Prime lens

Abstract

The embodiment of the invention discloses a prime lens. The prime lens comprises a first lens, a second lens, a third lens and a fourth lens which are sequentially arranged from an object space to an image space along an optical axis, the first lens has negative focal power, the second lens has positive focal power, the third lens has positive focal power, and the fourth lens has negative focal power; the object space surfaces and the image space surfaces of the first lens, the third lens and the fourth lens are aspheric surfaces; the object-side surface of the second lens is an aspheric surface, and the image-side surface is a binary diffraction surface. In the embodiment of the invention, the four optical lenses are adopted, and the focal power of each lens element and the relative position of each lens element are optimized, so that the design of the prime lens which is large in aperture, compact in structure and capable of meeting the imaging requirement is finally realized; the aperture of the prime lens is 1.4, the total optical length is not larger than 22.5 mm, the prime lens can be matched with a 1 / 2.7 ''sensor chip, and the comprehensive performance of the prime lens can meet the use requirement of a general sensor for security and protection monitoring.

Description

Technical Field

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

[0002] As the mainstream product in the security industry, 4mm fixed-focus lenses are very popular in the market. The number of 4mm lenses with large aperture on the market is generally 5 or more, and the aperture is F1.6. There are fewer lenses with a small number of lenses and a large aperture.

[0003] Therefore, designing a fixed-focus lens that uses 4 optical lenses, achieves a large aperture of F1.4, has a compact structure and takes imaging requirements into consideration will have a broad market prospect. Summary of the invention

[0004] The present invention provides a fixed-focus lens to realize a large-aperture fixed-focus lens with four lenses.

[0005] An embodiment of the present invention provides a fixed-focus lens, comprising a first lens, a second lens, a third lens, and a fourth lens arranged in sequence from the object side to the image side along an optical axis;

[0006] The first lens has negative optical power, the second lens has positive optical power, the third lens has positive optical power, and the fourth lens has negative optical power;

[0007] The object-side surface and the image-side surface of the first lens, the third lens and the fourth lens are all aspherical surfaces; the object-side surface of the second lens is an aspherical surface, and the image-side surface is a binary diffraction surface.

[0008] Optionally, the first lens is a meniscus lens, the second lens is a biconvex lens, the third lens is a biconvex lens, and the fourth lens is a meniscus lens or a biconcave lens.

[0009] Optionally, the first lens, the third lens and the fourth lens are all plastic lenses.

[0010] Optionally, the first lens satisfies the following condition: -0.740≤Φ1 / Φ≤-0.600;

[0011] Wherein, Φ1 is the optical focal length of the first lens, and Φ is the optical focal length of the fixed-focus lens.

[0012] Optionally, the second lens satisfies the following condition: 0.49≤Φ2 / Φ≤0.62;

[0013] Wherein, Φ2 is the optical focal length of the second lens, and Φ is the optical focal length of the fixed-focus lens.

[0014] Optionally, adjacent surfaces of the third lens and the fourth lens are glued to each other to form a glued lens group.

[0015] Optionally, the third lens and the fourth lens satisfy the following condition: 0.08≤(Φ3+Φ4) / Φ≤0.12;

[0016] Among them, Φ3 is the optical focal length of the third lens, Φ4 is the optical focal length of the fourth lens, and Φ is the optical focal length of the fixed-focus lens.

[0017] Optionally, the third lens and the fourth lens further satisfy the following condition: 72≤(Vd3+d4)≤85;

[0018] Wherein, Vd3 is the Abbe number of the third lens, and Vd4 is the Abbe number of the fourth lens.

[0019] Optionally, the fixed-focus lens meets the following conditions: 0.30 <T12 / TTL<0.38;

[0020] Wherein, T12 is the air distance between the image surface of the first lens and the object surface of the second lens on the optical axis, and TTL is the total optical length of the fixed-focus lens system.

[0021] Optionally, a stop is further included, wherein the stop is located between the first lens and the second lens.

