Prime lens

Abstract

The prime lens comprises a first lens, a second lens and a third lens which are sequentially arranged from an object plane to an image plane along an optical axis, the first lens is a negative focal power lens, the second lens is a positive focal power lens, and the third lens is a negative focal power lens; the object side surface of the first lens is a convex surface, and the image side surface is a concave surface; the object side surface of the second lens is a convex surface, and the image side surface is a convex surface; the object side surface of the third lens is a concave surface, and the image side surface is a concave surface. According to the technical scheme, the prime lens comprises the three lenses, so that the design of the prime lens with low cost, small size and light weight can be conveniently realized; and through reasonable distribution of the focal power and the surface type of each lens, the design of the prime lens capable of giving consideration to both a larger field of view and smaller low distortion is realized, and the distortion is less than or equal to 8% while the optimal object distance MTF (Modulation Transfer Function) of 140mm is greater than 0.15 at 200lp.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of optical devices, and in particular, to a fixed-focus lens. Background Art

[0002] With the development and increasing maturity of automatic identification technology, identification codes have gradually entered people's lives. Functions such as payment and information recognition in daily life are closely related to identification codes. The advantages of identification code technology, such as fast input speed, high reliability, flexible practicality, low cost and easy production, make it be used in more and more scenarios. Therefore, the research and development of a scanning lens that can be applied to most scanning devices and has a large depth of field and high resolution is more urgent.

[0003] However, in order to reduce aberration and distortion, the existing scanning lenses increase the number of lenses, resulting in common defects of different types or degrees, such as long overall length, large volume, too high cost, and narrow field of view. Therefore, the research and development of a scanning lens with small volume, low cost and low distortion is even more urgent. Summary of the Invention

[0004] The present invention provides a fixed-focus lens, realizing the design of a fixed-focus lens that can take into account small volume, low cost and low distortion.

[0005] The embodiments of the present invention provide a fixed-focus lens, including a first lens, a second lens and a third lens arranged in sequence along the optical axis from the object plane to the image plane;

[0006] The first lens is a negative-power lens, the second lens is a positive-power lens, and the third lens is a negative-power lens;

[0007] The first lens includes a first object side close to the object plane and a first image side close to the image plane. The first object side is a convex surface, and the first image side is a concave surface;

[0008] The second lens includes a second object side close to the object plane and a second image side close to the image plane. The second object side is a convex surface, and the second image side is a convex surface;

[0009] The third lens includes a third object side close to the object plane and a third image side close to the image plane. The third object side is a concave surface, and the third image side is a concave surface.

[0010] Optionally, -1.17 ≤ Φ1 / Φ ≤ -1.10, -0.88 ≤ Φ3 / Φ ≤ -0.78;

[0011] Wherein, Φ1 represents the optical power of the first lens, Φ3 represents the optical power of the third lens, and Φ represents the overall optical power of the fixed-focus lens.

[0012] Optionally, 1.85 ≤ f / h ≤ 2.05;

[0013] where f represents the effective focal length of the fixed-focus lens, and h represents the image-side semi-image height of the fixed-focus lens.

[0014] Optionally, 53° ≤ DFOV ≤ 59°;

[0015] where DFOV represents half of the maximum field of view angle of the fixed-focus lens.

[0016] Optionally, the fixed-focus lens further includes a diaphragm, and the diaphragm is disposed in the optical path between the first lens and the second lens;

[0017] wherein, the aperture NA of the diaphragm satisfies 0.9 < NA < 1.05.

[0018] Optionally, 1.3 ≤ CT2 / ET2 ≤ 2.2;

[0019] where CT2 represents the thickest thickness of the second lens, and ET2 represents the thinnest thickness of the second lens.

[0020] Optionally, 0.08 ≤ (CT1 + CT3) / TTL ≤ 0.140;

[0021] where CT1 represents the central thickness of the first lens; CT3 represents the central thickness of the third lens, and TTL represents the overall optical length of the fixed-focus lens.

[0022] Optionally, MAX(D1, D2, D3) ≤ 2.600;

[0023] where D1 represents the maximum aperture of the first lens, D2 represents the maximum aperture of the second lens, and D3 represents the maximum aperture of the third lens.

[0024] Optionally, 0.400 ≤ f / TTL ≤ 0.460;

[0025] where f represents the effective focal length of the fixed-focus lens, and TTL represents the overall optical length of the fixed-focus lens.

[0026] Optionally, the first lens, the second lens, and the third lens are all plastic aspherical lenses.

