Optical system and imaging apparatus having same

Abstract

[Problem] To obtain an optical system having high optical performance by properly correcting various aberrations including lateral chromatic aberration despite a wide-angle and a large aperture ratio. [Solution] An optical system comprises: a front lens group of positive refractive power arranged closest to the object side; a rear lens group of negative refractive power arranged closest to the image side; and an intermediate group of positive refractive power arranged between the front lens group and the rear lens group and containing one or more lens groups. The interval between adjacent lens groups changes during focusing, and a predetermined condition is satisfied.

Description

Optical system and imaging device having the same

[0001] The present invention relates to an optical system and an imaging device having the same, which is suitable for use in electronic cameras such as video cameras and digital still cameras, film cameras, broadcast cameras, and the like.

[0002] An imaging optical system used in an imaging device using an image sensor is required to have not only high image quality (high resolution) but also good image blur. As an imaging optical system that satisfies these requirements, an imaging optical system with a large aperture ratio is known.

[0003] Since imaging optical systems with a large aperture ratio have a shallow depth of field, it is necessary to effectively correct various aberrations, including chromatic aberration, in order to achieve high image quality and beautiful bokeh.

[0004] Conventionally, various imaging optical systems with a large aperture ratio that are designed to effectively correct various aberrations including chromatic aberration have been proposed (see Patent Document 1).

[0005] Patent Document 1 discloses an optical system that is composed of a first lens group with positive refractive power and a second lens group with positive refractive power, arranged in that order from the object side to the image side, in which the first lens group moves during focusing.

[0006] Japanese Patent Application Laid-Open No. 2019-74631

[0007] In an optical system with a large aperture ratio, the depth of field is shallow, so the impact of various aberrations such as axial chromatic aberration, chromatic aberration of magnification, spherical aberration, coma aberration, and astigmatism on image quality becomes greater.In order to effectively correct various aberrations such as spherical aberration, coma aberration, and astigmatism while aiming for overall compactness in an optical system with a large aperture ratio, it is effective to use a material with a high refractive index for the positive lens.

[0008] In Patent Document 1, the second lens group uses a material with a relatively high refractive index and low dispersion, and a small partial dispersion ratio, which makes it difficult to correct chromatic aberration of magnification when the angle of view is widened.

[0009] The present invention has been made in view of the above-mentioned problems, and aims to provide an optical system that has a wide angle and a large aperture ratio, yet effectively corrects various aberrations including lateral chromatic aberration, and easily achieves high optical performance.

[0010] An optical system according to one aspect of the present invention comprises a front lens group having positive refractive power and arranged closest to the object, a rear lens group having negative refractive power and arranged closest to the image, and an intermediate group having positive refractive power as a whole and including one or more lens groups and arranged between the front lens group and the rear lens group, wherein the spacing between adjacent lens groups changes during focusing, and wherein, when the refractive index of the positive lens Grp having the highest refractive index among the positive lenses arranged in the rear lens group is ndGrp and the Abbe number is vdGrp, the optical system satisfies the following conditional expressions: 1.750<ndGrp<1.800 20.0<vdGrp<25.0

[0011] Another aspect of the present invention is an optical system that includes, arranged in order from the object side to the image side, a front lens group with positive refractive power, an intermediate group including one or more lens groups and having positive refractive power as a whole, and a rear lens group with negative refractive power, in which the spacing between adjacent lens groups changes during focusing, and the rear lens group has at least one positive lens.

[0012] Other objects and features of the present invention are illustrated in the following examples.

[0013] According to the above-mentioned means, various aberrations including chromatic aberration of magnification can be well corrected despite the wide angle and large aperture ratio, and high optical performance can be easily obtained over the entire object distance range from infinity to close distances.

[0014] FIG. 1 is a cross-sectional view of the lens of the optical system of Example 1 when focused on an object at infinity. FIG. 2 is a longitudinal aberration diagram of the optical system of Example 1 when focused on an object at infinity. FIG. 3 is a cross-sectional view of the lens of Example 2 when focused on an object at infinity. FIG. 4 is a longitudinal aberration diagram of the optical system of Example 1 when focused on an object at infinity. FIG. 5 is a cross-sectional view of the lens of Example 3 when focused on an object at infinity. FIG. 6 is a longitudinal aberration diagram of the optical system of Example 3 when focused on an object at infinity. FIG. 7 is a cross-sectional view of the lens of Example 4 when focused on an object at infinity. FIG. 8 is a longitudinal aberration diagram of the optical system of Example 4 when focused on an object at infinity.

[0015] Hereinafter, embodiments of an optical system and an imaging apparatus having the same according to the present invention will be described with reference to the accompanying drawings.

[0016] Fig. 1 is a lens cross-sectional view of the optical system of Example 1 when focused on an object at infinity. Fig. 2 is a longitudinal aberration diagram of the optical system of Example 1 when focused on an object at infinity. Fig. 3 is a longitudinal aberration diagram of the optical system of Example 1 when focused on an object at a close distance (0.28 m).

[0017] Fig. 4 is a lens cross-sectional view of the optical system of Example 2 when focused on an object at infinity. Fig. 5 is a longitudinal aberration diagram of the optical system of Example 2 when focused on an object at infinity.

[0018] Fig. 6 is a longitudinal aberration diagram when the optical system of Example 2 is focused on a close object (0.28 m). Fig. 7 is a lens cross-sectional view when the optical system of Example 3 is focused on an object at infinity. Fig. 8 is a longitudinal aberration diagram when the optical system of Example 3 is focused on an object at infinity.

[0019] Fig. 9 is a longitudinal aberration diagram when the optical system of Example 3 is focused on a close object (0.28 m). Fig. 10 is a lens cross-sectional view when the optical system of Example 4 is focused on an object at infinity. Fig. 11 is a longitudinal aberration diagram when the optical system of Example 4 is focused on an object at infinity. Fig. 12 is a longitudinal aberration diagram when the optical system of Example 4 is focused on a close object (0.28 m).

[0020] FIG. 13 is a schematic diagram of the main parts of a camera (image pickup device) equipped with the optical system of this embodiment.

[0021] The optical system of each embodiment is a photographic lens system used in image pickup devices such as video cameras, digital cameras, and silver halide film cameras.

[0022] In the lens cross-sectional views, the left side is the object side (front) and the right side is the image side (rear). In the lens cross-sectional views, i indicates the order of the lens groups from the object side, and Li is the ith lens group.

[0023] SP denotes an aperture stop, which is arranged on the image side of the first lens unit L1.

[0024] IP is the image plane, and when used as the imaging optical system of a video camera or digital still camera, it is placed on the imaging surface of a solid-state imaging element (photoelectric conversion element) such as a CCD sensor or CMOS sensor, and in the case of a silver halide film camera, it is placed on the photosensitive surface corresponding to the film surface.