[0022] The technical solution of the embodiment of the present invention is to set four lenses to form a fixed-focus lens, wherein the focal lengths of the four lenses cooperate with each other to achieve a fixed-focus lens design with large aperture, compact structure and sturdy imaging requirements. Specifically, the focal length of the first lens is negative, which can make the object-side light smoothly enter the imaging system, so that the light enters the second lens at a smaller incident angle, reducing the proportion of higher-order aberrations. The focal length of the second lens is positive, which can further smoothly shrink the deflection angle of the light, so that the system has a looser tolerance sensitivity. The focal lengths of the third lens and the fourth lens are positive and negative, respectively, which is conducive to correcting system aberrations. In addition, the object surface and image surface of the first lens, the third lens and the fourth lens and the object surface of the second lens are all aspherical lenses, which can further correct system aberrations. The image surface of the second lens is set as a binary diffraction surface, which can be used to correct chromatic aberrations and is also conducive to improving image quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a structural schematic diagram of a fixed-focus lens provided in Embodiment 1 of the present invention;

[0024] Figure 2 yes Figure 1 Spherical aberration curve of the fixed focal length lens shown;

[0025] Figure 3 is a structural schematic diagram of a fixed-focus lens provided in Embodiment 2 of the present invention;

[0026] Figure 4 yes Figure 3 Spherical aberration curve of the fixed focal length lens shown;

[0027] Figure 5 is a structural schematic diagram of a fixed-focus lens provided in Embodiment 3 of the present invention;

[0028] Figure 6 yes Figure 5 Spherical aberration graph for the fixed focal length lens shown. DETAILED DESCRIPTION

[0029] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the accompanying drawings.

[0030] The terms used in the embodiments of the present invention are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. It should be noted that the directional words such as "upper", "lower", "left", "right" and the like described in the embodiments of the present invention are described at the angles shown in the accompanying drawings and should not be understood as limitations on the embodiments of the present invention. In addition, in the context, it is also necessary to understand that when it is mentioned that an element is formed "on" or "under" another element, it can not only be directly formed "on" or "under" another element, but also indirectly formed "on" or "under" another element through an intermediate element. The terms "first", "second", etc. are only used for descriptive purposes and do not represent any order, quantity or importance, but are only used to distinguish different components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0031] The term “including” and its variations used in the present invention are open inclusions, that is, “including but not limited to.” The term “based on” means “based at least in part on.” The term “one embodiment” means “at least one embodiment.”

[0032] It should be noted that the concepts such as "first" and "second" mentioned in the present invention are only used to distinguish the corresponding contents, and are not used to limit the order or interdependence.

[0033] It should be noted that the modifications of "one" and "plurality" mentioned in the present invention are illustrative rather than restrictive, and those skilled in the art should understand that, unless otherwise clearly indicated in the context, it should be understood as "one or more".

[0034] Figure 1 is a schematic diagram of the structure of a fixed-focus lens provided in Embodiment 1 of the present invention, with reference to Figure 1 The fixed-focus lens comprises a first lens 10, a second lens 20, a third lens 30 and a fourth lens 40 which are arranged in sequence from the object side to the image side along the optical axis;

[0035] The first lens 10 has negative optical power, the second lens 20 has positive optical power, the third lens 30 has positive optical power, and the fourth lens 40 has negative optical power;

[0036] The object-side surface and the image-side surface of the first lens 10 , the third lens 30 , and the fourth lens 40 are all aspherical surfaces; the object-side surface of the second lens 20 is an aspherical surface, and the image-side surface is a binary diffraction surface.

[0037] First of all, for optical lenses, the focal length is equal to the difference between the convergence of the image-side beam and the convergence of the object-side beam, which 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 refractive surface of a lens (i.e., a surface of a lens), can be used to characterize a lens, and can also be used to characterize a system formed by multiple lenses (i.e., a lens group).

[0038] In the fixed-focus lens provided in this embodiment, each lens can be arranged in a lens barrel ( Figure 1 (not shown) in the Figure 1 As shown, in the embodiment of the present invention, four lenses are arranged to form a fixed-focus lens, wherein the focal powers of the four lenses cooperate with each other to achieve a fixed-focus lens design with large aperture, compact structure and sturdy imaging requirements. Specifically, the focal power of the first lens 10 is negative, which can make the object light smoothly enter the imaging system, so that the light enters the second lens 20 at a smaller incident angle, reducing the proportion of high-order aberrations. The focal power of the second lens 20 is positive, which can further shrink the deflection angle of the light smoothly, so that the system has a looser tolerance sensitivity. The focal powers of the third lens 30 and the fourth lens 40 are positive and negative, respectively, which is conducive to correcting the aberration of the system. In addition, the object surface and image surface of the first lens 10, the third lens 30 and the fourth lens 40 and the object surface of the second lens 20 are all aspherical lenses, which are used to further correct the aberration of the system. The image surface of the second lens 20 is set as a binary diffraction surface, which is used to correct chromatic aberration and is also conducive to improving image quality.