[0027] The fixed-focus lens provided by the embodiment of the present invention realizes the design of a fixed-focus lens with miniaturization, low cost, and light weight by arranging the fixed-focus lens to include three lenses; and by setting the optical powers of the first lens to the third lens to be negative optical power, positive optical power, and negative optical power respectively, and at the same time setting the first lens to be a convex-concave lens, the second lens to be a biconvex lens, and the third lens to be a biconcave lens, through reasonable distribution of the optical powers and surface type structures of the lenses, the design of a fixed-focus lens that can take into account a larger field of view and smaller low distortion is realized, the imaging quality of the optical system is improved, and a good imaging effect of the fixed-focus lens is ensured.

[0028] It should be understood that the content described in this part is not intended to identify the key or important features of the embodiments of the present invention, nor is it used to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0030] Figure 1 is a schematic structural diagram of an optimal object distance lens provided by Embodiment 1 of the present invention;

[0031] Figure 2 is a schematic diagram of the light fan of an optimal object distance lens provided by Embodiment 1 of the present invention;

[0032] Figure 3 is a schematic diagram of the field curvature and distortion curve of an optimal object distance lens provided by Embodiment 1 of the present invention;

[0033] Figure 4 is a schematic MTF diagram of an optimal object distance lens provided by Embodiment 1 of the present invention;

[0034] Figure 5 is a schematic structural diagram of an optimal object distance lens provided by Embodiment 2 of the present invention;

[0035] Figure 6 is a schematic diagram of the light fan of an optimal object distance lens provided by Embodiment 2 of the present invention;

[0036] Figure 7 is a schematic diagram of the field curvature and distortion curve of an optimal object distance lens provided by Embodiment 2 of the present invention;

[0037] Figure 8 is a schematic MTF diagram of an optimal object distance lens provided by Embodiment 2 of the present invention;

[0038] Figure 9 It is a schematic structural diagram of an optimal object distance lens provided in Embodiment 3 of the present invention;

[0039] Figure 10 It is a schematic diagram of the light fan of an optimal object distance lens provided in Embodiment 3 of the present invention;

[0040] Figure 11 It is a schematic diagram of the field curvature distortion curve of an optimal object distance lens provided in Embodiment 3 of the present invention;

[0041] Figure 12 It is an MTF schematic diagram of an optimal object distance lens provided in Embodiment 3 of the present invention. Detailed implementation manners

[0042] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0043] Embodiment 1

[0044] Figure 1 It is a schematic structural diagram of an optimal object distance lens provided in Embodiment 1 of the present invention. As Figure 1 shown, the fixed-focus lens provided in Embodiment 1 of the present invention includes a first lens 110, a second lens 120, and a third lens 130 arranged in sequence along the optical axis from the object plane to the image plane. Among them, the first lens 110 is a negative-power lens, the second lens 120 is a positive-power lens, and the third lens 130 is a negative-power lens; the first lens 110 includes a first object side face close to the object plane and a first image side face close to the image plane. The first object side face is a convex face, and the first image side face is a concave face; the second lens 120 includes a second object side face close to the object plane and a second image side face close to the image plane. The second object side face is a convex face, and the second image side face is a convex face; the third lens 130 includes a third object side face close to the object plane and a third image side face close to the image plane. The third object side face is a concave face, and the third image side face is a concave face.

[0045] As Figure 1 shown, the fixed-focus lens includes a first lens 110, a second lens 120, and a third lens 130 arranged in sequence along the optical axis from the object plane to the image plane. The design of the three lenses of the fixed-focus lens ensures that the fixed-focus lens has a simple structure and is easy to implement a fixed-focus lens design with low cost, small volume, and light weight.

[0046] Furthermore, the focal length is equal to the difference between the convergence of the image-side light beam and the convergence of the object-side light beam, and it characterizes the ability of the optical system to deflect light. The larger the absolute value of the focal length, the stronger the ability to bend light, and the smaller the absolute value of the focal length, the weaker the ability to bend light. When the focal length is a positive number, the refraction of light is convergent; when the focal length is a negative number, the refraction of light is divergent. The focal length can be used to characterize a certain refractive surface of a lens (i.e., a surface of a lens), can be used to characterize a certain lens, and can also be used to characterize a system formed by multiple lenses (i.e., a lens group). The first lens 110 is a negative focal length lens, which can effectively deflect light incident at a large angle, ensuring that more light enters the optical system, thereby effectively increasing the field of view of the fixed-focus lens, and ensuring that the optical system has wide-angle characteristics. The second lens 120 is a positive power lens, so the second lens 120 can timely correct the larger aberrations generated by the first lens 110, especially can have a significant correction effect on the edge aberrations of the fixed focus lens, thereby improving the imaging resolution of the optical system.

