A medium telephoto lens

Abstract

The application discloses a medium-long focal length lens, which comprises a first lens with positive refractive power, a second lens with negative refractive power, a third lens with positive refractive power, a fourth lens with negative refractive power, a fifth lens with negative refractive power and a sixth lens with positive refractive power arranged in sequence along an optical axis from an object side to an image side; and the aperture number F of the medium-long focal length lens, the focal length f3 of the third lens and the focal length f4 of the fourth lens satisfy the condition: -3.460<=F / (f3+f4) <=23.254. The medium-long focal length lens can meet the requirement of a focal length of about 12 mm, has a relatively short total optical length and a relatively small volume, the aperture number F reaches about 1.35, and the requirement of 5MP high-definition image quality is ensured.

Description

Technical Field

[0001] This invention relates to the field of optical device technology, and in particular to a medium-long telephoto lens. Background Technology

[0002] Medium to long telephoto lenses are widely used in automotive radar, advanced driver assistance systems, and video recording equipment due to their strong telephoto capabilities. However, current technology limits the focal length of medium to long telephoto lenses, resulting in larger finished lenses and higher costs. Summary of the Invention

[0003] This invention provides a medium telephoto lens that, while meeting a focal length of approximately 12mm, has a shorter overall optical length and a smaller volume.

[0004] According to one aspect of the present invention, a medium-long telephoto lens is provided, comprising a first lens with positive optical power, a second lens with negative optical power, a third lens with positive optical power, a fourth lens with negative optical power, a fifth lens with negative optical power, and a sixth lens with positive optical power arranged sequentially along the optical axis from the object side to the image side. The aperture number F of the medium telephoto lens, the focal length f3 of the third lens, and the focal length f4 of the fourth lens satisfy the following condition: -3.460 ≤ F / (f3+f4) ≤ 23.254.

[0005] Optionally, the optical power of the first lens is φ1, the optical power of the second lens is φ2, the optical power of the third lens is φ3, the optical power of the fourth lens is φ4, the optical power of the fifth lens is φ5, the optical power of the sixth lens is φ6, and the optical power of the medium-telephoto lens is φ; wherein: 0.166≤φ1 / φ3≤0.391; -1.321≤φ2 / φ≤-0.002; 0.839≤φ3 / φ6≤1.364; -1.056≤φ5 / φ≤-0.639; 1.636≤φ6 / φ≤1.961.

[0006] Optionally, the refractive index of the first lens is n1, and the Abbe number of the first lens is v1; the refractive index of the second lens is n2, and the Abbe number of the second lens is v2; the refractive index of the third lens is n3, and the Abbe number of the third lens is v3; the refractive index of the fourth lens is n4, and the Abbe number of the fourth lens is v4; the refractive index of the fifth lens is n5, and the Abbe number of the fifth lens is v5; and the refractive index of the sixth lens is n6, and the Abbe number of the sixth lens is v6; wherein: 1.50≤n1≤2.02, 26.04≤v1≤37.56; 1.54≤n²≤1.67, 22.47≤v²≤25.23; 1.49≤n3≤1.57, 63.54≤v3≤81.06; 1.61≤n4≤1.62, 29.36≤v4≤37.04; 1.61≤n5≤1.70, 19.07≤v5≤26.63; 1.53≤n6≤1.61,49.46≤v6≤59.54。

[0007] Optionally, the first lens is a biconvex lens or a meniscus lens; The second lens is a meniscus lens.

[0008] Optionally, the sixth lens is a biconvex lens or a meniscus lens.

[0009] Optionally, the radius of curvature of the object surface of the second lens is R21, and the radius of curvature of the object surface of the fifth lens is R51; wherein, R21 / R51 < 0.

[0010] Optionally, the optical back focal length of the medium telephoto lens is BFL, and the total optical length of the medium telephoto lens is TTL; wherein, BFL / TTL ≥ 0.1.

[0011] Optionally, the first lens, the third lens, and the fourth lens are all glass spherical lenses; The second lens, the fifth lens, and the sixth lens are all plastic aspherical lenses.

[0012] Optionally, the third lens and the fourth lens are cemented lenses.