[0039] Continue to refer Figure 1 The fixed-focus lens further includes a stop STO, which is located between the first lens 10 and the second lens 20.

[0040] Those skilled in the art will know that the aperture STO is used to limit the light beam size in the optical system, and determines the amount of light that enters the photosensitive element through the lens, that is, it is used to control the light transmittance of the lens, that is, the aperture STO directly determines the size of the aperture of the optical lens. Setting the aperture STO between the first lens 10 and the second lens 20 essentially limits the specific position of the aperture STO at the waist of the entire optical system, thereby accurately controlling the light transmittance, expanding the light height of the central main light at the position of the aperture STO, and expanding the aperture to ensure the amount of light passing through the aperture STO and the brightness of the image; in addition, the aperture STO can block the far-axis light, effectively reduce the off-axis aberration, and ensure the clarity of the image. Therefore, the embodiment of the present invention can achieve an aperture of 1.4, a total optical length of no more than 22.5 mm, and can match a 1 / 2.7″ sensor chip. Its comprehensive performance meets the use requirements of general sensors for security monitoring.

[0041] In a specific embodiment, optionally, the first lens 10 is a meniscus lens, the second lens 20 is a biconvex lens, the third lens 30 is a biconvex lens, and the fourth lens 40 is a meniscus lens or a biconcave lens.

[0042] In a specific embodiment, optionally, the first lens 10 , the second lens 20 , the third lens 30 and the fourth lens 40 are all plastic lenses.

[0043] It can be understood that in the embodiment of the present invention, the surfaces of the first lens 10, the third lens 30 and the fourth lens 40 are all set as aspherical surfaces, and the use of plastics for preparation will reduce the difficulty and cost of preparation, which is conducive to reducing manufacturing costs and mass production. At the same time, the object surface of the second lens 20 is set as an aspherical surface, and the image surface is set as a binary diffraction surface, which can achieve the effect of correcting aberrations and chromatic aberrations by the aspherical surface and the binary diffraction surface. Exemplarily, since the second lens 20 needs to prepare an aspherical surface and a binary diffraction surface separately, it can be specifically prepared using acrylic materials to reduce the difficulty and cost of preparation.

[0044] In a specific embodiment, optionally, the first lens 10 satisfies the following condition: -0.740≤Φ1 / Φ≤-0.600; wherein Φ1 is the optical power of the first lens 10, and Φ is the optical power of the fixed-focus lens.

[0045] As can be seen from the above, in the embodiment of the present invention, the first lens 10 can be set as a convex-concave plastic aspheric lens with negative optical power. The optical power of the lens within this range can ensure that the object light is smoothly collected into the imaging system, so that the light enters the second lens 20 at a smaller incident angle, thereby reducing the proportion of higher-order aberrations.

[0046] In a specific embodiment, optionally, the second lens 20 satisfies the following condition: 0.49 ≤ Φ2 / Φ ≤ 0.62; where Φ2 is the optical power of the second lens 20, and Φ is the optical power of the fixed-focus lens.

[0047] As can be seen above, in the embodiment of the present invention, the second lens 20 can be set as a biconvex lens with positive optical power. The optical power of this lens within this range can ensure a further gentle angular contraction of light rays, making the system have a looser tolerance sensitivity.

[0048] In a specific embodiment, optionally, the adjacent surfaces of the third lens 30 and the fourth lens 40 are glued to form a glued lens group. Specifically, the third lens 30 and the fourth lens 40 can be bonded by glue.

[0049] Further optionally, the third lens 30 and the fourth lens 40 satisfy the following condition: 0.08 ≤ (Φ3 + Φ4) / Φ ≤ 0.12; where Φ3 is the optical power of the third lens 30, Φ4 is the optical power of the fourth lens 40, and Φ is the optical power of the fixed-focus lens.

[0050] As can be seen above, in the embodiment of the present invention, the third lens 30 and the fourth lens 40 can both be set as plastic aspherical lenses. When the optical powers of the third lens 30 and the fourth lens 40 satisfy this range, the optical power distribution of the system is reasonable, which is more conducive to the correction of system aberrations. Among them, the third lens 30 and the fourth lens 40 are bonded by glue.