[0047] Further, the first object side surface of the first lens 110 is a convex surface, and the first image side surface is a concave surface, that is, the object side surface of the first lens 110 is convex toward the object plane, and the image side surface is concave toward the image plane, that is, the first lens 110 is a lens with a convex-concave structure. And the first lens 110 can be a meniscus lens, which is conducive to the collection of light to ensure a larger field of view angle range of the fixed focus lens and achieve the characteristics of wide angle. The second object side surface of the second lens 120 is a convex surface, and the second image side surface is a convex surface, that is, the object side surface of the second lens 120 is convex toward the object plane, and the image side surface is convex toward the image plane, that is, the second lens 120 is a lens with a double convex structure. The third object side surface of the third lens 130 is a concave surface, and the third image side surface is a concave surface, that is, the object side surface of the third lens 130 is concave toward the object plane, and the image side surface is concave toward the image plane, that is, the third lens 130 is a lens with a double concave structure. The concave surface design of the image side surface of the third lens 130 can ensure good consistency between the central field of view and the edge field of view, and the edge field of view has less distortion than the central field of view, thereby ensuring the imaging effect of the fixed-focus lens.

[0048] In summary, the technical solution provided by the embodiment of the present invention facilitates the design of a low-cost, small-sized and light-weight fixed-focus lens by setting a fixed-focus lens to include three lenses; and by reasonably allocating the optical focal length and surface shape of each lens, a fixed-focus lens design that can take into account both a larger field of view and lower distortion is achieved.

[0049] Based on the above embodiment, the first lens 110 , the second lens 120 and the third lens 130 are all plastic aspherical lenses.

[0050] The characteristics of an aspherical lens are that the curvature changes continuously from the center of the lens to the periphery of the lens. Different from a spherical lens with a constant curvature from the center of the lens to the periphery of the lens, an aspherical lens has better curvature radius characteristics and has the advantages of improving distortion aberration and astigmatism aberration. After using an aspherical lens, it is possible to eliminate the aberration that appears during imaging as much as possible, thereby improving the imaging quality of the lens. For example, the first lens 110, the second lens 120, and the third lens 130 all use aspherical lenses to correct the off-axis point aberration of the system, optimize optical performances such as distortion and CRA, and improve the imaging quality. On this basis, the aspherical lens can be a plastic aspherical lens, which is beneficial to reducing the processing technology of the aspherical lens, and the cost of the aspherical lens is relatively low.

[0051] On the basis of the above embodiments, -1.17 ≤ Φ1 / Φ ≤ -1.10, -0.88 ≤ Φ3 / Φ ≤ -0.78; where Φ1 represents the optical power of the first lens 110, Φ3 represents the optical power of the third lens 130, and Φ represents the overall optical power of the fixed-focus lens. Limiting the optical powers of different lenses within this range can balance the lens distortion and improve the imaging quality. And by reasonably setting the optical powers of the first lens 110 and the third lens 130, it is possible to avoid the problem that the optical powers of the first lens 110 and the third lens 130 are too large or too small, ensuring that the convexity or concavity of the surface shapes of the first lens 110 and the third lens 113 is not too large or too small, ensuring the processing difficulty of the first lens 110 and the third lens 130, that is, ensuring the good processability of the fixed-focus lens.

[0052] On the basis of the above embodiments, 1.85 ≤ f / h ≤ 2.05; where f represents the effective focal length of the fixed-focus lens, and h represents the image-side semi-image height of the fixed-focus lens. That is, the effective focal length of the lens is directly proportional to the image-side semi-image height, so that the image-side semi-image height can be effectively controlled, thereby obtaining a reasonable field of view angle.

[0053] On the basis of the above embodiments, 53° ≤ DFOV ≤ 59°; where DFOV represents half of the maximum field of view angle of the fixed-focus lens. Since the larger the field of view angle, the higher the height of the light entering the fixed-focus lens and the larger the off-axis aberration, restricting 53° ≤ DFOV ≤ 59° can control the height of the light entering the fixed-focus lens, restrict the off-axis aberration, and at the same time meet the requirements of the target surface.

[0054] On the basis of the above embodiments, as Figure 1 shown, the fixed-focus lens further includes a diaphragm 140, and the diaphragm 140 is arranged in the optical path between the first lens 110 and the second lens 120; where the aperture NA of the diaphragm 140 satisfies 0.9 < NA < 1.05.

