Prime lens

Abstract

The utility model discloses a prime lens which comprises three lens groups. The first lens group with the positive focal power sequentially comprises two meniscus positive lenses, a meniscus negative lens and a set of triple cemented positive lenses, and an object side lens of the triple cemented positive lenses is an ultra-low dispersion convex lens. The second lens group with the negative focal power is configured to be a negative lens and can move along the central axis during focusing, and the focal length absolute value of the second lens group is 0.6 times that of the prime lens. And the third lens group with the positive focal power sequentially comprises a pair of doublet negative lenses, a piece of double-convex spherical positive lens and the other pair of doublet negative lenses. The prime lens does not contain an aspherical lens, is compact in structure and low in focusing power consumption, can be adaptive to a large aperture, and is suitable for a single-electric camera.

Description

Technical Field

[0001] This utility model belongs to the field of photographic lens technology, specifically relating to a fixed-focus lens that can be used in a mirrorless camera. Background Technology

[0002] Mirrorless cameras (SLT cameras) are currently popular non-professional digital cameras, equipped with full-frame or APS-C sensors and interchangeable lenses. High-quality lenses are expensive, and autofocus consumes a lot of power.

[0003] The internal focusing imaging lens disclosed in Chinese patents CN111443471A and CN111474691A is adapted to a single-lens reflex camera. It only moves one negative lens when focusing, resulting in fast focusing speed and low power consumption. Utility Model Content

[0004] The technical problem to be solved by this invention is how to improve a fixed-focus lens to adapt it to a mirrorless camera.

[0005] This utility model discloses a fixed-focus lens.

[0006] The fixed-focus lens includes, in sequence along the central axis, a first lens group with positive optical power, a second lens group with negative optical power, and a third lens group with positive optical power. It also includes an aperture stop. When focusing on an object from infinity to a nearby object, the first lens group, the aperture stop, and the third lens group are fixed relative to the image plane, while the second lens group moves along the central axis to the image plane.

[0007] The first lens group includes, along the central axis, the following components in sequence:

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

[0009] The second lens is configured as a positive lens, with a spherical convex surface on the object side and a spherical concave surface on the image side;

[0010] The third lens is configured as a negative lens, with a convex spherical surface on the object side and a concave spherical surface on the image side;

[0011] The fourth lens is configured as a biconvex spherical positive lens with a refractive index of 1.45~1.55 and an Abbe number greater than 80;

[0012] The fifth lens, configured as a biconcave spherical negative lens, is cemented together with the fourth lens; and

[0013] The sixth lens is configured as a biconvex spherical positive lens and is cemented together with the fifth lens;

[0014] The second lens group consists only of the seventh lens, which has a convex spherical surface on the object side and a concave spherical surface on the image side, and its absolute focal length is 0.6 times the absolute focal length of the fixed-focus lens.

[0015] The third lens group includes, along the central axis, the following components in sequence:

[0016] The eighth lens is configured as a biconcave spherical negative lens;

[0017] The ninth lens is configured as a biconvex spherical positive lens and is cemented together with the eighth lens;

[0018] The tenth lens is configured as a biconvex spherical positive lens;

[0019] The eleventh lens is configured as a biconvex spherical positive lens; and

[0020] The twelfth lens is configured as a biconcave spherical negative lens and is cemented together with the eleventh lens;

[0021] In some embodiments of this application, the focal length of the fixed-focus lens is 50-60 mm; furthermore, the second lens, the ninth lens, and the eleventh lens all have a refractive index of 1.59 and an Abbe number of 68.3.

[0022] The following beneficial effects can be obtained by implementing the technical solution of this utility model.

[0023] This utility model discloses a fixed-focus lens comprising three lens groups. The first lens group, with positive optical power, sequentially includes two meniscus positive lenses, one meniscus negative lens, and a set of cemented triplicate positive lenses, the object-side element of which is an extra-low dispersion convex lens. The second lens group, with negative optical power, consists of a single negative lens that can move along the central axis during focusing; its absolute focal length is 0.6 times the absolute focal length of the fixed-focus lens. The third lens group, with positive optical power, sequentially includes a cemented doublet negative lens, a biconvex spherical positive lens, and another cemented doublet negative lens. This fixed-focus lens does not contain aspherical lenses, has a compact structure, low focusing power consumption, can be adapted to large apertures, and is suitable for mirrorless cameras. Attached Figure Description

[0024] The accompanying figures should be used in conjunction with the detailed implementation section.