[0051] Further optionally, the third lens 30 and the fourth lens 40 also satisfy the following condition: 72 ≤ (Vd3 + d4) ≤ 85; where Vd3 is the Abbe number of the third lens 30, and Vd4 is the Abbe number of the fourth lens 40.

[0052] Among them, when the Abbe numbers of the third lens 30 and the fourth lens 40 satisfy this range, it is beneficial to the correction of system chromatic aberration, thereby further improving the image quality.

[0053] In a specific embodiment, optionally, the fixed-focus lens satisfies the following condition: 0.30 < T12 / TTL < 0.38; where T12 is the air gap on the optical axis from the image-side surface of the first lens 10 to the object-side surface of the second lens 20, and TTL is the total optical length of the fixed-focus lens system.

[0054] When the optical system ratio is within this range, the structure of the fixed-focus lens can be made more compact, the total optical length can be made not greater than 22.5 mm, it can be matched with a 1 / 2.7″ sensor chip, and the comprehensive performance meets the usage requirements of general sensors for security monitoring.

[0055] In the embodiment of the present invention, by adopting 4 optical lenses and optimizing the optical focal length of each lens element and the relative position of each lens element, the design of a fixed-focus lens with a large aperture, a compact structure and taking into account the imaging requirements is finally realized; the fixed-focus lens has an aperture of 1.4, a total optical length of no more than 22.5 mm, and can match a 1 / 2.7″ sensor chip, and its comprehensive performance meets the use requirements of a general sensor for security monitoring.

[0056] Based on the same concept above, the present invention provides three different specific embodiments, and the optical power relationship and related physical optical parameter design ranges thereof are shown in Table 1:

[0057] Table 1 Relationship between optical power and related physical optical parameters in various embodiments

[0058] Scope of protection Example 1 Example 2 Example 3 Lower limit Upper limit Φ1 / Φ -0.625 -0.662 -0.721 -0.740 -0.600 Φ2 / Φ 0.536 0.519 0.597 0.490 0.620 (Φ3+Φ4) / Φ 0.087 0.085 0.107 0.080 0.120 Vd3+Vd4 75.71 82.00 78.00 72.00 85.00 T12 / TTL 0.36 0.34 0.32 0.30 0.38

[0059] In the first embodiment of the present invention, reference is made to Figure 1 It can be seen that the structural composition of each component of the system and the shape and position of each component are crucial to the system. It can be seen from the figure that the optical system is composed of 4 optical lenses, among which the aperture STO is located between the first lens 10 and the second lens 20. A filter 50 is also provided along the object plane to the image plane; the filter 50 is located on the image side of the fourth lens 40, and the filter 50 can protect the photosensitive chip in the imaging sensor to ensure the imaging effect of the fixed focus lens. The third lens 30 and the fourth lens 40 are glued together, and the fourth lens 40 is a double concave lens. The fixed focus lens achieves a focal length f of 4.20 mm and an aperture F# of 1.40.

[0060] like Figure 1 The parameter design values ​​of each lens in the fixed-focus lens of the first embodiment are shown in Table 2:

[0061] Table 2: Design values ​​of each lens of the fixed focus lens in Example 1

[0062]

[0063]

[0064] The surface numbers in Table 2 are numbered according to the order of the surfaces of each lens. "STO" represents the aperture of a fixed-focus lens; "IMA" represents the image plane of a fixed-focus lens; the radius of curvature represents the degree of curvature of the lens surface, a positive value represents that the surface is bent toward the image plane, and a negative value represents that the surface is bent toward the object plane, where "Infinity" represents that the surface is a plane, the radius of curvature is infinite, and the distance 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, and a blank space represents that the current position is air, and the refractive index is 1; the Abbe number represents the dispersion characteristics of the material between the current surface and the next surface to light, and a blank space represents that the current position is air; the semi-aperture represents the effective diameter of the light of the lens; the k value represents the numerical value of the conic coefficient of the aspheric surface.

[0065] The aspheric cone coefficient can be defined by the following aspheric formula, but is not limited to the following representation method:

[0066]

[0067] Among them, z is the axial vector height of the aspheric surface in the Z direction; r is the height of the aspheric surface; c is the curvature of the fitted sphere, which is the inverse of the radius of curvature; k is the fitting cone coefficient; AE is the coefficient of the 4th, 6th, 8th, 10th, and 12th order terms of the aspheric polynomial.

