Prime lens

Abstract

The utility model discloses a prime lens which comprises three lens groups. In the direction of the central axis, the first lens group with positive focal power sequentially comprises two meniscus positive lenses, a pair of meniscus doublet lenses and another pair of doublet lenses, and the second lens group with negative focal power comprises a pair of doublet lenses. And the third lens group with the positive focal power sequentially comprises a set of triplet lens, three pairs of doublet lenses and an aspheric negative lens. During focusing, only the second lens group moves along the central axis. The prime lens adapts to a large aperture, is small in focusing electric power load, and can be used for a full-frame camera.

Description

TECHNICAL FIELD

[0001] The utility model belongs to photographic lens technical field, specifically relates to the fixed focus lens for full frame camera. BACKGROUND

[0002] The electronic photosensitive device of full frame camera is equivalent to 35mm film of 135 camera in photosensitive area, and the pixel unit reaches micron level, so the lens is required to be higher. If the aperture of the existing lens is greater than F / 1.4, all the lenses are usually moved during focusing, if only one group of lenses is moved for focusing, the maximum aperture is usually F / 1.4, for example, the SEL85F1.4GM type lens of Japan Sony company.

[0003] The imaging lens disclosed in patent CN08474925A of Japan Sony company has less total number of lenses, only one group of lenses in the middle is moved during focusing, and there is one super low dispersion lens. INVENTION CONTENTS

[0004] The utility model solves the technical problem of how to improve the fixed focus lens to adapt to full frame camera.

[0005] The utility model discloses a fixed focus lens.

[0006] The fixed focus lens can be used for full frame camera, and sequentially includes a first lens group with positive refractive power, a second lens group with negative refractive power and a third lens group with positive refractive power along the central axis direction, and the fixed focus lens also includes an aperture stop, when focusing from infinity to a close distance object, the first lens group, the aperture stop and the third lens group are fixed relative to the image surface, and the second lens group moves along the central axis to the image surface.

[0007] The first lens group sequentially includes:

[0008] The first lens is configured as a positive lens, the object side surface is a spherical convex surface, and the image side surface is a spherical concave surface.

[0009] The second lens is configured as a positive lens, the object side surface is a spherical convex surface, the image side surface is a spherical concave surface, the refractive index is 1.55~1.65, and the Abbe number is 66~71.

[0010] The third lens is configured as a positive lens, the object side surface is a spherical convex surface, the image side surface is a spherical concave surface, the refractive index is 1.55~1.65, and the Abbe number is 66~71.

[0011] The fourth lens is configured as a negative lens, the object side surface is a spherical convex surface, the image side surface is a spherical concave surface, and the fourth lens is glued with the third lens.

[0012] The fifth lens is configured as a double-convex spherical positive lens, the refractive index is 1.55~1.65, and the Abbe number is 66~71.

[0013] The sixth lens is configured as a biconcave spherical negative lens with an image-side curvature radius of 500~600mm, and is cemented to the fifth lens;

[0014] The second lens group includes, along the central axis, the following components in sequence:

[0015] The seventh lens is configured as a positive lens, with an object-side surface that is concave spherical and has an absolute radius of curvature greater than 1000 mm, and an image-side surface that is convex spherical; and

[0016] The eighth lens is configured as a biconcave spherical negative lens and is cemented together with the seventh lens;

[0017] The third lens group includes, along the central axis, the following components in sequence:

[0018] The ninth lens is configured as a biconvex spherical positive lens with an object-side surface curvature radius of 480~580mm, a refractive index of 1.55~1.65, and an Abbe number of 66~71.

[0019] The tenth lens is configured as a biconcave spherical negative lens and is cemented together with the ninth lens;

[0020] The eleventh lens is configured as a biconvex spherical positive lens and is cemented together with the tenth lens;

[0021] The twelfth lens is configured as a biconvex spherical positive lens;

[0022] The thirteenth lens is configured as a biconcave spherical negative lens and is cemented together with the twelfth lens;

[0023] The fourteenth lens is configured as a biconvex spherical positive lens;

[0024] The fifteenth lens is configured as a biconcave spherical negative lens and is cemented together with the fourteenth lens;

[0025] The sixteenth lens is configured as a biconvex spherical positive lens;

[0026] The seventeenth lens, configured as a biconcave spherical negative lens, is cemented together with the sixteenth lens; and

[0027] The eighteenth lens is configured as a negative lens, with an object-side surface that is a non-spherical concave surface with a radius of curvature of 450~550mm, and an image-side surface that is a non-spherical convex surface.