[0025] Figure 1 This is a cross-sectional view of the central axis of the fixed-focus lens in Embodiment 1;

[0026] Figure 2 This is the optical path diagram of the fixed-focus lens when focusing on infinity in Example 1. Figure 1 and Figure 2 The captions represent: IMG - image plane, FIT - filter protector, STO - aperture stop, Z1 - central axis, and T1 - movement range.

[0027] Figure 3a and Figure 3bThe images shown are the spherical aberration curve and transverse chromatic aberration curve of the fixed-focus lens calculated by ZEMAX software in Example 1. The calculation conditions are: incident light mixing range 425~658nm, incident pupil radius 18.0404mm, aperture F / 1.4, and infinity focus state.

[0028] Figure 4a and Figure 4b The graphs shown are two modulation transfer function (MTF) curves of a fixed-focus lens calculated by ZEMAX software in Example 1, with the calculation conditions as described above. Detailed Implementation

[0029] The embodiments are described below with reference to the accompanying drawings.

[0030] In this specification, unless otherwise specified, one embodiment, some embodiments and other embodiments are used to distinguish different embodiments and do not refer to all embodiments in general; the directions / positions indicated by top, bottom, center, edge, inner, outer, far, near, long, wide, vertical, horizontal, up, down, front, back, left, right, etc. are based on the observation angle of the accompanying drawings and should not be understood as the component / device being located in a specific position or facing a specific direction.

[0031] The lens grouping method described in this specification is for the convenience of illustrating the embodiments only. It is understood that other grouping methods may be used in optical design analysis, lens forming, and lens assembly.

[0032] Positive lens and positive optical power both refer to an optical system with a positive image-side focal length, which converges parallel incident light beams. Negative lens and negative optical power both refer to an optical system with a negative image-side focal length, which diverges parallel incident light beams. The light-transmitting surface of a lens can be simply referred to as the "surface." The side of the lens facing the object being photographed by the camera is called the "object-side surface," and the side of the lens facing the camera's image sensor is called the "image-side surface."

[0033] The aperture stop, also known as the "stop," is numbered along with the lens's light-transmitting surface.

[0034] Example 1

[0035] A fixed-focus lens is disclosed.

[0036] This fixed-focus lens has a focal length of 56mm and is compatible with a maximum aperture of F / 1.4 and an ASP-C format imaging sensor.

[0037] Please see Figure 1 , Figure 2 . Figure 1 The lens tube is not shown in the figure. The central axis Z1 (also known as the optical axis, principal axis, or principal optical axis) points from left to right towards the image plane IMG. Figure 2 In the image sensor (IMG), a filter protector (FIT) is located in front of the image sensor (IMG).

[0038] Table 1 below lists the lens data and positional relationships of this fixed-focus lens.

[0039]

[0040] In Table 1, the object-side surface of the first lens L1 is denoted as surface S1. Other light-transmitting surfaces (including the plane containing the aperture stop STO and the lens cementing surface) are numbered in ascending order along the central axis Z1. The image-side surface of the twelfth lens L12 is denoted as surface S21. "Radius of curvature R" represents the paraxial radius of curvature at the vertex of a surface. The i-th surface S... i and the (i+1)th surface S i+1 For lenses between two adjacent lenses: their "thickness" is the central axis thickness, and their "refractive index Nd" and "Abbe number Nd" are measured values ​​on the d-line. The distance between the image-side surface of the first lens and the object-side surface of the second lens on the central axis Z1 is called the central axis distance between them.

[0041] The 12 lenses of this fixed-focus lens are numbered in ascending order along the central axis, and are all glass lenses, which can be divided into three groups. In the lens barrel of this fixed-focus lens ( Figure 1 (Not shown) Along the central axis Z1, from the object side to the image side, the first lens group G1, the aperture stop STO, the second lens group G2, and the third lens group G3 are arranged in sequence.

[0042] During the focusing process of this fixed-focus lens, only the second lens group G2 can be moved back and forth along the central axis Z1 by a stepper motor. The first lens group G1, the aperture stop STO, and the third lens group G3 are all fixed in position relative to the image plane IMG. The distance between the aperture stop STO and the central axis of the surface S10 of the first lens group G1 is approximately 1.5 mm, and the distance between the central axis of the image plane IMG and the surface S21 of the third lens group G3 is approximately 19.7 mm.

[0043] The first lens group G1 has positive optical power and consists of six glass lenses: two meniscus positive lenses, one meniscus negative lens, and a set of cemented triplet positive lenses. These six glass lenses are arranged sequentially along the central axis Z1: lens L1, lens L2, lens L3, lens L4, lens L5, and lens L6. Lens L5 is a biconcave spherical negative lens, cemented together with lens L4 and lens L6 to form a set of cemented triplet positive lenses. Both the object-side and image-side surfaces of this cemented triplet positive lens are convex spherical.