[0068] The coefficient values ​​of each aspheric surface in the above embodiment 1 are shown in Table 3:

[0069] Table 3 Aspheric coefficients of lenses in Example 1

[0070] Surface number A B C D E S1 -9.14303E-03 6.12773E-04 -2.42705E-05 5.42658E-07 -5.21925E-09 S2 1.10915E-02 -2.24348E-03 3.50044E-04 -2.66272E-05 8.72243E-07 S4 -6.97433E-04 7.86560E-06 -6.54616E-07 0.00000E+00 0.00000E+00 S5 -2.83418E-05 2.11145E-05 -5.41376E-07 0.00000E+00 0.00000E+00 S6 -3.70683E-05 9.35707E-06 -5.54947E-07 -8.21319E-08 2.31722E-09 S7 8.34852E-04 2.07922E-04 -1.14097E-05 -6.53922E-08 1.27589E-08 S8 2.53423E-03 -1.96186E-05 2.29673E-05 -2.29213E-06 8.96378E-08

[0071] Among them, -9.14303E-03 means that the coefficient A of the surface number S1 is -9.14303*10 -3 , and so on. The diffraction surface coefficients can be defined by the following diffraction surface equation, but are not limited to the following representation method:

[0072] Φ=A1Y 2 +A2Y 4 +A3Y 6 ;

[0073] Among them, Φ is the phase of the diffraction surface; Y is the semi-aperture of the lens perpendicular to the optical axis; A1, A2, A3 are the diffraction surface phase coefficients.

[0074] The coefficient values ​​of the binary diffraction surface in the above embodiment 1 are shown in Table 4:

[0075] Table 4 Diffraction surface coefficients of the lens in Example 1

[0076] Surface number Diffraction order Maximum number of terms Normalized radius <![CDATA[A1]]> <![CDATA[A2]]> <![CDATA[A3]]> S5 +1 3 2.5 -63.2797 14.4713 -3.6531

[0077] Figure 2 yes Figure 1 The spherical aberration curve of the fixed focus lens shown is shown in Figure 2. Figure 2 , where the vertical direction represents the normalization of the aperture, 0 represents the optical axis, and the vertical vertex represents the maximum pupil radius; the horizontal direction represents the offset relative to the ideal focus, in millimeters (mm). The different linear curves in the figure represent different wavelengths of system imaging, which are represented by Figure 2 It can be seen that the axial aberrations at different wavelengths are all controlled within the range of (-0.05mm, +0.05mm), indicating that the spherical aberration of this fixed-focus lens at each wavelength is well controlled and can meet the needs of wide spectrum applications.

[0078] Figure 3 is a structural schematic diagram of a fixed-focus lens provided in Embodiment 2 of the present invention. In Embodiment 2 of the present invention, reference is made to Figure 3 It can be seen that the structural composition of each component of the system and the shape and position of each component are crucial to the system. It can be seen from the figure that the optical system is composed of 4 optical lenses, among which the aperture STO is located between the first lens 10 and the second lens 20. A filter 50 is also provided along the object plane to the image plane; the filter 50 is located on the image side of the fourth lens 40, and the filter 50 can protect the photosensitive chip in the imaging sensor to ensure the imaging effect of the fixed focus lens. The third lens 30 and the fourth lens 40 are glued together, and the fourth lens 40 is a concave-convex lens. The fixed focus lens achieves a focal length f of 4.20 mm and an aperture F# of 1.40.

[0079] like Figure 3 The parameter design values ​​of each lens in the fixed-focus lens of the second embodiment are shown in Table 5:

[0080] Table 5: Design values ​​of each lens in the fixed focus lens of Example 2

[0081]

[0082]

[0083] The surface numbers in Table 5 are numbered according to the order of the surfaces of each lens. "STO" represents the aperture of a fixed-focus lens; "IMA" represents the image plane of a fixed-focus lens; the radius of curvature represents the curvature of the lens surface, a positive value represents that the surface is bent toward the image plane, and a negative value represents that the surface is bent toward the object plane, where "Infinity" represents that the surface is a plane, the radius of curvature is infinite, and the distance 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, and a blank space represents that the current position is air, and the refractive index is 1; the Abbe number represents the dispersion characteristics of the material between the current surface and the next surface to light, and a blank space represents that the current position is air; the semi-aperture represents the effective diameter of the light of the lens; the k value represents the numerical value of the conic coefficient of the aspheric surface.

