Optical system and endoscope

Abstract

The utility model provides an optical system and an endoscope. The optical system is applied to the endoscope and comprises a first lens group, a second lens group and a third lens group which are sequentially arranged along an optical axis, the first lens group is close to an object side, and the third lens group is close to an image side; the positions of the first lens group and the third lens group along the optical axis are fixed, the first lens group and the third lens group comprise the same number of first single lenses and the same number of second single lenses, the first single lenses have positive focal power, and the second single lenses have negative focal power; the second lens group is movable between the first lens group and the third lens group along the optical axis to perform optical zooming. In the optical system, the positions of the first lens group, the third lens group and the image surface are not changed, only the second lens group needs to be moved, the movement is more convenient and flexible, and focusing can be completed more quickly. The symmetrical structure can help correct aberration brought by the second lens group. The operation or examination time can be shortened through the small focusing stroke.

Description

Technical Field

[0001] The utility model relates to the technical field of optical systems, in particular to an optical system applied to an endoscope and an endoscope. Background Art

[0002] In modern medical examinations, endoscopes are used to examine lesions within the human body. To diagnose lesions directly within the body, endoscopes with magnifying observation capabilities can be used. Endoscopes with magnifying observation capabilities contain a zoom objective lens. Under normal observation conditions, this lens has a wide field of view, suitable for overall observation. At close range, the lens achieves optical zoom by switching the internal lens position, resulting in a narrow field of view, suitable for localized observation.

[0003] With the development of science and technology, endoscopic camera systems are gradually adopting image sensors with ultra-high-definition resolution, such as 4K resolution. Endoscopic systems with ultra-high-definition resolution can help doctors detect tiny lesions, thereby improving the accuracy of diagnosis and preventing missed diagnosis. For endoscopic camera systems with ultra-high-definition resolution, especially for endoscopes with a large depth of field, in order to ensure imaging quality, the required imaging area of ​​the image sensor is larger. The traditional focusing method of the endoscope adapter adopts external focusing (focusing that changes the image distance), that is, the optical lens group as a whole moves away from or closer to the image sensor. The larger imaging area makes the optical focal length of the endoscope adapter larger, which increases the focusing stroke and time, resulting in a longer operation or examination time. Utility Model Content

[0004] In order to at least partially solve the problems existing in the prior art, according to one aspect of the present invention, an optical system is provided, which is applied to an endoscope, comprising a first lens group, a second lens group and a third lens group arranged in sequence along the optical axis, the first lens group is close to the object side, and the third lens group is close to the image side; the positions of the first lens group and the third lens group along the optical axis are fixed, the first lens group and the third lens group include the same number of first single lenses and the same number of second single lenses, the first single lenses have positive optical power, and the second single lenses have negative optical power; the second lens group is movable between the first lens group and the third lens group along the optical axis to perform optical zoom.

[0005] Preferably, each of the first lens group and the third lens group consists of a first single lens and a second single lens.

[0006] Preferably, the first single lens is a plano-convex lens, and the second single lens is a plano-concave lens.

[0007] Preferably, in the first lens group, the first single lens is located on the object side of the second single lens, with the convex surface of the first single lens facing the object side, and the concave surface of the second single lens facing the image side; in the third lens group, the first single lens is located on the image side of the second single lens, with the convex surface of the first single lens facing the image side, and the concave surface of the second single lens facing the image side.

[0008] Preferably, the second lens group includes a biconvex lens and a plano-concave lens.

[0009] Preferably, the biconvex lens and the plano-concave lens are cemented to form a doublet lens.

[0010] Preferably, the optical system satisfies the following condition: 8≤|f1 / f2|≤14. f1 is the focal length of the first lens group; f2 is the focal length of the second lens group.

[0011] Preferably, the movement range of the second lens group is set so that f12 satisfies the following conditional expression:

[0012] 2.5≤|f3 / f12|≤5.5. f3 is the focal length of the third lens group; f12 is the focal length of the first and second lens groups combined.

[0013] Preferably, the optical system satisfies the following condition: 3.5≤|f2 / f3|≤5.5, where f3 is the focal length of the third lens group and f2 is the focal length of the second lens group.

[0014] Preferably, the optical system further comprises an aperture stop arranged on the object side of the first lens group.

[0015] Preferably, the first lens group has positive refractive power, the second lens group has positive refractive power, and the third lens group has negative refractive power.

[0016] According to another aspect of the present invention, an endoscope is provided, which includes an image sensor and any one of the optical systems described above, wherein the image sensor is located on an image plane of the optical system.