[0055] Specifically, by setting the aperture stop 140, the propagation direction of the light beam can be adjusted, which is beneficial to improving the imaging quality. Moreover, in this fixed-focus lens, the aperture stop 140 can be located on the optical path between the first lens 110 and the second lens 120. Placing the aperture stop 140 in the middle of the fixed-focus lens can minimize the front and rear apertures of the fixed-focus lens. Additionally, the position of the aperture stop and the size of the light passing hole are directly related to the brightness, clarity, and the magnitude of certain aberrations of the image formed by the optical system. The smaller the light passing hole of the aperture stop, the smaller the spherical aberration, the clearer the image, and the larger the depth of field, but the weaker the brightness of the image. The larger the light passing hole, the stronger the brightness of the image, but the larger the spherical aberration, the worse the clarity of the image, and the smaller the depth of field. In the embodiment of the present invention, by reasonably setting the aperture size NA of the aperture stop 140 to satisfy 0.9 < NA < 1.05, a larger depth of field can be obtained within the required working range, and the brightness will not be too weak to make it impossible to clearly see the decoding object, thus improving the imaging effect of the fixed-focus lens.

[0056] Moreover, as Figure 1 shown, the fixed-focus lens provided by the embodiment of the present invention may further include a filter 150. The filter 150 is disposed on the image side of the third lens 130 and can filter out stray spectra to ensure the imaging quality.

[0057] Based on the above embodiment, 1.3 ≤ CT2 / ET2 ≤ 2.2; where CT2 represents the maximum thickness of the second lens, and ET2 represents the minimum thickness of the second lens. Since only the second lens 120 is a positive focal length lens in the fixed-focus lens provided by the embodiment of the present invention, the thickness ratio will be relatively large due to the front and rear negative focal length lenses. By effectively controlling the thickness ratio of the second lens 120, the lens life can be extended while reducing the cost.

[0058] Based on the above embodiment, 0.08 ≤ (CT1 + CT3) / TTL ≤ 0.140; where CT1 represents the central thickness of the first lens; CT3 represents the central thickness of the third lens, and TTL represents the optical total length of the fixed-focus lens. In this way, the size of the lens can be reduced, and further the size of the entire fixed-focus lens can be reduced to achieve a miniaturized and thin and light optical system.

[0059] Based on the above embodiment, MAX(D1, D2, D3) ≤ 2.600; where D1 represents the maximum aperture of the first lens, D2 represents the maximum aperture of the second lens, and D3 represents the maximum aperture of the third lens. Among them, MAX(D1, D2, D3) ≤ 2.600 represents the largest one among the maximum aperture of the first lens, the maximum aperture of the second lens, and the maximum aperture of the third lens. Limiting MAX(D1, D2, D3) ≤ 2.600 is beneficial to reducing the total length of the fixed-focus lens, thereby ensuring that the fixed-focus lens has a small size, achieving advantages such as a small overall volume and low cost.

[0060] Based on the above embodiments, 0.400 ≤ f / TTL ≤ 0.460; where f represents the effective focal length of the fixed-focus lens, and TTL represents the total optical length of the fixed-focus lens. Since the smaller the focal length, the larger the field of view angle required to achieve the same target surface, which brings greater off-axis aberration, and the larger the focal length, the greater the chromatic aberration generated by the system. The greater the chromatic aberration, the image quality cannot meet the design requirements. Therefore, it is limited that the effective focal length f of the fixed-focus lens and the total optical length TTL of the fixed-focus lens satisfy the following relational expression: 0.400 ≤ f / TTL ≤ 0.460, which can ensure that the focal length is within a reasonable range, avoid large off-axis aberration and chromatic aberration, and ensure the imaging effect of the fixed-focus lens.

[0061] As a feasible implementation manner, next, the parameters of each lens in the fixed-focus lens will be described.

[0062] Table 1 An optical design value of the fixed-focus lens in the first embodiment

[0063] Scope of protection Example 1 Lower limit Upper limit f / h 1.8695 1.850 2.050 |Maximum distortion| 5.708 3.950 5.800 NA 0.9220 0.9000 1.0500 CT2 / ET2 2.106 1.300 2.200 f / TTL 0.4553 0.400 0.4600 Φ1 / Φ -1.1137 -1.1000 -1.1700 Φ3 / Φ -0.8341 -0.7800 -0.8800 (CT1 + CT3) / TTL 0.1300 0.0800 0.1400 MAX(D1, D2, D3) 2.5100 2.400 2.600

[0064] Table 2 Design values of a fixed-focus lens

[0065]

[0066] In the above Table 2, the surface numbers are numbered according to the surface order of each lens. "S1" represents the object surface of the first lens, "S2" represents the image surface of the first lens, and so on. "STOP" represents the aperture of the lens. The radius of curvature represents the degree of curvature of the lens surface. A positive value represents that the surface bends towards the object side, and a negative value represents that the surface bends towards the image side. Among them, "infinity" represents that the surface is a plane and the radius of curvature is infinite. The thickness represents the central axial distance from the current surface to the next surface. The refractive index represents the ability of the material between the current surface and the next surface to deflect light. A space represents that the current position is air and the refractive index is 1. The Abbe number represents the dispersion characteristic of the material between the current surface and the next surface to light. A space represents that the current position is air. The semi-aperture represents half of the aperture size of the current surface.