[0084] The aspheric cone coefficient can be defined by the following aspheric formula, but is not limited to the following representation method:

[0085]

[0086] Among them, z is the axial vector height of the aspheric surface in the Z direction; r is the height of the aspheric surface; c is the curvature of the fitted sphere, which is the inverse of the radius of curvature; k is the fitting cone coefficient; AE is the coefficient of the 4th, 6th, 8th, 10th, and 12th order terms of the aspheric polynomial.

[0087] The coefficient values ​​of each aspheric surface in the above-mentioned embodiment 2 are shown in Table 6:

[0088] Table 6 Aspheric coefficients of lenses in Example 2

[0089]

[0090]

[0091] Among them, -1.00597E-02 means that the coefficient A of the surface number S1 is -1.00597*10 -2 , and so on. The diffraction surface coefficients can be defined by the following diffraction surface equation, but are not limited to the following representation method:

[0092] Φ=A1Y 2 +A2Y 4 +A3Y 6 ;

[0093] Among them, Φ is the phase of the diffraction surface; Y is the semi-aperture of the lens perpendicular to the optical axis; A1, A2, A3 are the diffraction surface phase coefficients.

[0094] The coefficient values ​​of the binary diffraction surface in the above-mentioned embodiment 2 are shown in Table 7:

[0095] Table 7 Diffraction surface coefficients of the lens in Example 2

[0096] Surface number Diffraction order Maximum number of terms Normalized radius <![CDATA[A1]]> <![CDATA[A2]]> <![CDATA[A3 <!-- 8 -->]]> S5 +1 3 2.5 -78.7182 28.6306 -6.6082

[0097] Figure 4 yes Figure 3 The spherical aberration curve of the fixed focus lens shown is shown in Figure 2. Figure 4 , where the vertical direction represents the normalization of the aperture, 0 represents the optical axis, and the vertical vertex represents the maximum pupil radius; the horizontal direction represents the offset relative to the ideal focus, in millimeters (mm). The different linear curves in the figure represent different wavelengths of system imaging, which are represented by Figure 4 It can be seen that the axial aberrations at different wavelengths are all controlled within the range of (-0.05mm, +0.05mm), indicating that the spherical aberration of this fixed-focus lens at each wavelength is well controlled and can meet the needs of wide spectrum applications.

[0098] Figure 5 is a structural schematic diagram of a fixed-focus lens provided in Embodiment 3 of the present invention. In Embodiment 3 of the present invention, reference is made to Figure 5 It can be seen that the structural composition of each component of the system and the shape and position of each component are crucial to the system. It can be seen from the figure that the optical system is composed of 4 optical lenses, among which the aperture STO is located between the first lens 10 and the second lens 20. A filter 50 is also provided along the object plane to the image plane; the filter 50 is located on the image side of the fourth lens 40, and the filter 50 can protect the photosensitive chip in the imaging sensor to ensure the imaging effect of the fixed focus lens. The third lens 30 and the fourth lens 40 are glued together, and the fourth lens 40 is a concave-convex lens. The fixed focus lens achieves a focal length f of 4.26 mm and an aperture F# of 1.41.

[0099] like Figure 5 The parameter design values ​​of each lens in the fixed-focus lens of the third embodiment are shown in Table 8:

[0100] Table 8: Design values ​​of each lens in the fixed focus lens in Example 3

[0101]

[0102]

[0103] The surface numbers in Table 8 are numbered according to the order of the surfaces of each lens. "STO" represents the aperture of a fixed-focus lens; "IMA" represents the image plane of a fixed-focus lens; the radius of curvature represents the curvature of the lens surface, a positive value represents that the surface is bent toward the image plane, and a negative value represents that the surface is bent toward the object plane, where "Infinity" represents that the surface is a plane, the radius of curvature is infinite, and the distance 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, and a blank space represents that the current position is air, and the refractive index is 1; the Abbe number represents the dispersion characteristics of the material between the current surface and the next surface to light, and a blank space represents that the current position is air; the semi-aperture represents the effective diameter of the light of the lens; the k value represents the numerical value of the cone coefficient of the aspheric surface.

[0104] The aspheric cone coefficient can be defined by the following aspheric formula, but is not limited to the following representation method:

[0105]

[0106] Among them, z is the axial vector height of the aspheric surface in the Z direction; r is the height of the aspheric surface; c is the curvature of the fitted sphere, which is the inverse of the radius of curvature; k is the fitting cone coefficient; AE is the coefficient of the 4th, 6th, 8th, 10th, and 12th order terms of the aspheric polynomial.