[0025] In the aberration diagrams, d and g represent the d-line and g-line, respectively. ΔM and ΔS represent the meridional image plane and the sagittal image plane, and lateral chromatic aberration is represented by the g-line.

[0026] ω is the half angle of view, and Fno is the F-number.

[0027] In Examples 1 to 4 shown in FIGS. 1, 4, 7 and 10, focusing is performed by moving the second lens unit L2 and the third lens unit L3 in the optical axis direction.

[0028] The arrows indicate the movement locus of each lens group during focusing from an object at infinity to a close object. Focusing may be performed by moving the entire optical system or any one of the lens groups.

[0029] In optical systems with large aperture ratios, the depth of field is shallow, which increases the impact of various aberrations on image quality, such as axial chromatic aberration, lateral chromatic aberration, spherical aberration, coma, and astigmatism. In optical systems with large aperture ratios, it is important to achieve good optical performance over the entire object distance while miniaturizing the entire lens system.

[0030] To solve this problem, it is important to properly set the refractive power and lens configuration of each lens group, as well as the movement conditions of each lens group during focusing. Without proper configuration, it becomes difficult to obtain an optical system that has high optical performance over the entire object distance while maintaining a large aperture ratio. Correcting lateral chromatic aberration becomes particularly difficult as the focal length becomes shorter.

[0031] In order to achieve good correction for aberrations such as chromatic aberration, spherical aberration, coma, and astigmatism while also achieving overall compactness, it is effective to use a material with a high refractive index, high dispersion, and a large partial dispersion ratio for the positive lens element located closer to the image side than the aperture stop.

[0032] The optical system according to the first embodiment will be described.

[0033] The optical system according to the first embodiment comprises a front lens group Lf having positive refractive power and positioned closest to the object, a rear lens group Lr having negative refractive power and positioned closest to the image, and an intermediate lens group Lm positioned between the front lens group Lf and the rear lens group Lr. The intermediate lens group Lm includes one or more lens groups and has positive refractive power as a whole, and the spacing between adjacent lens groups changes during focusing, and the following conditions are set:

[0034] The rear lens unit Lr has a positive lens, and is characterized by satisfying the following condition: 1.750<ndGrp<1.800 (1) where ndGrp is the refractive index of the positive lens Grp with the highest refractive index and vdGrp is the Abbe number.

[0035] Condition (1) defines the refractive index of the positive lens Grp, which has the highest refractive index, in the rear lens unit Lr.

[0036] If the refractive index of the positive lens becomes too high, exceeding the upper limit of conditional expression (1), the Petzval sum becomes large in the negative direction, making it difficult to correct curvature of field, whereas if the refractive index of the positive lens becomes too small, exceeding the lower limit, making it difficult to correct spherical aberration at the telephoto end.

[0037] The rear lens unit Lr has a positive lens, and is characterized in that it satisfies the following conditional expression: 20.0<vdGrp<25.0 (2) where vdGrp is the Abbe number of the positive lens Grp having the highest refractive index.

[0038] Condition (2) defines the Abbe number of the positive lens Grp having the highest refractive index in the rear lens unit Lr.

[0039] By satisfying conditional expression (2), axial chromatic aberration and lateral chromatic aberration can be corrected satisfactorily.

[0040] Next, an optical system according to a second embodiment will be described.

[0041] The optical system according to the second embodiment comprises, arranged in order from the object side to the image side, a front lens unit Lf with positive refractive power, an intermediate lens unit Lm including one or more lens units and having a positive refractive power as a whole, and a rear lens unit Lr with negative refractive power. The spacing between adjacent lens units changes during focusing. The movement of at least two lens units during focusing facilitates suppression of fluctuations in various aberrations that occur during focusing.

[0042] Furthermore, the fourth lens unit L4 has at least one positive lens, which makes it easier to correct chromatic aberration of magnification and the like that is likely to occur in the fourth lens unit L4.

[0043] In the optical system of each embodiment, it is preferable to satisfy one or more of the following conditions, in order to obtain the effects corresponding to each condition.

[0044] The front lens unit Lf preferably has two negative lenses in order from the object side.

[0045] The above-described configuration makes it easy to correct curvature of field and sagittal coma flare. In addition, the diameter of the front lens can be reduced, making it easy to make the lens smaller in diameter and lighter in weight.

[0046] When the partial dispersion ratio of the positive lens having the highest refractive index in the rear lens group is θgFGrp, it is preferable to satisfy the following condition: 0.600<θgFGrp<0.670 (3).

[0047] θgFGrp: partial dispersion ratio of the positive lens, which is defined by the following equation when the refractive index of the positive lens for the g-line is ngGrp, the refractive index of the positive lens for the F-line is nFGrp, and the refractive index of the positive lens for the C-line is nCGrp. θgFGrp=(ngGrp-nFGrp) / (nFGrp-nCGrp) Conditional equation (3) defines the anomalous dispersion of the positive lens with the highest refractive index in the rear lens group.

[0048] By satisfying conditional expression (3), in the correction of chromatic aberration, it becomes possible to achieve not only primary achromatism but also favorable correction of secondary spectrum.

[0049] If the partial dispersion ratio of the positive lens becomes too large, exceeding the upper limit of conditional expression (3), the axial chromatic aberration for the g-line will be overcorrected and will be too large on the negative side, whereas if the partial dispersion ratio of the positive lens becomes too small, exceeding the lower limit, the lateral chromatic aberration for the g-line will be undercorrected and will be too large on the positive side.

[0050] When the focal length of the entire optical system is f and the composite focal length of the intermediate unit Lm is fm, it is preferable to satisfy the following condition: 1.0<fm / f<2.0 (4).

[0051] Conditional expression (4) defines the composite focal length of the intermediate unit Lm. By satisfying conditional expression (4), it is possible to suppress aberration fluctuations during focusing. If the refractive power of the intermediate unit Lm becomes too strong by falling below the lower limit of conditional expression (4), it becomes difficult to correct spherical aberration, which is undesirable. If the refractive power of the intermediate unit Lm becomes too weak by exceeding the upper limit of conditional expression (4), the amount of movement of the intermediate unit Lm during focusing increases. This makes it difficult to shorten the overall lens length, which is undesirable.

[0052] When the focal length of the front lens unit Lf is ff, it is preferable to satisfy the following condition: 2.5<ff / f<5.0 (5).

[0053] Conditional expression (5) defines the refractive power of the front lens unit Lf. By satisfying conditional expression (5), it is possible to make the axial light rays incident on the focus lens unit closer to afocal, thereby making it possible to suppress aberration fluctuations during focusing. If the refractive power of the front lens unit Lf becomes too negative and deviates from the conditional expression, the lens diameter of the intermediate lens unit Lm becomes too large, and the focus lens unit becomes heavy.