[0067] Table 3 Design values of the aspheric conic coefficients of a fixed-focus lens

[0068]

[0069] "-3.0330E-003" represents -3.0330×10 -3 , and the rest of the coefficients are represented in this way.

[0070] The aspheric conic coefficients can be limited by the following aspheric formula, but are not limited to the following representation methods:

[0071]

[0072] Among them, z is the axial sagittal height in the Z direction of the aspheric surface; r is the height of the aspheric surface; c is the curvature of the fitted spherical surface, numerically the reciprocal of the radius of curvature R; k is the conic coefficient; A - G are the coefficients of the 4th, 6th, 8th, 10th, 12th, 14th, and 16th orders of the aspheric polynomial.

[0073] Furthermore, Figure 2 is a schematic diagram of the light fan of the optimal object distance lens provided in the first embodiment of the present invention. In the figure, the abscissa is the beam aperture and the ordinate is the lateral aberration. The most ideal curve is a straight line that coincides with the abscissa, indicating that all rays converge at the same point on the image plane. The corresponding interval on the ordinate of the curve is the maximum dispersion range of the beam on the ideal image plane. The light fan diagram can not only reflect the monochromatic aberration of different wavelengths but also represent the magnitude of the lateral chromatic aberration. From Figure 2 it can be seen that the system is well - approximated to the abscissa at each wavelength in each field of view, indicating that the lateral aberration of each wavelength of the system is well corrected. At the same time, there is no obvious dispersion among the wavelengths, indicating that the chromatic aberration of the system is also well corrected, thus ensuring that the optical system can meet the high - resolution imaging requirements.

[0074] Figure 3 is a schematic diagram of the field curvature and distortion curve of the optimal object distance lens provided in the first embodiment of the present invention. In the left - hand coordinate system in the figure, the horizontal coordinate represents the magnitude of the field curvature, with the unit of mm; the vertical coordinate represents the normalized image height, without a unit; where T represents the meridian and S represents the sagittal; from Figure 3 it can be seen that the field curvature of the lens provided in this embodiment is effectively controlled, that is, during imaging, the image quality at the center and the periphery has a small difference; in the right - hand coordinate system, the horizontal coordinate represents the magnitude of the distortion, with the unit of %; the vertical coordinate represents the normalized image height, without a unit; from Figure 3 it can be seen that the distortion of the lens provided in this embodiment is well corrected and the imaging distortion is small.

[0075] Figure 4 is a schematic diagram of the MTF of the optimal object distance lens provided in the first embodiment of the present invention. From the figure, it can be known that in the optical modulation function curve of the fixed - focus lens provided in the first embodiment of the present invention, the values at 200 line pairs / mm are all greater than 0.15, indicating that the fixed - focus lens has excellent resolution. The image quality from the central field of view to the edge field of view is very close, and the imaging has good consistency. The MTF curve is close to the diffraction limit.

[0076] In summary, for the fixed-focus lens provided in the first embodiment of the present invention, by reasonably setting parameters such as the optical power, surface shape, focal length, semi-image height, field of view angle, aperture diameter, lens thickness, maximum diameter, and total optical length of different lenses, a fixed-focus lens with large depth of field and small distortion characteristics is ensured to be realized. While the best object distance MTF can reach more than 0.15 at 200 lp when the best object distance is 140 mm, |distortion| ≤ 8% is achieved.

[0077] Embodiment 2

[0078] Figure 5 FIG. is a schematic structural diagram of an optimal object distance lens provided in the second embodiment of the present invention. As Figure 5 shown, the fixed-focus lens provided in the second embodiment of the present invention includes a first lens 110, a second lens 120, and a third lens 130 arranged in sequence along the optical axis from the object plane to the image plane. Among them, the first lens 110 is a negative-power lens, the second lens 120 is a positive-power lens, and the third lens 130 is a negative-power lens; the first lens 110 includes a first object side surface close to the object plane and a first image side surface close to the image plane. The first object side surface is a convex surface, and the first image side surface is a concave surface; the second lens 120 includes a second object side surface close to the object plane and a second image side surface close to the image plane. The second object side surface is a convex surface, and the second image side surface is a convex surface; the third lens 130 includes a third object side surface close to the object plane and a third image side surface close to the image plane. The third object side surface is a concave surface, and the third image side surface is a concave surface.