[0013] Optionally, the medium telephoto lens further includes: an aperture stop; The aperture stop is located in the optical path between the second lens and the third lens.

[0014] The medium-telephoto lens provided by this invention, by reasonably setting the number of lenses and the optical power of each lens, can achieve a short overall length and small size while meeting the requirements of a medium-telephoto focal length of about 12mm. At the same time, by setting the aperture number F of the medium-telephoto lens and the focal lengths f3 and f4 of the third lens to satisfy -3.460≤ F / (f3+f4)≤23.254, it can meet the performance requirements of a large aperture, while correcting the distortion curve, achieving low distortion performance, and ensuring the 5MP high-definition image quality requirement.

[0015] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of the present invention, nor is it intended to limit the scope of the invention. Other features of the invention will become readily apparent from the following description. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the structure of a medium-telephoto lens provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of another medium-telephoto lens provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the structure of another medium-telephoto lens provided in an embodiment of the present invention. Figure 4 To and Figure 2 A corresponding axial aberration curve for a medium to long telephoto lens; Figure 5 To and Figure 2 A field curvature distortion diagram corresponding to a medium to long telephoto lens; Figure 6 To and Figure 3 A corresponding axial aberration curve for a medium to long telephoto lens; Figure 7 To and Figure 3 The corresponding field curvature distortion diagram for a medium to long telephoto lens. Detailed Implementation

[0018] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0019] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0020] This invention provides a medium telephoto lens that, while satisfying the requirement of a medium telephoto focal length, has a relatively short overall length and a small volume. Figure 1 This is a structural schematic diagram of a medium-telephoto lens provided in an embodiment of the present invention, as shown below. Figure 1 As shown, the medium-telephoto lens provided in this embodiment of the invention includes a first lens 10 with positive optical power, a second lens 20 with negative optical power, a third lens 30 with positive optical power, a fourth lens 40 with negative optical power, a fifth lens 50 with negative optical power, and a sixth lens 60 with positive optical power, arranged sequentially from the object side to the image side along the optical axis; wherein, the aperture number F of the medium-telephoto lens and the focal length f3 of the third lens and the focal length f4 of the fourth lens satisfy: -3.460≤ F / (f3+f4)≤23.254.

[0021] Specifically, optical power is equal to the difference between the convergence of the image-side beam and the convergence of the object-side beam; it characterizes the ability of an optical system to deflect light. The larger the absolute value of the optical power, the stronger the bending ability of light; the smaller the absolute value, the weaker the bending ability. When the optical power is positive, the refraction of light is converging; when the optical power is negative, the refraction of light is diverging. Optical power can be used to characterize a single refractive surface of a lens (i.e., one surface of the lens), a single lens, or a system formed by multiple lenses (i.e., a lens group). In the medium-long telephoto lens provided in this embodiment, all lenses can be fixed in a single lens barrel (…). Figure 1(Not shown in the image). By rationally allocating the optical power of each lens, the balance of the incident angles of the front and rear lenses is ensured, reducing lens sensitivity and improving lens stability. This allows the lens to achieve a shorter length and smaller size while meeting the requirements of a 12mm medium telephoto lens. The aperture number is the ratio of the image-side focal length to the entrance pupil diameter of the optical system. By setting the aperture number F and the focal lengths f3 of the third lens 30 and f4 of the fourth lens 40 to -3.460 ≤ F / (f3+f4) ≤ 23.254, it is possible to ensure that the aperture number can reach the range of 1.35 to 1.6 while meeting the requirements of a shorter length and smaller size. This achieves large aperture performance and also better corrects distortion curves, achieving low distortion performance and meeting the requirements of 5MP high-definition image quality.

[0022] Optionally, the optical power of the first lens 10 is φ1, the optical power of the second lens 20 is φ2, the optical power of the third lens 30 is φ3, the optical power of the fourth lens 40 is φ4, the optical power of the fifth lens 50 is φ5, the optical power of the sixth lens 60 is φ6, and the optical power of the medium telephoto lens is φ; wherein: 0.166≤φ1 / φ3≤0.391; -1.321≤φ2 / φ≤-0.002; 0.839≤φ3 / φ6≤1.364; -1.056≤φ5 / φ≤-0.639; 1.636≤φ6 / φ≤1.961.