[0017] In summary, in this optical system, the positions of the first lens group, the third lens group, and the image plane remain unchanged. Only the second lens group needs to be moved, and there is no need to move the entire optical system closer to the image sensor. The movement is lighter and more flexible, so that focusing can be completed more quickly. In addition, the focal length of the second lens group will affect the focusing stroke. The smaller the focal length of the second lens group, the smaller the focusing stroke. However, the smaller the focal length of the second lens group, the greater the aberration caused. The first lens group and the third lens group use the same number and type of single lenses. This symmetrical structure can help correct the aberration caused by the second lens group. A smaller focusing stroke can shorten the duration of surgery or examination to a certain extent and improve user experience.

[0018] The Summary of the Utility Model introduces a series of simplified concepts, which will be further described in detail in the Detailed Description of the Utility Model. This Summary of the Utility Model does not intend to limit the key features and essential technical features of the claimed technical solution, nor does it intend to determine the scope of protection of the claimed technical solution.

[0019] The advantages and features of the present invention are described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The following drawings of the present invention are used as part of the present invention for understanding the present invention. The drawings show the embodiments of the present invention and their descriptions, and are used to explain the principles of the present invention. In the drawings,

[0021] Figure 1 FIG1 shows a schematic structural diagram of an optical system according to an exemplary embodiment of the present application in a farthest point observation state;

[0022] Figure 2 Shown according to Figure 1 A light path simulation diagram of the optical system of the embodiment shown in the farthest point observation state;

[0023] Figure 3 Shown according to Figure 1 A light path simulation diagram of the optical system of the embodiment shown in the closest point observation state;

[0024] Figure 4 Shown according to Figure 1 A modulation transfer function characteristic curve diagram of the optical system of the embodiment shown in the farthest point observation state;

[0025] Figure 5 Shown according to Figure 1 Field curvature and distortion curves of the optical system of the illustrated embodiment when viewed at the farthest point;

[0026] Figure 6 Shown according to Figure 1 A modulation transfer function characteristic curve diagram of the optical system of the embodiment shown in the closest point observation state;

[0027] Figure 7 Shown according to Figure 1 Field curvature and distortion curves of the optical system of the illustrated embodiment when observed at the closest point. DETAILED DESCRIPTION

[0028] In the following description, numerous details are provided to facilitate a thorough understanding of the present invention. However, those skilled in the art will appreciate that the following description merely illustrates preferred embodiments of the present invention, and that the present invention may be practiced without one or more of these details. Furthermore, to avoid confusion with the present invention, some technical features well known in the art have not been described in detail.

[0029] As mentioned above, for a larger imaging area, a longer focal length is required to ensure the imaging effect. For the optical system of an endoscope, since the object distance is different from that of a conventional camera, an optical system with a focal length greater than 25mm is usually called a telephoto system. According to the Gaussian imaging formula:

[0030]

[0031] The sum of the reciprocals of the object distance u and the phase distance v equals the reciprocal of the focal length. Therefore, for the same change in object distance, a long-focus optical system requires a much larger image distance adjustment than a short-focus optical system. This results in a longer external focusing stroke. In contrast, adjusting the internal focusing mechanism at a specific lens group in the system can effectively reduce the focusing stroke.

[0032] According to one aspect of the present invention, an optical system is provided. Figure 1 The schematic diagram of the optical system in its most distant observation state shows the structure. As shown, it includes a first lens group G1, a second lens group G2, and a third lens group G3, arranged sequentially along the optical axis. The first lens group G1 is positioned toward the object side, while the third lens group G3 is positioned toward the image side. The positions of the first and third lens groups G1 and G3 along the optical axis remain fixed. The first and third lens groups include the same number of first single lenses and the same number of second single lenses. The first single lenses have positive optical power, while the second single lenses have negative optical power. The second lens group G2 is movable along the optical axis to achieve zooming of the optical system.

[0033] Light from an object passes sequentially through the first lens group G1, the second lens group G2, and the third lens group G3 to form an image. The second lens group G2 is the focusing group, moving as the object distance changes. As the object distance decreases from infinity to close, the second lens group, acting as the focusing lens, gradually moves closer to the object. This allows for focusing on subjects at varying object distances by adjusting the position of the second lens group G2, resulting in a clear image on the image plane.