[0028] In some embodiments of this invention, the focal length of the fixed-focus lens can be selected to be 80-90mm. Furthermore, the second lens, the third lens, the fifth lens, and the ninth lens are made of the same optical material.

[0029] The following beneficial effects can be obtained by implementing the technical solution of this utility model.

[0030] This utility model discloses a fixed-focus lens comprising three lens groups. Along the central axis, the first lens group, with positive optical power, sequentially includes two meniscus positive lenses, a meniscus cemented doublet, and another cemented doublet. The second lens group, with negative optical power, includes a cemented doublet. The third lens group, with positive optical power, sequentially includes a set of cemented triplet lenses, three cemented doublet lenses, and an aspherical negative lens. During focusing, only the second lens group moves along the central axis. This fixed-focus lens is suitable for large apertures, has low focusing power consumption, and can be used in full-frame cameras. Attached Figure Description

[0031] The accompanying figures should be used in conjunction with the detailed implementation section.

[0032] Figure 1 This is a cross-sectional view of the central axis of the fixed-focus lens in Embodiment 1. The captions represent: IMG - image plane, FIT - filter protector, STO - aperture stop, Z1 - central axis, and T1 - movement range.

[0033] Figure 2 This is the optical path diagram of a fixed-focus lens focusing at infinity in Example 1.

[0034] Figure 3a and Figure 3b The images shown are the spherical aberration curve and transverse chromatic aberration curve of the fixed-focus lens calculated by ZEMAX software in Example 1. The calculation conditions are: incident light mixing range 430~658nm, incident pupil radius 34.3090mm, aperture F / 1.25, and infinity focus state.

[0035] Figure 4a and Figure 4b The graphs shown are two modulation transfer function (MTF) curves of a fixed-focus lens calculated by ZEMAX software in Example 1, with the calculation conditions as described above.

[0036] Figure 5 and Figure 6 This image is taken from the official website of Sony's SEL85F1.4GM prime lens. Figure 5 This is a structural diagram. Figure 6 This is a graph of the modulation transfer function. Detailed Implementation

[0037] The embodiments are described below with reference to the accompanying drawings.

[0038] In this specification, unless otherwise specified, one embodiment, some embodiments and other embodiments are used to distinguish different embodiments and do not refer to all embodiments in general; the directions / positions indicated by top, bottom, center, edge, inner, outer, far, near, long, wide, vertical, horizontal, up, down, front, back, left, right, etc. are based on the observation angle of the accompanying drawings and should not be understood as the component / device being located in a specific position or facing a specific direction.

[0039] The lens grouping method described in this specification is for the convenience of illustrating the embodiments only. It is understood that other grouping methods may be used in optical design analysis, lens forming, and lens assembly.

[0040] Positive lens and positive optical power both refer to an optical system with a positive image-side focal length, which converges parallel incident light beams. Negative lens and negative optical power both refer to an optical system with a negative image-side focal length, which diverges parallel incident light beams. The light-transmitting surface of a lens can be simply referred to as the "surface." The side of the lens facing the object being photographed by the camera is called the "object-side surface," and the side of the lens facing the camera's image sensor is called the "image-side surface."

[0041] The aperture stop, also known as the "stop," is numbered along with the lens's light-transmitting surface.

[0042] Example 1

[0043] A fixed-focus lens is disclosed.

[0044] This fixed-focus lens is designed for autofocus full-frame cameras, with a focal length of 85mm, and is compatible with a maximum aperture of F / 1.2~1.3 and an image plane of 42~44mm.