[0054] This makes it difficult to achieve rapid autofocusing. It is also difficult to correct axial chromatic aberration and spherical aberration, which is undesirable. If the refractive power of the front lens unit Lf becomes too strong in the positive direction and the conditional expression is violated, the refractive power of the intermediate lens unit Lm weakens, increasing the amount of movement during focusing. This makes it difficult to shorten the overall lens length. It is also difficult to correct axial chromatic aberration and spherical aberration, which is undesirable. It is preferable to satisfy the following conditional expression (6): −6.0<fr / f<−2.0 (6) where fr is the focal length of the rear lens unit Lr.

[0055] Conditional expression (6) defines the focal length fr of the rear lens unit Lr. By satisfying conditional expression (6), it becomes possible to suppress fluctuations in coma aberration during focusing.

[0056] By giving the rear lens unit Lr a negative refractive power, it is possible to make the optical system nearly symmetrical, making it easier to correct coma and chromatic aberration of magnification. Furthermore, it is possible to strengthen the refractive power of the middle lens unit Lm, which serves as the focusing lens unit, and reduce the amount of movement of the middle lens unit Lm during focusing. This makes it possible to suppress fluctuations in coma during focusing.

[0057] If the refractive power of the rear lens unit Lr becomes too strong, exceeding the upper limit of conditional expression (6), the refractive power of the intermediate lens unit Lm becomes too strong, making it difficult to correct spherical aberration.Furthermore, the entrance pupil position moves toward the image plane, making it difficult to ensure telecentricity of the light beam incident on the image sensor, which is undesirable.

[0058] If the refractive power of the rear lens unit Lr becomes too weak by going below the lower limit of conditional expression (6), the refractive power of the intermediate lens unit Lm becomes too weak, and the amount of movement of the intermediate lens unit Lm during focusing increases, which makes it difficult to shorten the overall lens length, which is undesirable.

[0059] When the back focus at infinity is sk, it is preferable to satisfy the following condition: −0.20<sk / fr<0.00 (7).

[0060] Conditional expression (7) defines the ratio between the focal length of the rear lens unit Lr and the back focal length sk when focusing at infinity. By satisfying conditional expression (7), it is possible to suppress fluctuations in coma aberration during focusing.

[0061] By providing the rear lens unit Lr with negative refractive power, it is possible to construct an optical system that is nearly symmetrical, making it easier to correct coma and chromatic aberration of magnification. In addition, it is possible to strengthen the refractive power of the intermediate lens unit Lm, which serves as the main focus lens unit, and reduce the amount of movement of the intermediate lens unit Lm during focusing.

[0062] This makes it possible to suppress fluctuations in coma aberration during focusing. If the upper limit of conditional expression (7) is exceeded and the refractive power of the rear unit Lr becomes too weak, it becomes difficult to reduce the diameter of the rear lens element. Furthermore, the symmetry of the refractive power arrangement of the optical system is lost, which is undesirable as it makes it difficult to correct curvature of field. If the lower limit of conditional expression (7) is exceeded and the refractive power of the rear unit Lr becomes too strong, the entrance pupil position moves toward the image plane, which is undesirable as it makes it difficult to ensure telecentricity of the light beam incident on the image sensor.

[0063] When the focal length of the positive lens Grp is taken as frp, it is preferable to satisfy the following condition: 1.0<frp / f<5.0 (8).

[0064] Conditional expression (8) defines the refractive power of the positive lens Grp, which has the highest refractive index among the positive lenses in the rear lens unit Lr. By satisfying conditional expression (8), it becomes possible to effectively correct axial chromatic aberration and chromatic aberration of magnification.

[0065] If the refractive power of the lens Grp becomes too strong by falling below the lower limit of conditional expression (8), it becomes difficult to correct the curvature of field.

[0066] If the upper limit is exceeded and the refractive power of the lens Grp becomes too weak, it becomes difficult to satisfactorily correct axial chromatic aberration and chromatic aberration of magnification, which is not preferable.

[0067] It is preferable to satisfy the following condition: −1.5<frp / fr<0.0 (9)

[0068] Conditional expression (9) defines the refractive power of the positive lens Grp, which has the highest refractive index among the positive lenses in the rear lens unit Lr. By satisfying conditional expression (9), it becomes possible to effectively correct axial chromatic aberration and lateral chromatic aberration.

[0069] If the refractive power of the lens Grp becomes too strong by falling below the lower limit of conditional expression (9), it becomes difficult to correct the curvature of field.

[0070] If the upper limit is exceeded and the refractive power of the lens Grp becomes too weak, it becomes difficult to satisfactorily correct axial chromatic aberration and chromatic aberration of magnification, which is not preferable.

[0071] The intermediate unit Lm preferably has, in order from the object side, a second lens unit L2 and a third lens unit L3 having a positive refractive power.

[0072] Dividing the focus lens group and using a floating focus makes it easier to suppress fluctuations in coma during focusing. Also, each lens group can be configured with fewer lenses, making it possible to reduce the weight of the focus lens group.

[0073] It is preferable that the second lens unit L2 and the third lens unit L3 move toward the object side during focusing from infinity to a close distance.

[0074] By moving both the second lens unit L2 and the third lens unit L3 toward the object side, it is possible to reduce changes in the field of view during focusing (focus breathing).

[0075] When the focal length of the second lens unit L2 is f2, it is preferable to satisfy the following condition: −0.60<fm / f2<0.10 (10).

[0076] Condition (10) defines the focal length of the second lens unit L2.

[0077] If the upper limit of conditional expression (10) is exceeded and the refractive power of the second lens unit L2 becomes positive, it becomes difficult to adopt a retrofocus type power arrangement, making it difficult to achieve a wide angle of view, and it also becomes difficult to suppress fluctuations in field curvature during focusing.

[0078] If the lower limit of conditional expression (10) is exceeded and the negative refractive power of the second lens unit becomes too strong, the performance change due to decentering of the second lens unit L2 and the third lens unit L3 becomes large, which is undesirable.

[0079] When the focal length of the third lens unit L3 is f3, it is preferable to satisfy the following condition: 0.5<fm / f3<1.3 (11).

[0080] Condition (11) defines the focal length of the third lens unit L3.

[0081] If the upper limit of conditional expression (11) is exceeded and the refractive power of the third lens group becomes too strong, it becomes difficult to suppress fluctuations in field curvature during focusing.

[0082] If the refractive power of the third lens group becomes too weak by going below the lower limit of conditional expression (11), the amount of movement of the third lens group during focusing becomes too large, which is undesirable because it increases the size of the optical system.

[0083] When the distance between the front lens unit Lf and the second lens unit L2 at infinity is d12, it is preferable to satisfy the following condition: 0.30<d12 / f<0.70 (12).