[0034] The first lens group and the third lens group can be symmetrical structures. This structure does not require that the first lens group and the third lens group are completely consistent or opposite in parameters and lens arrangement, but only requires that they have the same number of first single lenses and second single lenses. For example Figure 1As shown, the first lens group G1 includes lens L1 and lens L2, lens L1 has positive optical power, and lens L2 has negative optical power. Lens L1 can converge light from an object, and the aberrations generated are preliminarily corrected by lens L2. The third lens group also includes a lens L6 with positive optical power and a lens L5 with negative optical power, so that the first lens group and the third lens group form an optically symmetrical structure. The symmetrical structure can reduce the difficulty of calibrating the aberrations of the optical system, so that a smaller number of lenses can be used to achieve the same imaging effect. In an exemplary embodiment, the optical system can be composed of 6 lenses. In an embodiment not shown, the first lens group and the third lens group can include more lenses.

[0035] In summary, in this optical system, the positions of the first lens group G1, the third lens group G3, and the image plane r12 remain unchanged. It is only necessary to move the second lens group G2, without moving the entire optical system closer to the image sensor. The movement is lighter and more flexible, so that focusing can be completed more quickly. In addition, the focal length of the second lens group will affect the focusing stroke. The smaller the focal length of the second lens group, the smaller the focusing stroke. However, the smaller the focal length of the second lens group, the greater the aberration caused. The first lens group and the third lens group use the same number and type of single lenses. This symmetrical structure can help correct the aberration caused by the second lens group. A smaller focusing stroke can shorten the duration of surgery or examination to a certain extent and improve the user experience.

[0036] Preferably, each of the first and third lens groups consists of a first single lens and a second single lens. As described above, the symmetrical structural design can significantly reduce the generation of phase aberration, thereby eliminating the need for a large number of lenses for image correction. This significantly reduces the number of lenses, shortens the length of the optical system, and makes it more suitable for endoscopes. It can also reduce costs to a certain extent.

[0037] Preferably, the first single lens is a plano-convex lens, and the second single lens is a plano-concave lens.

[0038] Preferably, in the first lens group, the first single lens is located on the object side of the second single lens, with the convex surface of the first single lens facing the object side and the concave surface of the second single lens facing the image side. For the optical system, the incident light can be considered as parallel light. Figure 1 and Figure 2Parallel light entering single lens L1, the convex surface of a plano-convex lens, is refracted and converges toward the focal point of lens L1. Light at an angle enters lens L2, the flat surface of a plano-concave lens. It is refracted by lens L2 and emerges from the concave surface facing the image side. Similarly, in the third lens group G3, the first single lens is located on the image side of the second single lens, with its convex surface facing the image side and its concave surface facing the image side.

[0039] The above optical design can effectively reduce the difficulty of correcting aberrations (including field curvature and distortion). The specific effects are as follows: Figure 4-Figure 7 As shown in the figure, for different depths of field, the optical system has an excellent modulation transfer function, and the field curvature and distortion are also within the appropriate range.

[0040] like Figure 1 As shown, preferably, second lens group G2 comprises a biconvex lens and a plano-concave lens. Second lens group G2 also includes lens L3, which is a biconvex lens. Biconvex lenses are well-suited for applications with limited imaging. Because both the front and rear surfaces of a biconvex lens are convex spherical, lens L3 is symmetrical, minimizing aberrations. Furthermore, preferably, the radii of curvature on both sides of lens L3 are equal, which minimizes spherical aberration and eliminates coma, distortion, and chromatic aberration.

[0041] Preferably, a double convex lens and a plano-concave lens are glued together to form a double-cemented lens. Specifically, a double-cemented lens is an optical design in which two single lenses are bonded together by an adhesive. The two lenses are made of glasses with different refractive indices and dispersions, for example, one of them can be crown glass and the other can be flint glass. The refractive index and dispersion of the two lenses L3 and L4 can be specifically referred to Table 1. The image quality produced by such a combination is better than that of a single lens. The double-cemented lens is mainly used to reduce chromatic aberration, and can also reduce spherical aberration and other aberrations in the optical system. The dispersion effects of the two glasses will compensate (offset) each other to eliminate chromatic aberration, and the double-cemented lens can have the same focal length as the single lens.