[0045] Please see Figure 1 , Figure 1 This is a cross-sectional view of the central axis of the fixed-focus lens. Figure 1 The lens tube is not shown. The central axis Z1 (also known as the optical axis, principal axis, or principal optical axis) points from left to right towards the image plane IMG. A filter protector FIT is located in front of the image plane IMG.

[0046] Table 1, the aspherical expression f1, and Table 2, which lists the coefficients of the aspherical expression, show the lens data and positional relationship of this fixed-focus lens.

[0047]

[0048]

[0049]

[0050] In Table 1, the object-side surface of the first lens L1 is denoted as surface S1. Other light-transmitting surfaces (including the plane containing the aperture stop STO and the cementing surface of the cemented lens) are numbered in ascending order along the central axis Z1. The image-side surface of the eighteenth lens L18 is denoted as surface S29. "Radius of curvature R" represents the paraxial radius of curvature at the vertex of a surface. The i-th surface S... i and the (i+1)th surface S i+1For lenses between two adjacent lenses: their "thickness" is the central axis thickness, and their "refractive index Nd" and "Abbe number Nd" are measured values ​​on the d-line. The distance between the image-side surface of the first lens and the object-side surface of the second lens on the central axis Z1 is called the central axis distance between them.

[0051] This fixed-focus lens has 18 elements, all of which are glass lenses. They can be divided into three groups and numbered sequentially along the central axis. The lens barrel of this fixed-focus lens ( Figure 1 (Not shown) Along the central axis Z1, from the object side to the image side, the first lens group G1, the second lens group G2, the aperture stop STO, and the third lens group G3 are arranged in sequence.

[0052] During the focusing process of this fixed-focus lens, only the second lens group G2 can be moved back and forth along the central axis Z1 by a stepper motor. The first lens group G1, the aperture stop STO, and the third lens group G3 are all fixed in position relative to the image plane IMG. The central axis distance between the aperture stop STO and the surface S15 of the third lens group G3 is approximately 2.5 mm, and the central axis distance between the image plane IMG and the surface S29 of the third lens group G3 is approximately 20.3 mm.

[0053] The first lens group G1 has positive optical power and consists of six glass lenses, including two meniscus positive lenses, a meniscus cemented doublet, and another cemented doublet. These six glass lenses are arranged sequentially along the central axis Z1: lens L1, lens L2, lens L3, lens L4, lens L5, and lens L6. Lens L3 and lens L4 are cemented together to form a meniscus cemented doublet with a positive radius of curvature at the cemented surface. In the other cemented doublet, lens L5 and lens L6 are cemented together, the object-side surface is a convex spherical surface, and the image-side surface is a near-planar concave spherical surface with a negative radius of curvature at the cemented surface.

[0054] The second lens group G2 has negative optical power and consists of two glass lenses. These two glass lenses are the seventh lens L7 and the eighth lens L8, arranged sequentially along the central axis Z1. In a cemented doublet formed by cementing these two glass lenses, both the object-side and image-side surfaces are concave spherical surfaces, the object-side surface is nearly planar, and the radius of curvature of the cemented surface is negative.

[0055] The third lens group G3 has positive optical power and consists of ten glass lenses, including a set of cemented triplets, three sets of cemented doublets, and one aspherical negative lens. These ten glass lenses are arranged sequentially along the central axis Z1: lens L9, lens L10, lens L11, lens L12, lens L13, lens L14, lens L15, lens L16, lens L17, and lens L18. Lens L10 is a biconcave spherical negative lens, cemented together with lens L9 and lens L11 to form a set of cemented triplets. The object-side surface of this cemented triplet is a near-planar convex spherical surface, while the image-side surface is also convex spherical. The twelfth lens L12 is cemented with the thirteenth lens L13, the fourteenth lens L14 is cemented with the fifteenth object lens L15, and the sixteenth lens L16 is cemented with the seventeenth lens L17. In the three cemented doublet lenses formed in total, the object side is a convex spherical surface and the image side is a concave spherical surface. The radius of curvature of the cemented surfaces is negative.