[0084] Condition (12) defines the distance between the front lens unit Lf and the second lens unit L2 at infinity.

[0085] If the upper limit of conditional expression (12) is exceeded and the distance between the front lens unit Lf and the second lens unit L2 at infinity becomes too large, this is not preferable because it increases the size of the optical system, and the fluctuation in field curvature during focusing becomes too large.

[0086] If the lower limit of conditional expression (12) is exceeded and the distance between the front lens unit Lf and the second lens unit L2 at infinity becomes too narrow, it becomes impossible to ensure sufficient movement during focusing, making it difficult to shorten the minimum shooting distance. Also, the refractive power of the middle lens unit Lm, which is the focus lens unit, becomes too strong, making it difficult to reduce fluctuations in spherical aberration during focusing.

[0087] When the amount of movement of the front second lens unit L2 from infinity to the closest point is m2 and the amount of movement of the third lens unit L3 from infinity to the closest point is m3, it is preferable to satisfy the following condition: 0.9<m2 / m3<2.5 (13).

[0088] Condition (13) defines the ratio of the movement amounts of the second lens unit L2 and the third lens unit L3 during focusing.

[0089] If the upper limit of conditional expression (13) is exceeded and the amount of movement of the second lens group becomes too large relative to the third lens group, the amount of movement of the third lens group, which has a strong refractive power, becomes small, making it difficult to shorten the minimum shooting distance.

[0090] If the lower limit of conditional expression (13) is not reached and the amount of movement of the second lens group becomes too small relative to the third lens group, the fluctuation of coma aberration during focusing becomes too large.

[0091] The middle lens unit Lm has a positive lens, and preferably satisfies the following condition: 2.0<vdGmp / vdGrp<5.0 (14) where vdGmp is the Abbe number of the positive lens Gmp having the strongest refractive power.

[0092] Condition (14) defines the ratio of the Abbe numbers of the positive lens elements Gmp and Grp.

[0093] If the upper limit of conditional expression (14) is exceeded and the Abbe number of the positive lens Gmp becomes too large, the refractive index of Gmp becomes low, making it difficult to correct spherical aberration.

[0094] If the lower limit of conditional expression (14) is exceeded and the Abbe number of the positive lens Gmp becomes too small, it becomes difficult to correct axial chromatic aberration and chromatic aberration of magnification.

[0095] When the refractive index of the positive lens Gmp is ndGmp, it is preferable to satisfy the following condition: 0.70<ndGmp / ndGrp<1.00 (15).

[0096] Condition (15) defines the ratio of the refractive index of the positive lens Gmp to the refractive index of the positive lens Grp.

[0097] If the upper limit of conditional expression (15) is exceeded and the refractive index of the positive lens Gmp becomes too high, the Abbe number of Gmp becomes small, making it difficult to correct axial chromatic aberration and chromatic aberration of magnification.

[0098] If the lower limit of conditional expression (15) is exceeded and the refractive index of the positive lens Gmp becomes too low, it becomes difficult to correct spherical aberration.

[0099] The front lens unit Lf preferably has three or more positive lenses.

[0100] By using three or more positive lenses, it becomes easier to correct spherical aberration even if the refractive power is strong. Strong refractive power also makes it possible to reduce the diameter of the aperture and the focus lens group.

[0101] It is preferable that the optical system have four or fewer groups whose spacing between adjacent lens groups changes during zooming or focusing. Reducing the number of groups whose spacing changes simplifies the mechanical configuration and makes it easier to reduce the size of the lens in the outer diameter direction.

[0102] In each embodiment, it is preferable to set the numerical ranges of the above-mentioned conditional expressions (1) to (15) as follows:

[0103] 1.752<ndGrp<1.799...(1a) 22.2<vdGrp<24.9...(2a) 0.610<θgFGrp<0.660...(3a) 1.1<fm / f<1.8...(4a) 2.7<ff / f<4.5...(5a) -5.7<fr / f<-2.2...(6a) -0.18<sk / fr<-0.02...(7a) 1.2<frp / f<4.5...(8a) -1.4<frp / fr<-0.1...(9a) -0.50<fm / f2<0.07...(10a) 0.6<fm / f3<1.2...(11a) 0.35<d12 / f<0.65 (12a) 1.1<m2 / m3<2.2 (13a) 2.3<vdGmp / vdGrp<4.5 (14a) 0.75<ndGmp / ndGrp<0.97 (15a) It is more preferable to set the numerical ranges of the above-mentioned conditional expressions (1) to (15) as follows:

[0104] 1.754<ndGrp<1.798...(1b) 22.4<vdGrp<24.8...(2b) 0.620<θgFGrp<0.650...(3b) 1.2<fm / f<1.7...(4b) 2.8<ff / f<4.2...(5b) -5.5<fr / f<-2.3...(6b) -0.17<sk / fr<-0.04...(7b) 1.3<frp / f<4.0...(8b) -1.3<frp / fr<-0.2...(9b) -0.45<fm / f2<0.05...(10b) 0.7<fm / f3<1.1...(11b) 0.40<d12 / f<0.60 (12b) 1.2<m2 / m3<2.0 (13b) 2.7<vdGmp / vdGrp<4.0 (14b) 0.80<ndGmp / ndGrp<0.94 (15b) As described above, according to each embodiment, an optical system can be obtained that has a wide angle and a large aperture ratio, yet effectively corrects various aberrations including lateral chromatic aberration, and has high optical performance over the entire object distance range from infinity to close distances. [Example] In the surface data of each numerical embodiment, r represents the radius of curvature of each optical surface, and d (mm) represents the axial distance (distance on the optical axis) between the mth surface and the (m+1)th surface. Here, m is the surface number counted from the light incident side.

[0105] Furthermore, nd is the refractive index of each optical element at the d-line, and vd and θgf are the Abbe number and partial dispersion ratio between the g-line and the F-line of the optical element, respectively. The Abbe number vd and partial dispersion ratio θgf between the g-line and the F-line of a certain material can be expressed as follows. That is, let the g-line (wavelength 435.8 nm) be the Fraunhofer line. Furthermore, if the refractive indices at the d-line (wavelength 587.6 nm), F-line (wavelength 486.1 nm), and C-line (wavelength 656.3 nm) are Ng, Nd, NF, and NC, respectively, then vd = (Nd - 1) / (NF - NC) and θgf = (Ng - NF) / (NF - NC).

[0106] BF is the back focus. The back focus is the air-equivalent value of the distance from the lens surface located closest to the object in optical system L0 to the image plane. The total lens length is the sum of the back focus and the distance on the optical axis from the lens surface located closest to the object in optical system L0 to the lens surface closest to the image. (Aspherical surface data) shows the aspherical coefficients when the aspherical surface is expressed by the following formula.