[0023] In this way, by using the above combination of optical power for each lens, the overall optical length of the lens can be shortened to meet the 5MP resolution requirement, achieving small size and high pixel performance.

[0024] Optionally, the refractive index of the first lens 10 is n1, and the Abbe number of the first lens 10 is v1; the refractive index of the second lens 20 is n2, and the Abbe number of the second lens 20 is v2; the refractive index of the third lens 30 is n3, and the Abbe number of the third lens 30 is v3; the refractive index of the fourth lens 40 is n4, and the Abbe number of the fourth lens 40 is v4; the refractive index of the fifth lens 50 is n5, and the Abbe number of the fifth lens 50 is v5; and the refractive index of the sixth lens 60 is n6, and the Abbe number of the sixth lens 60 is v6; wherein: 1.50≤n1≤2.02, 26.04≤v1≤37.56; 1.54≤n²≤1.67, 22.47≤v²≤25.23; 1.49≤n3≤1.57, 63.54≤v3≤81.06; 1.61≤n4≤1.62, 29.36≤v4≤37.04; 1.61≤n5≤1.70, 19.07≤v5≤26.63; 1.53≤n6≤1.61,49.46≤v6≤59.54。

[0025] Thus, by using the above-mentioned combination of refractive index and dispersion coefficient for each lens, it is beneficial to correct transverse chromatic aberration and axial chromatic aberration, and ensure tolerance balance.

[0026] Optionally, the first lens is a biconvex lens or a meniscus lens; the second lens is a meniscus lens.

[0027] Among them, such as Figure 1 As shown, when the first lens 10 is a meniscus lens, the object-side surface of the first lens 10 convexes towards the object side, and the image-side surface of the first lens 10 is concave towards the image side; or, as... Figure 2 As shown, when the first lens 10 is a biconvex lens, the object-side surface of the first lens 10 convexes towards the object side, and the image-side surface of the first lens 10 convexes towards the image side.

[0028] refer to Figure 1 and Figure 2 In any of the accompanying drawings, when the second lens is a meniscus lens, the object-side surface of the second lens convexes towards the object side, and the image-side surface of the second lens is concave towards the image side.

[0029] In this way, by setting the shapes of the first and second lenses, it is possible to ensure that the focal length of the optical system reaches 12mm, thus achieving telephoto performance.

[0030] Optionally, the sixth lens is a biconvex lens or a meniscus lens.

[0031] Among them, such as Figure 2 As shown, when the sixth lens 60 is a meniscus lens, the object-side surface of the sixth lens 60 convexes towards the object side, and the image-side surface of the sixth lens 60 is concave towards the image side; or, as... Figure 3 As shown, when the sixth lens 60 is a biconvex lens, the object-side surface of the sixth lens 60 convexes towards the object side, and the image-side surface of the sixth lens 60 convexes towards the image side.

[0032] By setting the shape of the sixth lens, the overall optical length of the optical system can be shortened while ensuring that the optical system has long focal length performance, thus enabling the optical system to meet the requirements of small size.

[0033] Optional, see reference Figure 1-3 In any of the attached figures, the radius of curvature of the object surface of the second lens 20 is R21, and the radius of curvature of the object surface of the fifth lens 50 is R51; wherein, R21 / R51 < 0.

[0034] Specifically, by setting the radius of curvature R21 of the object-side surface of the second lens 20 and the radius of curvature R51 of the object-side surface of the fifth lens 50 to R21 / R51 < 0, when the second lens 20 is a meniscus lens, the fifth lens 50 can also be a meniscus lens, and the second lens 20 and the fifth lens 50 are curved in opposite directions. That is, if the object-side surface of the second lens 20 is convex towards the object side and the image-side surface of the second lens 20 is concave towards the image side, the object-side surface of the fifth lens 50 will be concave towards the object side and the image-side surface of the fifth lens 50 will be convex towards the image side. This is beneficial for correcting aberrations such as spherical aberration, astigmatism, coma, and field curvature.