[0079] Among them, the setting method of the above lens is the same as that in Embodiment 1 and will not be elaborated here.

[0080] As another feasible implementation manner, the specific parameters in the fixed-focus lens will be described below.

[0081] Table 4 An optical design value of the fixed-focus lens in Embodiment 2

[0082] Scope of protection Example 2 Lower limit Upper limit f / h 1.8821 1.850 2.050 |Maximum distortion| 4.946 3.950 5.800 NA 1.0300 0.9000 1.0500 CT2 / ET2 1.364 1.300 2.200 f / TTL 0.4047 0.400 0.4600 Φ1 / Φ -1.1317 -1.1000 -1.1700 Φ3 / Φ -0.8569 -0.7800 -0.8800 (CT1 + CT3) / TTL 0.0802 0.0800 0.1400 MAX(D1, D2, D3) 2.424 2.400 2.600

[0083] Table 5 Design values of a fixed-focus lens

[0084]

[0085] In Table 5 above, the surface numbers are numbered according to the surface order of each lens. "S1" represents the object-side surface of the first lens, "S2" represents the image-side surface of the first lens, and so on. "STOP" represents the aperture stop of the lens. The radius of curvature represents the degree of curvature of the lens surface. A positive value indicates that the surface is curved towards the object side, and a negative value indicates that the surface is curved towards the image side. Among them, "infinity" represents that the surface is a plane with an infinite radius of curvature. The thickness represents the central axial distance from the current surface to the next surface. The refractive index represents the ability of the material between the current surface and the next surface to deflect light. A space represents that the current position is air with a refractive index of 1. The Abbe number represents the dispersion characteristics of the material between the current surface and the next surface to light. A space represents that the current position is air. The semi-aperture represents half of the aperture size of the current surface.

[0086] Table 6 Design values of aspheric conic coefficients in a fixed-focus lens

[0087]

[0088] "-1.1745E-002" represents -1.1745×10 -2 , and the rest of the coefficients are expressed in this way.

[0089] The aspheric conic coefficient can be defined by the following aspheric formula, but is not limited to the following representation:

[0090]

[0091] Among them, z is the axial sagittal height in the Z direction of the aspheric surface; r is the height of the aspheric surface; c is the curvature of the fitted spherical surface, which is numerically the reciprocal of the radius of curvature; k is the fitted conic coefficient; A - F are the coefficients of the 4th, 6th, 8th, 10th, 12th, and 14th order terms of the aspheric polynomial.

[0092] Furthermore, Figure 6 is a schematic diagram of the light fan of an optimal object distance lens provided in the second embodiment of the present invention. In the figure, the abscissa is the beam aperture and the ordinate is the lateral aberration. The most ideal curve is a straight line coinciding with the abscissa, indicating that all rays converge at the same point on the image plane. The corresponding interval on the ordinate of the curve is the maximum dispersion range of the beam on the ideal image plane. The light fan diagram can not only reflect the monochromatic aberration of different wavelengths but also represent the magnitude of the lateral chromatic aberration. From Figure 6 it can be seen that the system closely adheres to the abscissa at each wavelength in each field of view, indicating that the lateral aberration of each wavelength of the system is well corrected. At the same time, there is no obvious dispersion among each wavelength, indicating that the chromatic aberration of the system is also well corrected, thereby ensuring that the optical system can meet the high-resolution imaging requirements.

[0093] Figure 7It is a schematic diagram of the field curvature and distortion curve of an optimal object distance lens provided by the second embodiment of the present invention. In the left coordinate system in the figure, the horizontal coordinate represents the magnitude of field curvature, with the unit of mm; the vertical coordinate represents the normalized image height, without a unit; where T represents the meridian and S represents the sagittal; from Figure 7 it can be seen that the field curvature of the lens provided in this embodiment is effectively controlled, that is, when imaging, the image quality at the center and the image quality at the periphery have a small difference; in the right coordinate system, the horizontal coordinate represents the magnitude of distortion, with the unit of %; the vertical coordinate represents the normalized image height, without a unit; from Figure 7 it can be seen that the distortion of the lens provided in this embodiment is well corrected, and the imaging distortion is small.

[0094] Figure 8 It is an MTF schematic diagram of an optimal object distance lens provided by the second embodiment of the present invention. From the figure, it can be known that in the optical modulation function curve of the fixed-focus lens provided by the second embodiment of the present invention, the values at 200 line pairs / mm are all greater than 0.15, indicating that the fixed-focus lens has excellent resolution, the image quality from the central field of view to the edge field of view is very close, the imaging has good consistency, and the MTF curve is close to the diffraction limit.