[0107]

[0108] where x: displacement from the reference plane in the optical axis direction h: height in the direction perpendicular to the optical axis R: radius of the base quadratic surface k: conic constant C ｎ : nth-order aspherical coefficients Note that the "E-Z" symbol means "10 －Ｚ In the lens cross-sectional view of FIG. 1, L1 denotes a first lens group (front lens group Lf) having a positive refractive power, L2 denotes a second lens group, L3 denotes a third lens group having a positive refractive power, and L4 denotes a fourth lens group (rear lens group Lr) having a negative refractive power.

[0109] The first lens unit L1 includes, in order from the object side, a negative meniscus lens having a meniscus shape with a convex surface facing the object side and an aspheric surface formed on the image side, and a cemented negative lens formed by cementing together a negative meniscus lens with a convex surface facing the object side and a positive meniscus lens with a convex surface facing the object side.

[0110] Furthermore, it is composed of a biconcave negative lens, a cemented positive lens formed by cementing a biconvex positive lens and a negative meniscus lens with its concave surface facing the object side, a biconvex positive lens, and a cemented negative lens formed by cementing a biconcave negative lens and a positive meniscus lens with its convex surface facing the object side.

[0111] The second lens unit L2 is composed of a negative meniscus lens with a concave surface facing the object side, and a positive meniscus lens with a convex surface facing the object side.

[0112] The third lens group L3 is composed of a biconvex positive lens (positive lens Gmp) with an aspherical surface formed on the object-side surface, and a biconvex positive lens with an aspherical surface formed on the image-side surface.

[0113] The fourth lens unit L4 is composed of a positive meniscus lens (positive lens Grp) with a convex surface facing the object side, a negative meniscus lens with a convex surface facing the object side, and a cemented negative lens formed by cementing a biconvex positive lens and a biconcave negative lens.

[0114] SP denotes an aperture stop, which is arranged on the image side of the first lens unit L1.

[0115] Focusing is performed by moving the intermediate lens unit Lm toward the object side.

[0116] During focusing from infinity to a close distance, the second lens unit L2 moves toward the object side as shown by the arrow, and the third lens unit L3 moves toward the object side while increasing the distance between it and the second lens unit L2.

[0117] Numerical values ​​corresponding to Example 1 are hereinafter referred to as Numerical Example 1.

[0118] (Numerical Example 1) Unit: mm Surface data Surface number rd nd νd Pitch diameter θgf 1 49.250 2.80 1.58313 59.4 54.49 2* 25.748 10.41 45.24 3 91.875 1.40 1.51633 64.1 44.82 4 27.517 6.24 2.00069 25.5 40.51 5 32.273 13.09 37.51 6 -31.122 1.40 1.51742 52.4 37.10 7 167.205 0.20 39.18 8 77.066 15.83 1.76385 48.5 40.96 9 -28.044 1.50 1.85478 24.8 42.44 10 -49.833 0.20 45.41 11 52.217 9.24 2.00100 29.1 46.69 12 -175.727 2.91 45.73 13 -83.034 1.30 1.77047 29.7 43.66 14 25.756 10.87 1.59522 67.7 38.86 15 326.441 2.29 38.39 16(Aperture) ∞ (Variable) 38.01 17 -46.467 1.40 1.77047 29.7 34.83 18 -268.162 0.20 35.79 19 46.945 3.88 1.59522 67.7 37.18 20 98.394 (variable) 36.93 21* 48.067 7.58 1.59522 67.7 36.94 22 -186.725 2.69 36.81 23 251.279 3.25 1.76450 49.1 37.23 24* -109.420 (variable) 37.21 25 45.812 5.15 1.79631 22.6 37.98 0.6411 26 424.301 0.20 37.52 27 71.316 1.20 1.61340 44.3 36.40 28 26.464 4.46 33.82 29 72.240 10.39 1.43875 94.7 33.86 30 -27.204 1.20 1.85478 24.8 33.70 31 5790.447 13.45 35.75 Image surface ∞ Aspheric surface data Surface 2 K = 0.00000e+00 A 4=-1.16066e-06 A 6=-3.03244e-09 A 8=-4.46652e-13 A10= 3.41317e-15 A12=-1.75072e-17 21st side K = 0.00000e+00 A 4=-1.70638e-06 A 6= 4.96073e-09 A 8= 1.12011e-11 A10=-2.59322e-14 A12= 2.53729e-17 Surface 24 K = 0.00000e+00 A 4= 6.13353e-06 A 6= 5.14223e-09 A 8=-1.07393e-13 A10= 1.29861e-14 A12= 8.20741e-18 Data Focal length 34.00 F-number 1.24 Half angle of view 32.47 Image height 21.64 Lens length 154.96 BF 13.45 Infinity 0.28m d16 17.85 6.06 d20 1.20 4.43 d24 1.19 9.76 Lens Group Data Lens Group Initial Surface Focal Length 1 1 135.15 2 17 -143.94 3 21 41.37 4 25 -180.31 [Example 2] In the lens cross-sectional view of Figure 4, L1 is the first lens group (front lens group Lf) with positive refractive power, L2 is the second lens group, L3 is the third lens group with positive refractive power, and L4 is the fourth lens group (rear lens group Lr) with negative refractive power.

[0119] The first lens unit L1 includes, in order from the object side, a negative meniscus lens having a meniscus shape with a convex surface facing the object side and an aspheric surface formed on the image side, and a cemented negative lens formed by cementing together a negative meniscus lens with a convex surface facing the object side and a positive meniscus lens with a convex surface facing the object side.

[0120] Furthermore, it is composed of a biconcave negative lens, a cemented positive lens formed by cementing a biconvex positive lens and a negative meniscus lens with its concave surface facing the object side, a biconvex positive lens, and a cemented negative lens formed by cementing a biconcave negative lens and a biconvex positive lens.

[0121] The second lens unit L2 is composed of a negative meniscus lens with a concave surface facing the object side, and a positive meniscus lens with a convex surface facing the object side.

[0122] The third lens group L3 is composed of a biconvex positive lens (positive lens Gmp) with an aspherical surface formed on the object-side surface, and a biconvex positive lens with an aspherical surface formed on the image-side surface.

[0123] The fourth lens unit L4 is composed of a positive meniscus lens (positive lens Grp) with a convex surface facing the object side, a negative meniscus lens with a convex surface facing the object side, and a cemented negative lens formed by cementing a biconvex positive lens and a biconcave negative lens.

[0124] SP denotes an aperture stop, which is arranged on the image side of the first lens unit L1.

[0125] Focusing is performed by moving the intermediate lens unit Lm toward the object side. When focusing from infinity to a close distance, the second lens unit L2 moves toward the object side as shown by the arrow. The third lens unit L3 moves toward the object side while increasing the distance between it and the third lens unit L3.