[0044] The second lens group G2, with a focal length of -34.42mm, is composed of a single glass lens and can move along the central axis during focusing. Its absolute focal length is 0.61 times that of the fixed-focus lens. The object-side surface of the aforementioned single glass lens is nearly flat, and its radius of curvature is positive.

[0045] The third lens group G3 has positive optical power and consists of five glass lenses: a cemented doublet negative lens, a biconvex spherical positive lens, and another cemented doublet negative lens. These five glass lenses are arranged sequentially along the central axis Z1: lens L8, lens L9, lens L10, lens L11, and lens L12. In the cemented doublet negative lens formed by cementing lens L8 and lens L9, the object-side surface is concave spherical, and the image-side surface is convex spherical; the cemented surface is nearly planar, and the radius of curvature is positive. In the cemented doublet negative lens formed by cementing lens L11 and lens L12, the object-side surface is convex spherical, and the image-side surface is concave spherical; the radius of curvature of the cemented surface is negative.

[0046] All 12 glass lenses are arranged sequentially along the central axis Z1 to form the three lens groups mentioned above, and are installed inside the lens barrel. The lens barrel is equipped with a stepper motor and a power input interface.

[0047] Figure 1 The second lens group G2 is located at the object-side boundary of its movement range T1, approximately 1.95 mm away from the central axis of the aperture stop STO. At this time, the fixed-focus lens is in focus at infinity. Please refer to the optical path diagram. Figure 2 When this fixed-focus lens focuses on a close-up object, the second lens group G2 moves along the central axis to the image plane IMG, without exceeding the image-side boundary of its movement range T1. Because only a single lens element moves, the focusing electrical load of this fixed-focus lens is low, reducing camera power consumption.

[0048] The specific focal length of each glass lens, each set of cemented lenses, and the three lens groups of this fixed-focus lens can be calculated using the data in Table 1. Details of each lens are as follows.

[0049] The first lens L1 is configured as a positive lens, with a spherical convex surface on the object side and a radius of curvature of +47.393mm, a spherical concave surface on the image side and a radius of curvature of +244.881mm, a central axis thickness of 4.651mm, a refractive index of 2.00, and an Abbe number of 25.4.

[0050] The second lens L2 is configured as a positive lens, with a spherical convex surface on the object side and a radius of curvature of +31.581mm, a spherical concave surface on the image side and a radius of curvature of +51.220mm, a central axis thickness of 3.811mm, a refractive index of 1.59, an Abbe number of 68.3, and a central axis distance of 0.150mm from the first lens L1.

[0051] The third lens L3 is configured as a negative lens. Its object side is a spherical convex surface with a radius of curvature of +138.198 mm, and its image side is a spherical concave surface with a radius of curvature of +21.779 mm. Its central axis thickness is 1.000 mm, its refractive index is 1.74, its Abbe number is 27.8, and its central axis distance from that of the second lens L2 is 1.641 mm.

[0052] The fourth lens L4 is configured as a biconvex spherical positive lens with an object-side radius of curvature of +36.985mm, an image-side radius of curvature of -27.954mm, a central axis thickness of 7.970mm, a refractive index of 1.50, an Abbe number of 81.6, and a central axis distance of 2.379mm from the third lens L3.

[0053] The fifth lens, L5, is configured as a biconcave spherical negative lens with an object-side radius of curvature of -27.954 mm, an image-side radius of curvature of +21.807 mm, a central axis thickness of 1.50 mm, a refractive index of 1.67, and an Abbe number of 32.2. It is cemented together with the fourth lens, L4.

[0054] The sixth lens, L6, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +21.807mm, an image-side radius of curvature of -87.127mm, a central axis thickness of 7.405mm, a refractive index of 1.91, and an Abbe number of 35.3. It is cemented with the fifth lens, L5, and the distance between the lens and the central axis of the aperture stop, STO, is 1.500mm.

[0055] The seventh lens, L7, is configured as a negative lens. Its object-side surface is a convex spherical surface with a radius of curvature of +992.649 mm, and its image-side surface is a concave spherical surface with a radius of curvature of +20.896 mm. Its central axis thickness is 0.800 mm, its refractive index is 1.62, its Abbe number is 63.4, and its focal length is -34.417 mm. When in infinity focus, its distance from the central axis of the aperture stop STO is 1.945 mm.

[0056] The eighth lens, L8, is configured as a biconcave spherical negative lens with an object-side radius of curvature of -18.414 mm, an image-side radius of curvature of +218.495 mm, a central axis thickness of 1.000 mm, a refractive index of 1.74, and an Abbe number of 27.8. When in infinity focusing state, its central axis distance from that of the seventh lens, L7, is 11.705 mm.