[0035] Optionally, the optical back focal length of the medium-telephoto lens is BFL, and the total optical length of the medium-telephoto lens is TTL; wherein, BFL / TTL ≥ 0.1. This helps to reduce the lens size and adapt to a 1 / 2.7′ target surface sensor.

[0036] Optional, continue to refer to Figure 1-3 In any of the attached figures, the first lens 10, the third lens 30, and the fourth lens 40 are all glass spherical lenses; the second lens 20, the fifth lens 50, and the sixth lens 60 are all plastic aspherical lenses.

[0037] Among them, the cost of plastic lenses is much lower than that of glass lenses, and the performance of plastic lenses and glass lenses compensate for each other. By setting three plastic aspherical lenses and three glass spherical lenses, it is possible to ensure good image quality and low cost, and also to ensure that medium and long telephoto lenses can still be used normally in high and low temperature environments.

[0038] Optional, continue to refer to Figure 1-3 In any of the attached figures, the third lens 30 and the fourth lens 40 are cemented lenses.

[0039] The use of cemented lenses effectively reduces the air gap between the third lens 30 and the fourth lens 40, thereby reducing the overall length of the lens. Furthermore, cemented lenses can minimize or eliminate chromatic aberration, allowing for adequate correction of various aberrations in medium to long telephoto lenses. This improves resolution and optimizes optical performance such as distortion and CRA while maintaining a compact structure. It also reduces light loss caused by inter-lens reflections, increasing illumination and thus improving image quality and sharpness. Additionally, the use of cemented lenses reduces the number of assembly components between the two lenses, simplifying the assembly process in lens manufacturing, reducing costs, and minimizing tolerance sensitivity issues such as tilting / eccentricity of lens units during assembly.

[0040] Optional, continue to refer to Figure 1-3In any of the accompanying drawings, the medium telephoto lens also includes an aperture stop 70; the aperture stop 70 is located in the optical path between the second lens 20 and the third lens 30. By adding the aperture stop 70, the propagation direction of the light beam can be adjusted, which is beneficial to improving image quality. Furthermore, by placing the aperture stop 70 in an appropriate position, it helps to improve relative illumination and reduce CRA.

[0041] The medium-telephoto lens provided by this invention, through the reasonable setting of the number of lenses and the optical power of each lens, achieves a short overall length and small size while meeting the requirements of a medium-telephoto focal length of approximately 12mm. Simultaneously, by setting the aperture F of the medium-telephoto lens to the focal lengths f3 and f4 of its third and fourth lenses to satisfy -3.460 ≤ F / (f3+f4) ≤ 23.254, it ensures that the aperture can reach the range of 1.35 to 1.6 while maintaining a short length and small size, achieving large aperture performance. Furthermore, it can better correct distortion curves, achieving low distortion performance and meeting the requirements of 5MP high-definition image quality.

[0042] As a feasible implementation method, the design parameters of various lenses in a medium to long telephoto lens are described below as an example.

[0043] In one exemplary embodiment, Table 1 is related to Figure 2 The specific parameters of each lens in the corresponding medium to long telephoto lens.

[0044] Table 1 shows another parameter design for each lens in the telephoto lens. The medium telephoto lens in this embodiment has a focal length f of 11.62mm, an aperture F of 1.6, an optical total length TTL of less than or equal to 22.355mm, a field of view of 31.3°, and optical distortion as low as 1.35%.

[0045] Table 2 shows the design parameters of the surface type, radius of curvature, thickness, material, semi-diameter, and K-coefficient of each lens in a medium-telephoto lens corresponding to Table 1.