[0126] Numerical values ​​corresponding to Example 2 are hereinafter referred to as Numerical Example 2.

[0127] (Numerical Example 2) Unit: mm Surface data Surface number rd nd νd Pitch diameter θgf 1 62.837 2.80 1.58313 59.4 55.11 2* 28.303 9.96 45.48 3 136.845 1.40 1.51633 64.1 44.99 4 37.622 2.93 2.00069 25.5 41.31 5 50.790 10.90 40.28 6 -36.110 1.40 1.51742 52.4 39.18 7 131.738 0.20 41.17 8 78.795 12.72 1.76385 48.5 42.73 9 -37.220 1.50 1.85478 24.8 43.95 10 -66.905 0.20 46.45 11 61.160 7.16 2.00100 29.1 47.00 12 -308.481 5.13 45.91 13 -100.536 1.30 1.77047 29.7 43.27 14 29.265 11.46 1.59522 67.7 38.86 15 -119.475 1.00 38.43 16(Aperture) ∞ (Variable) 38.01 17 -38.857 1.40 1.77047 29.7 35.03 18 -106.130 0.20 36.38 19 45.912 3.15 1.59522 67.7 38.09 20 85.580 (Variable) 37.90 21* 65.526 6.78 1.55332 71.7 36.31 22 -153.621 3.46 35.73 23 500.204 3.28 1.76450 49.1 35.94 24* -90.398 (Variable) 35.56 25 36.959 5.00 1.77830 23.9 35.63 0.6248 26 149.776 0.20 35.19 27 67.697 1.20 1.61340 44.3 34.52 28 25.062 9.23 31.81 29 143.338 7.77 1.43875 94.7 31.96 30 -29.960 1.20 1.85478 24.8 32.25 31 288.732 14.94 34.68 Image surface ∞ Aspheric surface data Surface 2 K = 0.00000e+00 A 4=-3.81875e-07 A 6=-1.77711e-09 A 8= 6.18998e-12 A10=-1.59599e-14 A12= 1.44372e-17 21st side K = 0.00000e+00 A 4= 5.75900e-07 A 6= 6.03605e-09 A 8= 5.42166e-12 A10=-9.22343e-15 A12= 7.97029e-18 Surface 24 K = 0.00000e+00 A 4= 5.65424e-06 A 6= 5.91887e-09 A 8=-9.92448e-13 A10= 2.08698e-14 A12=-2.51766e-18 Various data Focal length 42.38 F-number 1.52 Half angle of view 27.04 Image height 21.64 Lens length 152.96 BF 14.94 Infinity 0.28m d16 22.71 6.96 d20 1.20 5.21 d24 1.20 12.94 Lens Group Data Lens Group Initial Surface Focal Length 1 1 123.27 2 17 -158.07 3 21 47.82 4 25 -102.07 [Example 3] In the lens cross-sectional view of Figure 7, L1 is the first lens group (front lens group Lf) with positive refractive power, L2 is the second lens group, L3 is the third lens group with positive refractive power, and L4 is the fourth lens group (rear lens group Lr) with negative refractive power.

[0128] The first lens unit L1 includes, in order from the object side, a negative meniscus lens having a meniscus shape with a convex surface facing the object side and an aspheric surface formed on the image side, and a cemented negative lens formed by cementing together a negative meniscus lens with a convex surface facing the object side and a positive meniscus lens with a convex surface facing the object side.

[0129] Furthermore, it is composed of a biconcave negative lens, a cemented positive lens formed by cementing a biconvex positive lens and a negative meniscus lens with its concave surface facing the object side, a biconvex positive lens, and a cemented negative lens formed by cementing a biconcave negative lens and a biconvex positive lens.

[0130] The second lens unit L2 is composed of a negative meniscus lens with a concave surface facing the object side, and a positive meniscus lens with a convex surface facing the object side.

[0131] The third lens group L3 is composed of a biconvex positive lens (positive lens Gmp) with an aspherical surface formed on the object-side surface, and a biconvex positive lens with an aspherical surface formed on the image-side surface.

[0132] The fourth lens unit L4 is composed of a biconvex positive lens (positive lens Grp), a negative meniscus lens with a convex surface facing the object side, and a cemented negative lens formed by cementing a biconvex positive lens and a negative meniscus lens with a concave surface facing the object side.

[0133] SP denotes an aperture stop, which is arranged on the image side of the first lens unit L1.

[0134] Focusing is performed by moving the intermediate lens unit Lm toward the object side.

[0135] During focusing from infinity to a close distance, the second lens unit L2 moves toward the object side as shown by the arrow, and the third lens unit L3 moves toward the object side while increasing the distance between it and the third lens unit L3.

[0136] Numerical values ​​corresponding to Example 3 are hereinafter referred to as Numerical Example 3.

[0137] (Numerical Example 3) Unit: mm Surface data Surface number rd nd νd Effective diameter θgf 1 54.657 2.80 1.58313 59.4 56.17 2* 25.434 11.70 45.10 3 113.067 1.40 1.51633 64.1 44.62 4 26.749 4.42 2.00069 25.5 40.27 5 32.139 15.23 38.73 6 -31.675 1.40 1.51742 52.4 37.92 7 219.944 0.28 40.47 8 88.731 14.47 1.76385 48.5 41.49 9 -28.600 1.50 1.85478 24.8 41.75 10 -47.807 3.69 44.28 11 56.321 11.48 2.00100 29.1 46.20 12 -162.905 2.82 44.32 13 -79.829 1.30 1.77047 29.7 42.56 14 27.506 10.42 1.59522 67.7 38.76 15 -11558.899 1.68 38.40 16(Aperture) ∞ (Variable) 38.00 17 -45.259 1.40 1.77047 29.7 35.46 18 -204.255 0.20 36.46 19 50.234 4.88 1.59522 67.7 37.79 20 841.378 (Variable) 37.61 21* 48.125 7.93 1.49700 81.5 36.64 22 -339.413 0.20 35.02 23 66.186 4.20 1.76450 49.1 34.51 24* -544.156 (Variable) 33.86 25 49.370 4.43 1.75575 24.7 33.98 0.6291 26 -5095.117 0.19 33.57 27 128.162 1.20 1.61340 44.3 32.89 28 23.425 5.04 30.45 29 110.349 9.26 1.43875 94.7 30.62 30 -23.798 1.20 1.85883 30.0 30.99 31 -121.247 13.45 33.73 Image surface ∞ Aspheric surface data Surface 2 K = 0.00000e+00 A 4=-5.19506e-07 A 6=-3.96549e-09 A 8= 9.07553e-12 A10=-1.67303e-14 A12= 3.06982e-18 21st side K = 0.00000e+00 A 4= 2.97105e-06 A 6= 6.99504e-09 A 8= 1.64275e-11 A10=-3.89622e-14 A12= 4.86661e-17 Surface 24 K = 0.00000e+00 A 4= 1.06794e-05 A 6= 3.72319e-09 A 8= 2.78295e-11 A10=-6.58023e-14 A12= 1.80633e-16 Data Focal Length 30.00 F-Number 1.24 Half Angle of View 35.79 Image Height 21.64 Lens Length 93.73 BF 13.45 Infinity 0.28m d16 14.39 6.18 d20 1.20 4.70 d24 1.20 5.90 Lens Group Data Lens Group Initial Surface Focal Length 1 1 108.48 2 17 -531.24 3 21 41.93 4 25 -90.76 [Example 4] In the lens cross-sectional view of Figure 1, L1 is the first lens group (front lens group Lf) with positive refractive power, L2 is the second lens group, L3 is the third lens group with positive refractive power, and L4 is the fourth lens group (rear lens group Lr) with negative refractive power.