[0095] In summary, for the fixed-focus lens provided by the second embodiment of the present invention, by reasonably setting parameters such as the optical power, surface shape, focal length, semi-image height, field of view angle, aperture diameter, lens thickness, maximum aperture, and overall optical length of different lenses, it is ensured to realize a fixed-focus lens with the characteristics of large depth of field and small distortion. While being able to achieve that the MTF at the optimal object distance of 140 mm is greater than 0.15 at 200 lp, |distortion| ≤ 8% is realized.

[0096] Embodiment Three

[0097] Figure 9 It is a schematic structural diagram of an optimal object distance lens provided by the third embodiment of the present invention. As Figure 9 shown, the fixed-focus lens provided by the third embodiment of the present invention includes a first lens 110, a second lens 120, and a third lens 130 arranged in sequence along the optical axis from the object plane to the image plane. Among them, the first lens 110 is a negative optical power lens, the second lens 120 is a positive optical power lens, and the third lens 130 is a negative optical power lens; the first lens 110 includes a first object side surface close to the object plane and a first image side surface close to the image plane. The first object side surface is a convex surface, and the first image side surface is a concave surface; the second lens 120 includes a second object side surface close to the object plane and a second image side surface close to the image plane. The second object side surface is a convex surface, and the second image side surface is a convex surface; the third lens 130 includes a third object side surface close to the object plane and a third image side surface close to the image plane. The third object side surface is a concave surface, and the third image side surface is a concave surface.

[0098] Among them, the setting method of the above lens is the same as that in Embodiment One, and will not be elaborated here.

[0099] As another feasible implementation, the specific parameters in the fixed-focus lens are described below.

[0100] Table 7 An optical design value of the fixed-focus lens in the third embodiment

[0101] Scope of protection Example 3 Lower limit Upper limit f / h 2.0227 1.850 2.050 |Maximum distortion| 3.962 3.950 5.800 NA 1.0256 0.9000 1.0500 CT2 / ET2 1.334 1.300 2.200 f / TTL 0.4221 0.400 0.4600 Φ1 / Φ -1.1603 -1.1000 -1.1700 Φ3 / Φ -0.7980 -0.7800 -0.8800 (CT1 + CT3) / TTL 0.0961 0.0800 0.1400 MAX(D1, D2, D3) 2.540 2.400 2.600

[0102] Table 8 Design values of a fixed-focus lens

[0103]

[0104] In Table 8 above, the surface numbers are numbered according to the surface order of each lens. "S1" represents the object-side surface of the first lens, "S2" represents the image-side surface of the first lens, and so on. "STOP" represents the aperture of the lens. The radius of curvature represents the degree of curvature of the lens surface. A positive value represents that the surface bends towards the object side, and a negative value represents that the surface bends towards the image side. Among them, "infinity" represents that the surface is a plane and the radius of curvature is infinite. The thickness represents the central axial distance from the current surface to the next surface. The refractive index represents the ability of the material between the current surface and the next surface to deflect light. A space represents that the current position is air and the refractive index is 1. The Abbe number represents the dispersion characteristic of the material between the current surface and the next surface to light. A space represents that the current position is air. The semi-aperture represents half of the aperture size of the current surface.

[0105] Table 9 Design values of the aspheric conic coefficients of a fixed-focus lens

[0106]

[0107] "-7.5340E-003" represents -7.5340×10 -3 , and the rest of the coefficients are all represented in this way.

[0108] The aspheric conic coefficient can be defined by the following aspheric formula, but is not limited to the following representation method

[0109]

[0110] Among them, z is the axial sagittal height in the Z direction of the aspheric surface; r is the height of the aspheric surface; c is the curvature of the fitted spherical surface, numerically the reciprocal of the radius of curvature; k is the fitted conic coefficient; A - F are the coefficients of the 4th, 6th, 8th, 10th, 12th, and 14th order terms of the aspheric polynomial.

[0111] Furthermore, Figure 10It is a schematic diagram of the light fan of an optimal object distance lens provided in the third embodiment of the present invention. In the figure, the abscissa is the beam aperture, and the ordinate is the lateral aberration. The most ideal curve is a straight line that coincides with the abscissa, indicating that all light rays converge at the same point on the image plane. The corresponding interval on the ordinate of the curve is the maximum dispersion range of the beam on the ideal image plane. The light fan diagram can not only reflect the monochromatic aberration of different wavelengths but also represent the magnitude of the lateral chromatic aberration. From Figure 10 it can be seen that the system closely approaches the abscissa at each wavelength in each field of view, indicating that the lateral aberration of each wavelength of the system is well corrected. At the same time, there is no obvious dispersion among the wavelengths, indicating that the chromatic aberration of the system is also well corrected, thus ensuring that the optical system can meet the requirements of high-resolution imaging.