[0046] Table 2 shows another design parameter for each lens in the telephoto lens. refer to Figure 2The medium-telephoto lens provided in this embodiment includes a first lens 10, a second lens 20, a third lens 30, a fourth lens 40, a fifth lens 50, and a sixth lens 60 arranged sequentially along the optical axis from the object plane to the image plane. Furthermore, a flat glass plate (not shown in the figure) can be disposed between the image plane and the sixth lens 60; this flat glass plate serves to filter light and provide protection. Table 2 shows the optical physical parameters of each lens in the medium-telephoto lens provided in this embodiment, including surface type, radius of curvature, thickness, and material. The surface number is determined by the order of the surfaces of each lens. For example, "1" represents the object-side surface of the first lens 10, "2" represents the image-side surface of the first lens 10, "3" represents the object-side surface of the second lens 20, "4" represents the image-side surface of the second lens 20, and so on. "13" represents the object-side surface of the flat glass, "14" represents the image-side surface of the flat glass, and "IMA" represents the image plane. The radius of curvature represents the degree of curvature of the lens surface. A positive value indicates that the surface bends towards the image plane, and a negative value indicates that the surface bends towards the object plane. The thickness represents the central axial distance from the current surface to the next surface. The units for both the radius of curvature and the thickness are millimeters (mm).

[0047] The equation for the shape of the aspherical surface in the medium-to-long telephoto lens of this embodiment is: Where Z represents the distance sag from the vertex of the aspherical surface at a height of y along the optical axis; c = 1 / R, where R represents the paraxial radius of curvature of the aspherical surface shape; K represents the conic coefficient; and A, B, C, D, E, and F represent higher-order aspherical coefficients. For example, Table 3 shows the coefficients related to... Figure 2 The aspherical surface parameters corresponding to each lens in the medium to long telephoto lens are shown.

[0048] Table 3 lists the aspherical surface parameters of each lens in the telephoto lens. Where -1.836162E-05 indicates that the coefficient A of surface number 3 is -1.836162 × 10⁻⁵. -5 .

[0049] Figure 4 To and Figure 2 The corresponding axial aberration curve for a medium to long telephoto lens. Figure 5 To and Figure 2 The corresponding field curvature distortion diagram for a medium to long telephoto lens, by Figures 4-5 It can be seen that the medium-telephoto lens provided in this embodiment has low distortion and high pixel performance while meeting the requirements of low cost, small size, short optical length, and a focal length of 12mm.

[0050] In yet another exemplary embodiment, Table 4 is related to Figure 3 The specific parameters of each lens in the corresponding medium to long telephoto lens.

[0051] Table 4 shows another parameter design for each lens in a telephoto lens. The medium telephoto lens in this embodiment has a focal length f of up to 12mm, an aperture f of up to 1.35, a total optical length TTL of less than or equal to 22.375mm, a field of view of up to 30.76°, and optical distortion as low as -0.29%.

[0052] Table 5 shows the design parameters of the surface type, radius of curvature, thickness, material, semi-diameter, and K-coefficient of each lens in a medium-telephoto lens corresponding to Table 4.

[0053] Table 5 lists one design parameter for each lens in the telephoto lens. refer to Figure 3 The medium-telephoto lens provided in this embodiment includes a first lens 10, a second lens 20, a third lens 30, a fourth lens 40, a fifth lens 50, and a sixth lens 60 arranged sequentially along the optical axis from the object plane to the image plane. Furthermore, a flat glass plate (not shown in the figure) can be disposed between the image plane and the sixth lens 60; this flat glass plate serves to filter light and provide protection. Table 5 shows the optical physical parameters of each lens in the medium-telephoto lens provided in this embodiment, including surface type, radius of curvature, thickness, and material. The surface number is determined by the order of the surfaces of each lens. For example, "1" represents the object-side surface of the first lens 10, "2" represents the image-side surface of the first lens 10, "3" represents the object-side surface of the second lens 20, "4" represents the image-side surface of the second lens 20, and so on. "13" represents the object-side surface of the flat glass, "14" represents the image-side surface of the flat glass, and "IMA" represents the image plane. The radius of curvature represents the degree of curvature of the lens surface. A positive value indicates that the surface bends towards the image plane, and a negative value indicates that the surface bends towards the object plane. The thickness represents the central axial distance from the current surface to the next surface. The units for both the radius of curvature and the thickness are millimeters (mm).

[0054] The equation for the shape of the aspherical surface in the medium-to-long telephoto lens of this embodiment is: Where Z represents the distance sag from the vertex of the aspherical surface at a height of y along the optical axis; c = 1 / R, where R represents the paraxial radius of curvature of the aspherical surface shape; K represents the conic coefficient; and A, B, C, D, E, and F represent higher-order aspherical coefficients. For example, Table 6 shows the coefficients related to... Figure 3The aspherical surface parameters corresponding to each lens in the medium to long telephoto lens are shown.