[0042] In the above technical solution, the second lens group G2 uses a double-cemented lens, which has a simple structure and sufficient aberration correction capability, among which the correction performance of positional chromatic aberration makes it more suitable for imaging in the visible light band. And compared with a double-separated lens, or two single lenses fixed to each other by a maintaining structure, the two lenses will not have the problem of relative eccentricity and tilt when the lens is assembled. The lens group is used as a focusing lens group and needs to be moved in the lens barrel for focusing. The use of double-cemented lenses with relatively fixed positions can reduce assembly errors and improve the yield rate. Furthermore, reducing assembly errors can not only reduce aberrations, but also ensure the consistency of aberrations generated when the second lens group is in different positions, thereby reducing the difficulty of correcting aberrations. On the other hand, the weight of the second lens group G2 can also be reduced, thereby reducing inertia, making the movement speed faster and the response better when focusing.

[0043] Preferably, the first lens group and the second lens group have positive optical focal length, and the third lens group has negative optical focal length. In order to reduce the length of the lens barrel, the positive lens with convergence ability needs to bear a larger deflection angle. The first lens group and the second lens group can be combined to form a lens with positive optical focal length. Therefore, by reasonably setting the focal length of the first lens group and the second lens group, it is possible to avoid one of the first lens group or the second lens group from bearing too large a deflection angle, which makes subsequent aberration correction more difficult. The first lens group and the third lens group are fixed, while the second lens group moves. At the same time, the design of the first lens group and the second lens group having positive optical focal length and the third lens group having negative optical focal length can reduce the travel required for focusing.

[0044] As mentioned above, both the first and second lens groups have positive optical power, which is not conducive to correcting field curvature, astigmatism, and spherical aberration. Technical knowledge in this field indicates that positive optical power lenses produce negative spherical aberration. Therefore, a third lens group with negative optical power (i.e., divergent power) can be used to correct spherical aberration, field curvature, and astigmatism. Preferably, the first and third lens groups work together to correct spherical and chromatic aberration; the positive and negative lenses work together to correct coma, astigmatism, magnification, chromatic aberration, and field curvature.

[0045] Combining the conditional equations and table below, we can specifically deduce the relationship between the first and third lens groups. In other words, the ability of the first and third lens groups to correct spherical and chromatic aberrations in conjunction with each other is based on their symmetrical structures, closely related to the focal length relationship between them, and the front and back surface curvature radii and spacing of the individual lenses included in the first and third lens groups.

[0046] Preferably, the optical system of this embodiment satisfies the following conditional expression: 8≤f1 / f2≤14, where f1 represents the focal length of the first lens group, and f2 represents the focal length of the second lens group. By satisfying this conditional expression, the optical system can better correct aberrations, so that each lens group bears the responsibility of correcting aberrations in a balanced manner, while avoiding excessive lateral size of the lens group. When the value of this conditional expression is too low, below the lower limit, the object-side incident light will be refracted by the first lens group at an excessively large deflection angle, which is not conducive to aberration correction and will result in excessive sensitivity to optical system tolerances. When the value of this conditional expression is too high, above the upper limit, the aberration correction burden of the second and third lens groups will be increased, while the length of the optical system will be significantly increased, which is not conducive to system miniaturization and reducing the amount of focusing.

[0047] Preferably, the optical system satisfies the following conditional expression: 2.5≤|f3 / f12|≤5.5. f3 is the focal length of the third lens group, and f12 is the focal length of the first lens group and the second lens group combined. As described above, the first lens group and the second lens group are both lens groups with positive optical power, and are equivalent to lenses with positive optical power after combination. By reasonably setting the focal length of f3 and the focal length of f12, the length of the optical system can be shortened. Specifically, for example, when the value of the conditional expression is lower than the lower limit, the third lens group cannot effectively correct the residual aberration of the optical system; when the value of the conditional expression is higher than the upper limit, the lateral size of the third lens group will become larger, and the total length of the optical system will increase, which is not conducive to the miniaturization of the system.

[0048] Preferably, the optical system satisfies the following conditional formula:

[0049] 3.5≤|f2 / f3|≤5.5

[0050] f3 is the focal length of the third lens group, and f2 is the focal length of the second lens group. By satisfying this conditional expression, the optical system can effectively correct aberrations while preventing the lateral dimensions of the lens groups from becoming excessively large or increasing the overall length of the optical system. If the value of this conditional expression is below the lower limit, the third lens group will not be able to effectively correct the residual aberrations of the optical system. If the value of this conditional expression is above the upper limit, the lateral dimensions of the third lens group will increase, increasing the overall length of the optical system and hindering system miniaturization.