[0056] All 18 glass lenses are arranged sequentially along the central axis Z1 to form the three lens groups mentioned above, and are installed inside the lens barrel. The lens barrel is equipped with a stepper motor and a power input interface.

[0057] Figure 1 The second lens group G2 is located at the object-side boundary of its movement range T1, approximately 21.5 mm away from the central axis of the aperture stop STO. At this point, the fixed-focus lens is in focus at infinity. Please refer to the optical path diagram. Figure 2 When this fixed-focus lens focuses on a close-up object, the second lens group G2 moves along the central axis to the image plane IMG, without exceeding the image-side boundary of its movement range T1. Because only the two cemented lenses move, the focusing electrical load of this fixed-focus lens is low, reducing camera power consumption.

[0058] The specific focal lengths of each glass lens, each set of cemented lenses, and the three lens groups of this fixed-focus lens can be calculated using the data in Tables 1 and 2. Details of each lens are as follows.

[0059] The first lens L1 is configured as a positive lens, with a spherical convex surface on the object side and a radius of curvature of +76.011 mm, a spherical concave surface on the image side and a radius of curvature of 215.365 mm, a central axis thickness of 10.300 mm, a refractive index of 1.85, and an Abbe number of 23.8.

[0060] The second lens L2 is configured as a positive lens, with a spherical convex surface on the object side and a radius of curvature of +61.477mm, a spherical concave surface on the image side and a radius of curvature of +78.556mm, a central axis thickness of 5.600mm, a refractive index of 1.59, an Abbe number of 68.3, and a central axis distance of 0.150mm from the first lens L1.

[0061] The third lens L3 is configured as a positive lens, with a spherical convex surface on the object side and a radius of curvature of +47.222mm, a spherical concave surface on the image side and a radius of curvature of +82.800mm, a central axis thickness of 8.200mm, a refractive index of 1.59, an Abbe number of 68.3, and a central axis distance of 0.150mm from the second lens L2.

[0062] The fourth lens, L4, is configured as a negative lens. Its object side is a spherical convex surface with a radius of curvature of +82.800mm, and its image side is a spherical concave surface with a radius of curvature of +35.941mm. Its central axis thickness is 1.500mm, its refractive index is 1.85, and its Abbe number is 25.2. It is cemented together with the third lens, L3.

[0063] The fifth lens, L5, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +53.336 mm, an image-side radius of curvature of -174.338 mm, a central axis thickness of 9.50 mm, a refractive index of 1.59, an Abbe number of 68.3, and a central axis distance of 3.946 mm from the fourth lens, L4.

[0064] The sixth lens, L6, is configured as a biconcave spherical negative lens with an object-side radius of curvature of -174.338 mm, an image-side radius of curvature of +568.332 mm, a central axis thickness of 1.500 mm, a refractive index of 2.00, and an Abbe number of 25.5. It is cemented together with the fifth lens, L5.

[0065] The seventh lens L7 is configured as a positive lens, with a concave spherical object side with a radius of curvature of -1204.786 mm, a convex spherical image side with a radius of curvature of -163.974 mm, a central axis thickness of 3.400 mm, a refractive index of 1.92, and an Abbe number of 20.9. When focusing at infinity, its central axis distance from that of the sixth lens L6 is 2.138 mm.

[0066] The eighth lens, L8, is configured as a biconcave spherical negative lens with an object-side radius of curvature of -163.974 mm, an image-side radius of curvature of +46.971 mm, a central axis thickness of 1.00 mm, a refractive index of 1.70, and an Abbe number of 55.5. It is cemented with the seventh lens, L7, and when focusing at infinity, the distance between its central axis and the aperture stop STO is 21.473 mm.

[0067] The ninth lens, L9, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +533.957 mm, an image-side radius of curvature of -35.626 mm, a central axis thickness of 7.500 mm, a refractive index of 1.59, and an Abbe number of 68.3. The central axis distance between it and the aperture stop STO is 2.489 mm.

[0068] The tenth lens, L10, is configured as a biconcave spherical negative lens with an object-side radius of curvature of -35.626 mm, an image-side radius of curvature of +35.626 mm, a central axis thickness of 2.000 mm, a refractive index of 1.77, and an Abbe number of 29.7. It is cemented to the ninth lens, L9.