[0055] Table 6 shows another type of aspherical surface parameters for each lens in telephoto lenses. Where -1.143869E-03 indicates that the coefficient A of surface number 3 is -1.143869 × 10 -3 . Figure 6 To and Figure 3 The corresponding axial aberration curve for a medium to long telephoto lens. Figure 7 To and Figure 3 The corresponding field curvature distortion diagram for a medium to long telephoto lens, by Figures 6-7 It can be seen that the medium-telephoto lens provided in this embodiment has low distortion and high pixel performance while meeting the requirements of low cost, small size, short optical length, and a focal length of 12mm.

[0056] The specific embodiments described above do not constitute a limitation on the scope of protection of this 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 principles of this invention should be included within the scope of protection of this invention.

Claims

1. A medium-telephoto lens, characterized in that, The lens comprises a first lens with positive optical power, a second lens with negative optical power, a third lens with positive optical power, a fourth lens with negative optical power, a fifth lens with negative optical power, and a sixth lens with positive optical power, arranged sequentially from the object side to the image side along the optical axis; the medium-telephoto lens has six lenses with optical power. The aperture F of the medium telephoto lens satisfies the following condition: -3.460mm, the focal length f3 of the third lens, and the focal length f4 of the fourth lens. -1 ≤ F / (f3+f4)≤23.254mm -1 ; The first lens has an optical power of φ1, the second lens has an optical power of φ2, the third lens has an optical power of φ3, the fifth lens has an optical power of φ5, the sixth lens has an optical power of φ6, and the medium-telephoto lens has an optical power of φ; wherein: 0.166≤φ1 / φ3≤0.391; -1.321≤φ2 / φ≤-0.002; 0.839≤φ3 / φ6≤1.364; -1.056≤φ5 / φ≤-0.639; 1.636≤φ6 / φ≤1.

961.

2. The medium telephoto lens according to claim 1, characterized in that, The first lens has a refractive index of n1 and an Abbe number of v1; the second lens has a refractive index of n2 and an Abbe number of v2; the third lens has a refractive index of n3 and an Abbe number of v3; the fourth lens has a refractive index of n4 and an Abbe number of v4; the fifth lens has a refractive index of n5 and an Abbe number of v5; and the sixth lens has a refractive index of n6 and an Abbe number of v6; wherein: 1.50≤n1≤2.02, 26.04≤v1≤37.56; 1.54≤n²≤1.67, 22.47≤v²≤25.23; 1.49≤n3≤1.57, 63.54≤v3≤81.06; 1.61≤n4≤1.62, 29.36≤v4≤37.04; 1.61≤n5≤1.70, 19.07≤v5≤26.63; 1.53≤n6≤1.61,49.46≤v6≤59.54。 3. The medium telephoto lens according to claim 1, characterized in that, The first lens is a biconvex lens or a meniscus lens; The second lens is a meniscus lens.

4. The medium telephoto lens according to claim 1, characterized in that, The sixth lens is a biconvex lens or a meniscus lens.

5. The medium telephoto lens according to claim 1, characterized in that, The radius of curvature of the object surface of the second lens is R21, and the radius of curvature of the object surface of the fifth lens is R51; wherein, ≤R21 / R51≤ .

6. The medium telephoto lens according to claim 1, characterized in that, The optical back focal length of the medium-telephoto lens is BFL, and the total optical length of the medium-telephoto lens is TTL; wherein, ≤BFL / TTL≤ .

7. The medium telephoto lens according to claim 1, characterized in that, The first lens, the third lens, and the fourth lens are all glass spherical lenses; The second lens, the fifth lens, and the sixth lens are all plastic aspherical lenses.

8. The medium telephoto lens according to claim 1, characterized in that, The third lens and the fourth lens are cemented lenses.

9. The medium telephoto lens according to claim 1, characterized in that, Also includes: Aperture; The aperture stop is located in the optical path between the second lens and the third lens.

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