[0051] Preferably, if Figure 1 As shown, the optical system may further include an aperture stop ST disposed on the object side of the first lens group. The aperture stop ensures that the entrance pupil of the endoscope adapter is consistent with the exit pupil of the rigid scope, allowing the optical system to be more effectively integrated with other structures, such as endoscopes. For example, the aperture stop may limit the entrance pupil diameter to 4.8 mm. For example, the exit pupil angle 2w of the eyepiece in a conventional rigid scope may be 10-20 degrees.

[0052] exist Figure 1In the specific embodiment shown, r1-r11 represent the surfaces of each lens from the object side to the image side, respectively. Among them, r6 refers to both the image-side surface of lens L3 and the object-side surface of lens L4, with the two lens surfaces cemented together to form a single surface (ignoring the cementation thickness). r12 represents the image plane. As shown in the table below, lens L1 is a plano-convex lens, and lens L2 is a plano-concave lens; the two lenses form the first lens group G1. Lens L3 is a biconvex lens, and lens L4 is a plano-concave lens; the two lenses are cemented together to form a doublet, forming the second lens group G2. Lens L5 is a plano-concave lens, and lens L6 is a plano-convex lens; the two lenses form the third lens group G3.

[0053] Table 1

[0054]

[0055] Among them, the thickness parameter is not the lens thickness in a narrow sense. Take the thickness of surface r1 of 2.31mm as an example, that is, the distance r1-r2 is 2.31mm. Since the air gap is also considered an optical element in the optical system, in other words, the thickness shown in the table is equivalent to the distance from this surface to the next surface, regardless of whether there is air or an optical lens between the two surfaces. It can be concluded that D4 is the distance between the surface r4 of lens L2 close to the image side and the surface r5 of lens L3 close to the object side. Similarly, D7 is the distance between the surface r7 of lens L4 close to the image side and the surface r8 of lens L5 close to the object side. When the second lens group is away from the object side, D4 ​​increases and D7 decreases; when the second lens group is close to the object side, on the contrary, the sum of D4 and D7 is a constant. As mentioned above, when the object distance approaches from infinity to close distance, the second lens group as the focusing lens group gradually approaches the object side. Therefore, the specific focusing parameters are as follows:

[0056] Table 2

[0057] Object distance D4(mm) D7(mm) Infinity 5.62 2.21 200mm 0.72 7.11

[0058] When the object distance is infinite, the focal length of the optical system is 46.63mm; when the object distance is 200mm, the focal length is 42.48mm, a focal length change of 8.91%. As the object distance changes, the second lens group shifts to form a clear image on the image plane. While the focal length of the optical system changes during this change in object distance, the change is minimal. Consequently, the F-number of the optical system does not change significantly. Furthermore, by rationally configuring the parameters of the first, second, and third lens groups, internal focusing can be achieved over a focal length range of 20mm-50mm.

[0059] Through the above design, the length of the optical system can also be controlled within a smaller range. For example, the length of the lens barrel for mounting the lens can be 53.43 mm, which can be applied to, for example, the endoscope system mentioned below. The imaging height of the optical system is 6 mm, which can meet the large size requirements of ultra-high-definition image sensors (such as 4K level). On the one hand, the optical system can use only 6 lenses, and the structure is relatively simple. On the other hand, the lenses of the optical system of the embodiment of the present application all use spherical lenses, and measurement can be carried out by optical and tactile measurement. Compared with aspheric lenses, spherical lenses are less difficult to process and have lower costs. The accuracy of the measurement results is relatively high during measurement, which can meet the production needs of industrialization.

[0060] When the object distance is infinite, the position of the second lens group in the optical structure is as follows: Figure 2 As shown, it is located in the first position. At this time, the modulation transfer function (MTF) value of the optical system is as follows Figure 4 As shown, field curvature and distortion are Figure 5 When the object distance is 200mm, the position of the second lens group in the optical structure is as follows: Figure 3 As shown, it is located at the second position. At this time, the modulation transfer function (MTF) value of the optical system is as follows Figure 6 As shown, field curvature and distortion are Figure 7 As shown in the figure, the system distortion is less than 2%. For applications with short object distances and small field of view angles (such as endoscopes), this can achieve good results.

[0061] According to another aspect of the present application, an endoscope is provided, comprising an optical system and an imaging chip, wherein the imaging chip is arranged at the image plane of the optical system. The optical system is the above-mentioned optical system. By adopting the above-mentioned optical system, the lens can be moved to a minimum extent to achieve a continuous change of the focal length from 20mm to 50mm with a depth of field DOF of 200mm-infinity. Compared with the external focusing structure, the focusing amount is reduced to 1 / 3 of the original amount, and during the focusing process, the focal length change is less than 10%. Therefore, the endoscope of this embodiment can make up for the shortcomings of the existing technology under the premise of adopting a larger imaging chip.