[0069] The eleventh lens, L11, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +35.626mm, an image-side radius of curvature of -135.604mm, a central axis thickness of 8.600mm, a refractive index of 1.73, and an Abbe number of 54.7. It is cemented together with the tenth lens, L10.

[0070] The twelfth lens, L12, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +46.752mm, an image-side radius of curvature of -181.209mm, a central axis thickness of 6.500mm, a refractive index of 1.73, an Abbe number of 54.7, and a central axis distance of 0.150mm from the eleventh lens, L11.

[0071] The thirteenth lens, L13, is configured as a biconcave spherical negative lens with an object-side radius of curvature of -181.209 mm, an image-side radius of curvature of +50.080 mm, a central axis thickness of 1.500 mm, a refractive index of 1.77, and an Abbe number of 29.7. It is cemented together with the twelfth lens, L12.

[0072] The fourteenth lens, L14, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +62.431 mm, an image-side radius of curvature of -71.562 mm, a central axis thickness of 6.700 mm, a refractive index of 2.00, an Abbe number of 29.1, and a central axis distance of 0.893 mm from that of the thirteenth lens, L11.

[0073] The fifteenth lens, L15, is configured as a biconcave spherical negative lens with an object-side radius of curvature of -71.562 mm, an image-side radius of curvature of +46.084 mm, a central axis thickness of 1.500 mm, a refractive index of 1.62, and an Abbe number of 36.3. It is cemented together with the fourteenth lens, L14.

[0074] The sixteenth lens, L16, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +56.199mm, an image-side radius of curvature of -97.979mm, a central axis thickness of 6.400mm, a refractive index of 2.00, an Abbe number of 25.5, and a central axis distance of 0.903mm from the fifteenth lens, L15.

[0075] The seventeenth lens, L17, is configured as a biconcave spherical negative lens with an object-side radius of curvature of -97.979 mm, an image-side radius of curvature of +43.506 mm, a central axis thickness of 4.000 mm, a refractive index of 1.49, and an Abbe number of 70.4. It is cemented together with the sixteenth lens, L16.

[0076] The eighteenth lens, L18, is configured as a negative lens. Its object side is an aspherical concave surface with a radius of curvature of -119.605 mm, and its image side is an aspherical convex surface with a radius of curvature of -500.00 mm. Its central axis thickness is 2.500 mm, its refractive index is 1.81, its Abbe number is 40.7, its central axis distance from the seventeenth lens, L17 is 5.208 mm, and its central axis distance from the image plane, IMG, is approximately 20.3 mm.

[0077] The second lens L2, the third lens L3, the fifth lens L5, and the ninth lens L9 of this fixed-focus lens have the same refractive index and Abbe number, and use the same grade of optical glass.

[0078] In Table 1, the specific shapes of the two aspherical surfaces S28 and S29 are characterized by the aspherical expression f1 and the coefficients in Table 2. In the aspherical expression f1, Y is the radial coordinate, i.e., the distance from the central axis in the direction perpendicular to the central axis; Z(Y) is the axial coordinate, i.e., the distance from the vertex of the lens surface in the direction parallel to the central axis; R is the paraxial radius of curvature at the vertex of the lens surface; K is the conic constant; and Ai is the i-th order aspherical coefficient.

[0079] For the spherical aberration curves along the perpendicular axis (i.e., radial and Y-axis directions) of this fixed-focus lens under the following conditions: incident light mixing range of 430~658nm, incident pupil radius of 34.309mm, aperture of F / 1.25, and infinity focus. Figure 3a Please refer to the calculated vertical color difference curve. Figure 3b . Figure 3a The spherical aberration of all curves is in the range of -0.03 to 0.03 mm. Figure 3b The color difference of all curves is in the range of -2.5 to 5 micrometers.