[0112] Figure 11 It is a schematic diagram of the field curvature and distortion curve of an optimal object distance lens provided in the third embodiment of the present invention. In the left coordinate system in the figure, the horizontal coordinate represents the magnitude of the field curvature, with the unit of mm; the vertical coordinate represents the normalized image height, without a unit; where T represents the meridian and S represents the sagittal; from Figure 11 it can be seen that the field curvature of the lens provided in this embodiment is effectively controlled, that is, when imaging, the image quality at the center and the periphery has a small difference; in the right coordinate system, the horizontal coordinate represents the magnitude of the distortion, with the unit of %; the vertical coordinate represents the normalized image height, without a unit; from Figure 11 it can be seen that the distortion of the lens provided in this embodiment is well corrected, and the imaging distortion is small.

[0113] Figure 12 It is a schematic diagram of the MTF of an optimal object distance lens provided in the third embodiment of the present invention. From the figure, it can be known that in the optical modulation function curve of the fixed-focus lens provided in the embodiment of the present invention, the value at 200 line pairs / mm is greater than 0.15, indicating that the fixed-focus lens has excellent resolving power, and the image quality from the central field of view to the edge field of view is very close, and the imaging has good consistency, and the MTF curve is close to the diffraction limit.

[0114] In summary, the fixed-focus lens provided in the third embodiment of the present invention, by reasonably setting parameters such as the optical power, surface shape, focal length, semi-image height, field of view angle, aperture diameter, lens thickness, maximum aperture, and total optical length of different lenses, ensures the realization of a fixed-focus lens with the characteristics of large depth of field and small distortion. While achieving an MTF greater than 0.15 at 200 lp for the optimal object distance of 140 mm, it realizes |distortion| ≤ 8%.

[0115] The above specific embodiments do not constitute a limitation on the protection scope of the present invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A fixed-focus lens, characterized in that, It includes a first lens, a second lens, and a third lens arranged in sequence along the optical axis from the object plane to the image plane; The first lens is a negative-power lens, the second lens is a positive-power lens, and the third lens is a negative-power lens; The first lens includes a first object side face close to the object plane and a first image side face close to the image plane. The first object side face is a convex face, and the first image side face is a concave face; The second lens includes a second object side face close to the object plane and a second image side face close to the image plane. The second object side face is a convex face, and the second image side face is a convex face; The third lens includes a third object side face close to the object plane and a third image side face close to the image plane. The third object side face is a concave face, and the third image side face is a concave face.

2. The fixed-focus lens according to claim 1, wherein, -1.17 ≤ Φ1 / Φ ≤ -1.10, -0.88 ≤ Φ3 / Φ ≤ -0.78; Wherein, Φ1 represents the optical power of the first lens, Φ3 represents the optical power of the third lens, and Φ represents the overall optical power of the fixed-focus lens.

3. The fixed-focus lens according to claim 1, wherein 1.85 ≤ f / h ≤ 2.05; Where f represents the effective focal length of the fixed-focus lens, and h represents the semi-image height on the image side of the fixed-focus lens.

4. The fixed-focus lens according to claim 1, characterized in that, 53° ≤ DFOV ≤ 59°; Where DFOV represents half of the maximum field of view angle of the fixed-focus lens.

5. The fixed-focus lens according to claim 1, characterized in that, The fixed-focus lens further includes a diaphragm, and the diaphragm is disposed in the optical path between the first lens and the second lens; Wherein, the aperture NA of the diaphragm satisfies 0.9 < NA < 1.

05.

6. The fixed-focus lens according to claim 1, wherein 1.3 ≤ CT2 / ET2 ≤ 2.2; Where CT2 represents the thickest thickness of the second lens, and ET2 represents the thinnest thickness of the second lens.

7. The fixed-focus lens according to claim 1, characterized in that, 0.08 ≤ (CT1 + CT3) / TTL ≤ 0.140; Where CT1 represents the central thickness of the first lens; CT3 represents the central thickness of the third lens, and TTL represents the overall optical length of the fixed-focus lens.

8. The fixed-focus lens according to claim 1, wherein MAX(D1, D2, D3) ≤ 2.600; Where D1 represents the maximum aperture of the first lens, D2 represents the maximum aperture of the second lens, and D3 represents the maximum aperture of the third lens.

9. The fixed-focus lens according to claim 1, wherein 0.400 ≤ f / TTL ≤ 0.460; Where f represents the effective focal length of the fixed-focus lens, and TTL represents the overall optical length of the fixed-focus lens.

10. The fixed-focus lens according to claim 1, characterized in that, The first lens, the second lens, and the third lens are all plastic aspherical lenses.

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