[0107] The coefficient values ​​of each aspheric surface in the above-mentioned embodiment 3 are shown in Table 9:

[0108] Table 9 Aspheric coefficients of the lens in Example 3

[0109]

[0110]

[0111] Among them, -9.84138E-03 means that the coefficient A of the surface number S1 is -9.84138*10 -3 , and so on. The diffraction surface coefficients can be defined by the following diffraction surface equation, but are not limited to the following representation method:

[0112] Φ=A1Y 2 +A2Y 4 +A3Y 6 ;

[0113] Among them, Φ is the phase of the diffraction surface; Y is the semi-aperture of the lens perpendicular to the optical axis; A1, A2, A3 are the diffraction surface phase coefficients.

[0114] The coefficient values ​​of the binary diffraction surface in the above-mentioned embodiment 3 are shown in Table 10:

[0115] Table 10 Diffraction surface coefficients of the lens in Example 3

[0116] Surface number Diffraction order Maximum number of terms Normalized radius <![CDATA[A1]]> <![CDATA[A2]]> <![CDATA[A3]]> S5 +1 3 2.5 -58.5508 19.7396 -5.2386

[0117] Figure 6 yes Figure 5 The spherical aberration curve of the fixed focus lens shown is shown in Figure 2. Figure 6 , where the vertical direction represents the normalization of the aperture, 0 represents the optical axis, and the vertical vertex represents the maximum pupil radius; the horizontal direction represents the offset relative to the ideal focus, in millimeters (mm). The different linear curves in the figure represent different wavelengths of system imaging, which are represented by Figure 6 It can be seen that the axial aberrations at different wavelengths are all controlled within the range of (-0.05mm, +0.05mm), indicating that the spherical aberration of this fixed-focus lens at each wavelength is well controlled and can meet the needs of wide spectrum applications.

[0118] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, combinations and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A fixed-focus lens, characterized in that: It includes a first lens, a second lens, a third lens and a fourth lens which are arranged in sequence from the object side to the image side along the optical axis; The first lens has negative optical power, the second lens has positive optical power, the third lens has positive optical power, and the fourth lens has negative optical power; The object-side surface and the image-side surface of the first lens, the third lens and the fourth lens are all aspherical surfaces; The object-side surface of the second lens is an aspherical surface, and the image-side surface is a binary diffraction surface.

2. The fixed-focus lens according to claim 1, characterized in that: The first lens is a meniscus lens, the second lens is a biconvex lens, the third lens is a biconvex lens, and the fourth lens is a meniscus lens or a biconcave lens.

3. The fixed-focus lens according to claim 1, characterized in that: The first lens, the third lens and the fourth lens are all plastic lenses.

4. The fixed-focus lens according to claim 1, characterized in that: The first lens satisfies the following conditions: -0.740≤Φ1 / Φ≤-0.600; Wherein, Φ1 is the optical focal length of the first lens, and Φ is the optical focal length of the fixed-focus lens.

5. The fixed-focus lens according to claim 1, wherein: The second lens satisfies the following condition: 0.49≤Φ2 / Φ≤0.62; Wherein, Φ2 is the optical focal length of the second lens, and Φ is the optical focal length of the fixed-focus lens.

6. The fixed-focus lens according to claim 1, wherein: Adjacent surfaces of the third lens and the fourth lens are cemented to each other to form a cemented lens group.

7. The fixed-focus lens according to claim 1, wherein: The third lens and the fourth lens satisfy the following condition: 0.08≤(Φ3+Φ4) / Φ≤0.12; Among them, Φ3 is the optical focal length of the third lens, Φ4 is the optical focal length of the fourth lens, and Φ is the optical focal length of the fixed-focus lens.

8. The fixed-focus lens according to claim 7, characterized in that: The third lens and the fourth lens also satisfy the following condition: 72≤(Vd3+d4)≤85; Wherein, Vd3 is the Abbe number of the third lens, and Vd4 is the Abbe number of the fourth lens.

9. The fixed-focus lens according to claim 1, wherein: The fixed focus lens meets the following conditions: 0.30 <T12 / TTL<0.38; Wherein, T12 is the air distance between the image surface of the first lens and the object surface of the second lens on the optical axis, and TTL is the total optical length of the fixed-focus lens system.

10. The fixed-focus lens according to claim 1, wherein: Also included is an aperture stop located between the first lens and the second lens.

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