[0138] The first lens unit L1 includes, in order from the object side, a negative meniscus lens having a meniscus shape with a convex surface facing the object side and an aspheric surface formed on the image side, and a cemented negative lens formed by cementing together a negative meniscus lens with a convex surface facing the object side and a positive meniscus lens with a convex surface facing the object side.

[0139] Furthermore, it is composed of a biconcave negative lens, a cemented positive lens formed by cementing a biconvex positive lens and a negative meniscus lens with its concave surface facing the object side, a biconvex positive lens, and a cemented negative lens formed by cementing a biconcave negative lens and a positive meniscus lens with its convex surface facing the object side.

[0140] The second lens unit L2 is composed of a negative meniscus lens with a concave surface facing the object side, and a positive meniscus lens with a convex surface facing the object side.

[0141] The third lens group L3 is composed of a biconvex positive lens (positive lens Gmp) with an aspherical surface formed on the object-side surface, and a biconvex positive lens with an aspherical surface formed on the image-side surface.

[0142] The fourth lens unit L4 is composed of a positive meniscus lens (positive lens Grp) with a convex surface facing the object side, a negative meniscus lens with a convex surface facing the object side, and a negative meniscus lens with a concave surface facing the object side.

[0143] SP denotes an aperture stop, which is arranged on the image side of the first lens unit L1.

[0144] Focusing is performed by moving the intermediate lens unit Lm toward the object side. When focusing from infinity to a close distance, the second lens unit L2 moves toward the object side as shown by the arrow. The third lens unit L3 moves toward the object side while increasing the distance between it and the third lens unit L3.

[0145] Numerical values ​​corresponding to Example 4 are hereinafter referred to as Numerical Example 4.

[0146] (Numerical Example 4) Unit: mm Surface data Surface number rd nd νd Pitch diameter θgf 1 51.616 2.80 1.58313 59.4 56.10 2* 26.628 8.90 46.44 3 57.709 1.40 1.76385 48.5 46.12 4 23.785 7.12 2.00069 25.5 40.76 5 33.469 16.86 39.08 6 -35.492 1.40 1.51742 52.4 36.89 7 582.720 1.01 38.85 8 75.366 17.97 1.76385 48.5 44.20 9 -28.721 1.50 1.85478 24.8 45.33 10 -57.261 0.19 48.30 11 60.154 8.68 2.00100 29.1 48.14 12 -151.485 3.38 47.37 13 -80.629 1.30 1.77047 29.7 44.02 14 26.819 10.01 1.59522 67.7 39.01 15 238.603 2.51 38.47 16(Aperture) ∞ (Variable) 38.01 17 -30.758 1.40 1.77047 29.7 34.72 18 -47.438 0.20 36.10 19 45.828 4.04 1.59522 67.7 37.22 20 145.005 (Variable) 36.92 21* 91.312 7.65 1.49700 81.5 37.42 22 -64.145 4.45 38.04 23 84.332 3.29 1.76450 49.1 38.21 24* -826.769 (Variable) 37.84 25 62.960 3.06 1.79631 22.6 37.91 0.6411 26 182.404 0.19 37.62 27 43.277 1.20 1.61340 44.3 36.61 28 25.661 9.65 34.55 29 -67.750 1.20 1.77047 29.7 34.68 30 -502.271 13.45 35.73 Image surface ∞ Aspheric surface data Surface 2 K = 0.00000e+00 A4=-4.53964e-07 A6=-2.81876e-09 A8= 5.57099e-12 A10=-9.85220e-15 A12= 2.29695e-18 Surface 21 K = 0.00000e+00 A 4=-6.77933e-07 A 6= 5.55648e-10 A 8= 9.21152e-12 A10=-3.25029e-14 A12= 2.86790e-17 Surface 24 K = 0.00000e+00 A4= 6.90962e-06 A6= 1.59512e-09 A8= 8.82941e-12 A10=-8.70854e-15 A12= 1.07094e-17 Data Focal length 34.97 F-number 1.24 Half angle of view 31.75 Image height 21.64 Total lens length 88.52 BF 13.45 Infinity 0.28m d16 17.78 8.56 d20 1.20 4.15 d24 1.19 7.45 Group data Group First surface Focal length 1 1 125.63 2 17 1567.48 3 21 45.30 4 25 -97.66 [Modification] In contrast to Example 1, the second lens unit L2 may be configured with a single positive or negative lens. By configuring the second lens unit L2 with a single lens, it becomes easier to reduce the weight of the second lens unit L2.

[0147] In the optical systems of each embodiment, it is advisable to vapor-deposit a fluorine coating on the object-side lens surface of the lens positioned closest to the object and the image-side lens surface of the lens positioned closest to the image. Because the object-side lens surface of the lens positioned closest to the object and the image-side lens surface of the lens positioned closest to the image are prone to contact with the outside world, vapor-depositing a fluorine coating can improve water and oil repellency, suppress flare, and achieve high optical performance.

[0148] In particular, since the lens surface closest to the object side of the lens positioned closest to the object side has a large diameter, it is preferable to vapor-deposit a fluorine coating on it.

[0149] In the cemented lenses arranged in the optical systems of the examples, it is preferable that the positive lens and negative lens constituting at least one cemented lens are bonded with an adhesive having an axial thickness of 0.005 mm or more and 0.05 mm or less. If it is less than 0.005 mm, the adhesive is prone to peeling, and if it is more than 0.03 mm, the axial distance from the lens surface closest to the object to the lens surface closest to the image becomes long, resulting in a long overall lens length. It is more preferable that it satisfies the requirement of 0.008 mm or more and 0.02 mm or less.

[0150] At least one lens arranged in the optical system of each example is provided with an anti-reflection coating for preventing reflection, and the anti-reflection coating is composed of multiple films. Here, when the refractive index of the film closest to the air interface with respect to the d-line is Nd, it is preferable that the anti-reflection coating PC has Nd of 1.32 or less.