[0080] Please see Figure 4a and Figure 4b The diagram shows the modulation transfer function (MTF) curve of the fixed-focus lens calculated under the above conditions. Figure 4a It can be seen that the MTF values ​​of each curve decrease relatively uniformly within the frequency range of 0~30 lp / mm. Many of the upper curves (curve a, curve b, ... and curve k) are distributed within a relatively narrow range of the vertical axis, with only the 21 mm-sagittal curve (curve p) eventually dropping below 0.4. (Comparison) Figure 4b and Figure 6 It can be seen that the image quality of this prime lens at an aperture of F / 1.25 is close to that of the Sony SEL85F1.4GM prime lens with a maximum aperture of F / 1.4. Therefore, this prime lens can be used with full-frame cameras, and its maximum aperture range is F / 1.2 to F / 1.3.

[0081] In this fixed-focus lens, the first lens group consists of two meniscus positive lenses, a meniscus cemented doublet, and another cemented doublet. The second lens group is a cemented doublet. The third lens group consists of a set of triplet lenses, three cemented doublet lenses, and an aspherical negative lens. During focusing, only the second lens group moves along the central axis. This fixed-focus lens exhibits good resolution and low optical distortion at high light transmission (aperture f / 1.2~1.3), improving the vignetting problem of large-aperture lenses. Furthermore, it has a compact structure and low focusing electrical load.

[0082] The seven sets of cemented lenses in this fixed-focus lens have significant differences in dispersion coefficients between the positive and negative lenses. This allows for the correction of axial chromatic aberration, spherical aberration, and distortion aberrations, while also reducing the tolerance sensitivity of image quality and facilitating manufacturing and assembly. By adding cemented lenses, the axial length, the number of aspherical lenses, and the number of ultra-low dispersion lenses can be controlled.

[0083] As a replaceable component of a full-frame camera, for other aspects of the structure and function of this fixed-focus lens, please refer to paragraphs 0110 to 0122 on pages 10-11 of the specification of patent CN10847825A, or refer to commercially available camera fixed-focus lenses.

[0084] In other embodiments, the lenses described above can be made of other types of lens materials, the stepper motor can be replaced by a voice coil motor or an ultrasonic motor, and the total focal length of the fixed-focus lens can be any value within the range of 80-90mm. Based on the lens data of Embodiment 1, the surface shape, central axis thickness, and central axis spacing of each lens can be adaptively adjusted to achieve clear imaging and other technical requirements of the camera. Specifically:

[0085] The first lens L1 has a refractive index of 1.80~1.90 and an Abbe number of 21~26;

[0086] The second lens L2, the third lens L3, the fifth lens L5, and the ninth lens L9 have refractive indices of 1.55 to 1.65 and Abbe numbers of 66 to 71, respectively, and can be made of the same optical material.

[0087] The fourth lens, L4, has a refractive index of 1.80 to 1.90 and an Abbe number of 22.5 to 27.5.

[0088] The sixth lens, L6, has a side curvature radius of 500-600 mm, a refractive index of 1.95-20.5, and an Abbe number of 23-28.

[0089] The seventh lens, L7, has an object-side surface curvature radius of 480~580mm, a refractive index of 1.87~1.97, and an Abbe number of 18~23.

[0090] The eighth lens, L8, has a refractive index of 1.65 to 1.75 and an Abbe number of 53 to 58.

[0091] The tenth lens L10 and the thirteenth lens L13 have a refractive index of 1.72~1.82 and an Abbe number of 27~32, and can be made of the same optical material;

[0092] The eleventh lens L11 and the twelfth lens L12 have refractive indices of 1.65~1.75 and Abbe numbers of 50~60, respectively, and can be made of the same optical material;

[0093] The fourteenth lens, L14, has a refractive index of 1.95 to 2.05 and an Abbe number of 26.5 to 31.5.

[0094] The fifteenth lens, L15, has a refractive index of 1.57~1.67 and an Abbe number of 33.5~38.5.

[0095] The sixteenth lens, L16, has a refractive index of 1.95 to 2.05 and an Abbe number of 23 to 28.

[0096] The seventeenth lens, L17, has a refractive index of 1.45 to 1.55 and an Abbe number of 68 to 73.

