the selected preferred direction (V1; V2). 3 7. Method according to one of the preceding claims, wherein a Snellen E is used as the adapted optotype, in which the connecting line connecting the three parallel E lines is arranged parallel to the selected preferred direction (V1; V2).

8. Method according to claim 6 or 7, wherein the adapted optotype is displayed at least once rotated 90° clockwise to the preferred direction (V1; V2) and at least once rotated 90° counterclockwise to the preferred direction (V1; V2), and wherein the test subject is asked to differentiate these two differently rotated adapted optotypes from one another as part of a visual task.

9. Method according to one of the preceding claims, wherein a hatched area is used as the adapted optotype, in which the hatching lines are arranged perpendicular to the selected preferred direction (V1; V2). 10.Method according to one of the preceding claims, wherein in addition to the adapted optotype at least one further optotype is displayed whose grey value corresponds approximately to an average grey value of the adapted optotype, and the test subject is asked to differentiate the displayed optotypes from one another as part of a visual task.

11. Method according to one of the preceding claims, wherein the applied optical power is varied at least up to a limiting refraction for the selected preferred direction, from which limiting refraction the test subject recognizes the directional feature of the adapted optotype.

12. Method according to one of claims 1 to 10, wherein the dimension of the directional feature of the adapted optotype is varied at least up to a limiting dimension up to which the test subject recognizes the directional feature of the adapted optotype.

13. Method according to one of the preceding claims, wherein as. 4 Visual acuity characteristics, at least one visual acuity and / or at least one sensitivity and / or at least one visual acuity-refraction value pair and / or at least one refraction value is determined.

14. Method according to one of the preceding claims, wherein the subject is presented with at least one visual task dependent on the displayed adjusted optotype, to which the subject answers by providing active and / or passive feedback.

15. Method according to one of the preceding claims, wherein the subject's visual acuity in the selected preferred direction (V1; V2) is determined with two different optical powers applied, and a sensitivity of the subject is determined therefrom.

16. Method according to one of the preceding claims, wherein a subjective and / or objective refraction is performed, and the subject's refractive error data are derived from the subject's refractive error determined thereby. 17.Method according to one of the preceding claims, wherein the visual acuity of the subject is determined as the visual acuity characteristic and converted into a different visual acuity type.

18. Method according to one of the preceding claims, wherein the at least one adapted optotype is displayed without correction and / or without complete correction of the optical cylinder correction required by the subject and nevertheless sharply represented for the subject.

19. Using adapted optotypes which each have a directional feature which is arranged parallel to a preferred direction (V1; V2) which either corresponds to an axial position which is assigned to an optical cylinder correction required by a subject, or which is rotated by 90° to this axial position, or the preferred direction (V1; V2) is derived from wavefront data by means of a point spread function in order to. 5 Determine the visual acuity characteristics of the subject for the selected preferred direction (V1; V2) taking into account at least one dimension of the directional feature of the adjusted optotype.

20. A device for determining visual acuity characteristics of a subject who has at least one astigmatic refractive error, comprising: - a selection module that selects a preferred direction, wherein this preferred direction (V1; V2) either corresponds to an axial position associated with an optical cylinder correction required by the subject, or is rotated by 90° to this axial position, or the preferred direction (V1; V2) is derived from wavefront data using a point spread function; - a refraction unit configured to apply an optical power in the selected preferred direction to the subject;- a display module with a display, which displays at least one adapted optotype with a directional feature on the display such that the directional feature of the adapted optotype is arranged parallel to the preferred direction (V1; V2); and - a visual acuity characteristic determination module, which determines the visual acuity characteristics of the subject for the selected preferred direction (V1; V2) taking into account at least one dimension of the directional feature of the adapted optotype and the applied optical power.

21. The device according to claim 20, with an eye-tracking unit, which tracks at least one eye of the subject when the at least one adapted optotype is displayed.

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