[0151] By setting Nd to 1.32 or less, the difference in refractive index from air can be reduced, which makes it possible to further reduce light reflection and reduce ghosting.

[0152] Specific examples of the configuration of the antireflection film PC include, but are not limited to, multilayer films formed using a wet method, as described in JP-A-2012-230211, JP-A-2014-95877, etc. More preferably, by setting Nd to 1.30 or less, ghosts can be further reduced.

[0153] Here, it is preferable to provide an anti-reflection film PC on the image-side lens surface of the negative lens arranged in the optical system, the negative lens having a concave surface facing the image side. Light reflected by a negative lens having a concave surface facing the image side is likely to be reflected at a large angle relative to the normal direction of the lens surface of the negative lens having a concave surface facing the image side, and therefore the reflectance is likely to be high. Furthermore, light reflected by a negative lens having a concave surface facing the image side is likely to be focused on the image plane, making ghost images noticeable. Therefore, by providing an anti-reflection film PC on the image-side lens surface of a negative lens having a concave surface facing the image side, ghost images can be reduced.

[0154] The various values ​​in each numerical example are summarized in Table 1 below.

[0155]

[0156] Next, an example in which the optical system of each example is used as an imaging optical system will be described with reference to Fig. 13. In Fig. 13, 10 is a diagram showing an example of an imaging device, 11 is an imaging optical system configured using the optical system of this embodiment, and 12 is a solid-state imaging element (photoelectric conversion element) such as a CCD sensor or CMOS sensor that receives the subject image formed by the imaging optical system 11. Also, 13 is a recording means that records the subject image received by the imaging element 12, and 14 is a finder for observing the subject image displayed on a display element (not shown). The display element is configured using a liquid crystal panel or the like, and displays the subject image formed on the imaging element 12.

[0157] In this way, by applying the optical system of this embodiment to an optical device such as a digital camera, an optical device with high optical performance can be realized.

[0158] This embodiment can also be applied to an SLR (Single Lens Reflex) camera that does not have a quick return mirror.

[0159] The optical system of this embodiment can also be applied to a video camera.

[0160] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments and various modifications and changes are possible within the scope of the invention.

Claims

1. An optical system comprising a front lens group having positive refractive power and located closest to the object, a rear lens group having negative refractive power and located closest to the image, and an intermediate group having positive refractive power as a whole and including one or more lens groups and located between the front lens group and the rear lens group, wherein the spacing between adjacent lens groups changes during focusing, and wherein the optical system satisfies the following conditional expressions: 1.750<ndGrp<1.800 20.0<vdGrp<25.0, where ndGrp is the refractive index of the positive lens Grp having the highest refractive index among the positive lenses located in the rear lens group and vdGrp is the Abbe number.

2. The optical system according to claim 1, wherein the front lens group has two negative lenses arranged in this order from the object side to the image side.

3. The optical system according to claim 1 or 2, wherein the following condition is satisfied: 0.600<θgFGrp<0.670, where θgFGrp is the partial dispersion ratio of the positive lens Grp.

4. An optical system according to any one of claims 1 to 3, characterized in that the following condition is satisfied: 1.0<fm / f<2.0, where f is the focal length of the entire system and fm is the composite focal length of the intermediate group.

5. An optical system according to any one of claims 1 to 4, characterized in that the following condition is satisfied: 2.5<ff / f<5.0, where f is the focal length of the entire system and ff is the focal length of the front lens group.

6. An optical system according to any one of claims 1 to 5, characterized in that the following condition is satisfied: -6.0<fr / f<-2.0, where f is the focal length of the entire system and fr is the focal length of the rear lens group.

7. An optical system according to any one of claims 1 to 6, characterized in that the following condition is satisfied: -0.20<sk / fr<0.00, where sk is the back focus when focused at infinity and fr is the focal length of the rear lens group.

8. An optical system according to any one of claims 1 to 7, characterized in that the following condition is satisfied: 1.0<frp / f<5.0, where f is the focal length of the entire system and frp is the focal length of the positive lens Grp.

9. An optical system according to any one of claims 1 to 8, characterized in that the following condition is satisfied: -1.5<frp / fr<0.0, where fr is the focal length of the rear lens group and frp is the focal length of the positive lens Grp.

10. The optical system according to any one of claims 1 to 9, characterized in that the intermediate group comprises, in order from the object side, a second lens group and a third lens group having a positive refractive power.

11. The optical system according to claim 10, wherein the second lens group and the third lens group each move toward the object side during focusing from infinity to a close distance.

12. The optical system according to claim 10 or 11, characterized in that the following condition is satisfied: -0.6<fm / f2<0.1, where fm is the composite focal length of the intermediate lens group and f2 is the focal length of the second lens group.

13. An optical system according to any one of claims 10 to 12, characterized in that the following condition is satisfied: 0.5<fm / f3<1.3, where fm is the composite focal length of the intermediate lens group and f3 is the focal length of the third lens group.

14. An optical system according to any one of claims 10 to 13, characterized in that the following condition is satisfied: 0.30<d12 / f<0.70, where f is the focal length of the entire system and d12 is the distance on the optical axis between the front lens group and the second lens group L2 when focused at infinity.

15. An optical system according to any one of claims 10 to 14, characterized in that the following condition is satisfied: 0.9<m2 / m3<2.5, where m2 is the amount of movement of the second lens group from infinity to the closest point, and m3 is the amount of movement of the third lens group from infinity to the closest point.

16. An optical system according to any one of claims 1 to 15, characterized in that the intermediate group has a positive lens, and satisfies the following condition: 1.0<vdGmp / vdGrp<5.0, where vdGmp is the Abbe number of the positive lens Gmp with the strongest refractive power.

17. The optical system according to claim 16, wherein the following condition is satisfied: 0.7<ndGmp / ndGrp<1.0, where ndGmp is the refractive index of the positive lens Gmp.

18. The optical system according to any one of claims 1 to 17, characterized in that the front lens group has a stop arranged closest to the image side.

19. An optical system according to any one of claims 1 to 18, characterized in that the front lens group has three or more positive lenses.

20. An optical system according to any one of claims 1 to 19, characterized in that the optical system has four or less groups in which the spacing between adjacent lens groups changes during zooming or focusing.

21. An optical system comprising, arranged in order from the object side to the image side, a front lens group with positive refractive power, an intermediate group including one or more lens groups and having positive refractive power as a whole, and a rear lens group with negative refractive power, wherein the spacing between adjacent lens groups changes during focusing, and wherein the rear lens group has at least one positive lens.

22. An imaging device comprising the optical system according to any one of claims 1 to 21 and an imaging element for receiving an image formed by said optical system.

Images