[0097] The eighteenth lens, L18, has a refractive index of 1.75 to 1.85 and an Abbe number of 38 to 43.

[0098] Furthermore, the image-side radius of curvature of the sixth lens L6 is 500-600 mm, the object-side radius of curvature of the seventh lens L7 is greater than 1000 mm, the object-side radius of curvature of the ninth lens L9 is 480-580 mm, and the object-side radius of curvature of the eighteenth lens L18 is 450-550 mm. That is, each of these four lenses has one image-side radius of curvature (S in Table 1). 10 S 11 S 15 and S 29 The lens itself is approximately a plane.

[0099] All the embodiments, application examples, and technical analyses described above are intended to introduce the technical concept and features of this utility model, enabling those skilled in the art to implement the technical solution of this utility model, and do not constitute any limitation on the scope of protection of this utility model. Simple modifications and equivalent transformations to the above embodiments are all within the scope of protection of the claims of this utility model.

Claims

1. A fixed-focus lens for use in a full-frame camera, comprising, in sequence along a central axis, a first lens group with positive optical power, a second lens group with negative optical power, and a third lens group with positive optical power, and further comprising an aperture stop, wherein when focusing from infinity to a nearby object, the first lens group, the aperture stop, and the third lens group are fixed relative to the image plane, and the second lens group moves along the central axis to the image plane. Its features are, The first lens group includes, sequentially along the central axis: The first lens is configured as a positive lens, with a spherical convex surface on the object side and a spherical concave surface on the image side; The second lens is configured as a positive lens, with a spherical convex surface on the object side and a spherical concave surface on the image side, a refractive index of 1.55~1.65, and an Abbe number of 66~71; The third lens is configured as a positive lens, with a spherical convex surface on the object side and a spherical concave surface on the image side, a refractive index of 1.55~1.65, and an Abbe number of 66~71; The fourth lens is configured as a negative lens, with a convex spherical surface on the object side and a concave spherical surface on the image side, and is cemented together with the third lens; The fifth lens is configured as a biconvex spherical positive lens with a refractive index of 1.55~1.65 and an Abbe number of 66~71; and The sixth lens is configured as a biconcave spherical negative lens with an image-side curvature radius of 500~600mm, and is cemented to the fifth lens; The second lens group includes, along the central axis, the following components in sequence: The seventh lens is configured as a positive lens, with an object-side surface that is concave spherical and has an absolute radius of curvature greater than 1000 mm, and an image-side surface that is convex spherical; and The eighth lens is configured as a biconcave spherical negative lens and is cemented together with the seventh lens; The third lens group includes, along the central axis, the following components in sequence: The ninth lens is configured as a biconvex spherical positive lens with an object-side surface curvature radius of 480~580mm, a refractive index of 1.55~1.65, and an Abbe number of 66~71. The tenth lens is configured as a biconcave spherical negative lens and is cemented together with the ninth lens; The eleventh lens is configured as a biconvex spherical positive lens and is cemented together with the tenth lens; The twelfth lens is configured as a biconvex spherical positive lens; The thirteenth lens is configured as a biconcave spherical negative lens and is cemented together with the twelfth lens; The fourteenth lens is configured as a biconvex spherical positive lens; The fifteenth lens is configured as a biconcave spherical negative lens and is cemented together with the fourteenth lens; The sixteenth lens is configured as a biconvex spherical positive lens; The seventeenth lens, configured as a biconcave spherical negative lens, is cemented together with the sixteenth lens; and The eighteenth lens is configured as a negative lens, with an object-side surface that is a non-spherical concave surface with a radius of curvature of 450~550mm, and an image-side surface that is a non-spherical convex surface.

2. The fixed-focus lens according to claim 1, characterized in that, The focal length is 80~90mm.

3. The fixed-focus lens according to claim 2, characterized in that, The second lens, the third lens, the fifth lens, and the ninth lens are made of the same optical material.

4. The fixed-focus lens according to claim 3, characterized in that, The first lens has an object-side radius of curvature of +76.011 mm, an image-side radius of curvature of +215.365 mm, a central axis thickness of 10.300 mm, a refractive index of 1.85, and an Abbe number of 23.