[0062] The endoscope of this embodiment can greatly reduce the focusing amount while meeting the ultra-high resolution requirements of the market endoscopic camera system, shortening the doctor's operation time, thereby effectively improving the doctor's user experience and having high clinical application value.

[0063] In the description of the present invention, it should be understood that the directions or positional relationships indicated by directional words such as "front", "back", "up", "down", "left", "right", "horizontal", "vertical", "vertical", "horizontal", "top", "bottom", etc. are usually based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description. Unless otherwise specified, these directional words do not indicate or imply that the device or element referred to must have a specific direction or be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the scope of protection of the present invention; the directional words "inside" and "outside" refer to the inside and outside relative to the outline of each component itself.

[0064] For ease of description, area-relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the regional positional relationship between one or more components or features shown in the figures and other components or features. It should be understood that area-relative terms include not only the orientation of the components as described in the figures, but also different orientations in use or operation. For example, if the components in the drawings are inverted as a whole, the situation where the components are "above other components or features" or "above other components or features" will include the situation where the components are "below other components or structures" or "below other components or structures". Thus, the exemplary term "above" may include both the orientations "above" and "below". In addition, these components or features may also be positioned at other different angles (e.g., rotated 90 degrees or other angles), and this document is intended to include all of these situations.

[0065] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, parts, components and / or combinations thereof.

[0066] It should be noted that the terms "first," "second," and the like in the specification and claims of the present invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate, such that the embodiments of the present invention described herein can be implemented in an order other than that illustrated or described herein.

[0067] The present invention has been described through the above embodiments. However, it should be understood that the above embodiments are for illustrative and illustrative purposes only and are not intended to limit the present invention to the described embodiments. Furthermore, those skilled in the art will appreciate that the present invention is not limited to the above embodiments and that various variations and modifications may be made based on the teachings of the present invention, all of which fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. An optical system, which is applied to an endoscope, characterized in that: The lens comprises a first lens group, a second lens group and a third lens group arranged in sequence along the optical axis, wherein the first lens group is close to the object side and the third lens group is close to the image side; The positions of the first lens group and the third lens group along the optical axis are fixed, the first lens group and the third lens group include the same number of first single lenses and the same number of second single lenses, the first single lenses have positive refractive power, and the second single lenses have negative refractive power; The second lens group is movable along the optical axis between the first lens group and the third lens group to perform optical zoom.

2. The optical system according to claim 1, wherein: Each of the first lens group and the third lens group is composed of a first single lens and a second single lens.

3. The optical system according to claim 1, wherein: The first single lens is a plano-convex lens, and the second single lens is a plano-concave lens.

4. The optical system according to claim 3, wherein: In the first lens group, the first single lens is located on the object side of the second single lens, with the convex surface of the first single lens facing the object side and the concave surface of the second single lens facing the image side; In the third lens group, the first single lens is located on the image side of the second single lens, with the convex surface of the first single lens facing the image side and the concave surface of the second single lens facing the image side.

5. The optical system according to claim 1, wherein: The second lens group includes a biconvex lens and a plano-concave lens.

6. The optical system according to claim 5, characterized in that The biconvex lens and the plano-concave lens are cemented together to form a doublet lens.

7. The optical system according to claim 1, wherein: The optical system satisfies the following conditional formula: 8≤|f1 / f2|≤14 f1 is the focal length of the first lens group; f2 is the focal length of the second lens group.

8. The optical system according to claim 1, wherein: The movement range of the second lens group is set so that f12 satisfies the following conditional expression: 2.5≤|f3 / f12|≤5.5 f3 is the focal length of the third lens group; f12 is the focal length of the first lens group and the second lens group combined.

9. The optical system according to claim 1, wherein: The optical system satisfies the following conditional formula: 3.5≤|f2 / f3|≤5.5 f3 is the focal length of the third lens group; f2 is the focal length of the second lens group.

10. The optical system according to claim 1, wherein: The optical system further includes an aperture stop disposed on the object side of the first lens group.

11. The optical system according to any one of claims 1 to 10, characterized in that The first lens group has positive refractive power, the second lens group has positive refractive power, and the third lens group has negative refractive power.

12. An endoscope, characterized in that: include: The optical system according to any one of claims 1 to 11; as well as An image sensor is located on the image plane of the optical system.

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