8. The second lens has an object-side radius of curvature of +61.477 mm, an image-side radius of curvature of +78.556 mm, a central axis thickness of 5.600 mm, a refractive index of 1.59, and an Abbe number of 68.

3. The third lens has an object-side radius of curvature of +47.222 mm, an image-side radius of curvature of +82.800 mm, a central axis thickness of 8.200 mm, a refractive index of 1.59, and an Abbe number of 68.

3. The fourth lens has an object-side radius of curvature of +82.800 mm, an image-side radius of curvature of +35.941 mm, a central axis thickness of 1.500 mm, a refractive index of 1.85, and an Abbe number of 25.

2. The fifth lens has an object-side radius of curvature of +53.336 mm, an image-side radius of curvature of -174.338 mm, a central axis thickness of 9.50 mm, a refractive index of 1.59, and an Abbe number of 68.

3. The sixth lens has an object-side radius of curvature of -174.338 mm, an image-side radius of curvature of +568.332 mm, a central axis thickness of 1.500 mm, a refractive index of 2.00, and an Abbe number of 25.

5.

5. The fixed-focus lens according to claim 4, characterized in that, The ninth lens has an object-side radius of curvature of +533.957 mm, an image-side radius of curvature of -35.626 mm, a central axis thickness of 7.500 mm, a refractive index of 1.59, and an Abbe number of 68.

3. The tenth lens has an object-side radius of curvature of -35.626 mm, an image-side radius of curvature of +35.626 mm, a central axis thickness of 2.000 mm, a refractive index of 1.77, and an Abbe number of 29.

7. The eleventh lens has an object-side radius of curvature of +35.626 mm, an image-side radius of curvature of -135.604 mm, a central axis thickness of 8.600 mm, a refractive index of 1.73, and an Abbe number of 54.

7. The twelfth lens has an object-side radius of curvature of +46.752 mm, an image-side radius of curvature of -181.209 mm, a central axis thickness of 6.500 mm, a refractive index of 1.73, and an Abbe number of 54.

7. The thirteenth lens has an object-side radius of curvature of -181.209 mm, an image-side radius of curvature of +50.080 mm, a central axis thickness of 1.500 mm, a refractive index of 1.77, and an Abbe number of 29.

7. The fourteenth lens has an object-side radius of curvature of +62.431 mm, an image-side radius of curvature of -71.562 mm, a central axis thickness of 6.700 mm, a refractive index of 2.00, and an Abbe number of 29.

1. The fifteenth lens has an object-side radius of curvature of -71.562 mm, an image-side radius of curvature of +46.084 mm, a central axis thickness of 1.500 mm, a refractive index of 1.62, and an Abbe number of 36.

3. The sixteenth lens has an object-side radius of curvature of +56.199 mm, an image-side radius of curvature of -97.979 mm, a central axis thickness of 6.400 mm, a refractive index of 2.00, and an Abbe number of 25.

5. The seventeenth lens has an object-side radius of curvature of -97.979 mm, an image-side radius of curvature of +43.506 mm, a central axis thickness of 4.000 mm, a refractive index of 1.49, and an Abbe number of 70.

4. The eighteenth lens has an object-side radius of curvature of -119.605 mm, an image-side radius of curvature of -500.00 mm, a central axis thickness of 2.50 mm, a refractive index of 1.81, and an Abbe number of 40.

7.

6. The fixed-focus lens according to claim 5, characterized in that, The seventh lens has an object-side radius of curvature of -1204.786 mm, an image-side radius of curvature of -163.974 mm, a central axis thickness of 3.400 mm, a refractive index of 1.92, and an Abbe number of 20.

9. The eighth lens has an object-side radius of curvature of -163.974 mm, an image-side radius of curvature of +46.971 mm, a central axis thickness of 1.00 mm, a refractive index of 1.70, and an Abbe number of 55.

5.

7. The fixed-focus lens according to claim 6, characterized in that, The aperture stop is positioned between the second lens group and the third lens group.

Images