[0057] The ninth lens, L9, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +218.495mm, an image-side radius of curvature of -22.422mm, a central axis thickness of 6.000mm, a refractive index of 1.59, and an Abbe number of 68.3. It is cemented together with the eighth lens, L8.

[0058] The tenth lens, L10, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +110.916 mm, an image-side radius of curvature of -47.839 mm, a central axis thickness of 5.057 mm, a refractive index of 2.00, an Abbe number of 25.4, and a central axis distance of 0.150 mm from the ninth lens, L9.

[0059] The eleventh lens, L11, is configured as a biconvex spherical positive lens with an object-side radius of curvature of +50.292mm, an image-side radius of curvature of -22.764mm, a central axis thickness of 10.000mm, a refractive index of 1.59, an Abbe number of 68.3, and a central axis distance of 0.150mm from that of the tenth lens, L10.

[0060] The twelfth lens, L12, is configured as a biconcave spherical negative lens with an object-side radius of curvature of -22.764 mm, an image-side radius of curvature of +58.653 mm, a central axis thickness of 1.500 mm, a refractive index of 1.60, and an Abbe number of 38.0. It is cemented to the eleventh lens, L11, and the distance between the lens and the central axis of the image plane IMG is approximately 19.7 mm.

[0061] The second lens L2, the ninth lens L9, and the eleventh lens L11 of this fixed-focus lens have the same refractive index and Abbe number, and can use the same grade of optical glass.

[0062] For the spherical aberration curves along the perpendicular axis (i.e., radial and Y-axis directions) of this fixed-focus lens under the following conditions: incident light mixing range of 425~658nm, incident pupil radius of 18.0404mm, aperture of F / 1.4, and infinity focus. Figure 3a Please refer to the calculated vertical color difference curve. Figure 3b . Figure 3a The spherical aberration of all curves is in the range of -0.03 to 0.08 mm. Figure 3b The color difference of all curves is within the range of -4.0 to 2.5 micrometers.

[0063] Please see Figure 4a and Figure 4b The diagram shows the modulation transfer function (MTF) curve of the fixed-focus lens calculated under the above conditions. Figure 4a It can be seen that the MTF values ​​of each curve decrease relatively uniformly within the frequency range of 0~30 lp / mm, with most curves distributed within a narrow range of the vertical axis, and only the 14.2 mm-sagittal curve ultimately slightly exceeding 0.5. From Figure 4b It can be seen that the image quality of this fixed-focus lens at an aperture of F / 1.4 can meet the requirements of APS-C cameras.

[0064] This fixed-focus lens has only 12 spherical lens elements, including three sets of cemented lenses. The positive and negative lenses of the cemented lenses have a large difference in dispersion coefficient, which can correct axial chromatic aberration, spherical aberration, and distortion aberration. It can also control the axial length, and reduce the tolerance sensitivity of image quality, making it easier to manufacture and assemble.

[0065] In this fixed-focus lens, the first lens group comprises two meniscus positive lenses, one meniscus negative lens, and a set of cemented triplet positive lenses, the object-side element of which is an extra-low dispersion convex lens. The second lens group consists of a single negative lens that can move along the central axis during focusing; its absolute focal length is 0.6 times the absolute focal length of the fixed-focus lens. The third lens group comprises a cemented doublet negative lens, a biconvex spherical positive lens, and another cemented doublet negative lens. This fixed-focus lens does not contain aspherical lenses, has a compact structure, low focusing power consumption, is compatible with large apertures, and is suitable for mirrorless cameras.

[0066] As a replaceable component for APS-C cameras, especially mirrorless cameras, for other aspects of the structure and function of this fixed-focus lens, please refer to paragraphs 0110 to 0122 on pages 10-11 of the specification of patent CN10847825A, or refer to commercially available fixed-focus lenses for standard focal length cameras.

[0067] In other embodiments, the lens can be made of other types of optical materials, the stepper motor can be replaced by a voice coil motor or an ultrasonic motor, and the total focal length of the fixed-focus lens can be any value within the range of 50-60mm. Based on the lens data of Embodiment 1, the surface shape, central axis thickness, and central axis spacing of each lens can be adaptively adjusted to achieve clear imaging and other technical requirements of the camera. Specifically:

[0068] The refractive index of the first lens L1 is 1.95~2.05, and the Abbe number is 23~28;

[0069] The second lens L2, the ninth lens L9, and the eleventh lens L11 can be made of the same optical material with a refractive index of 1.55~1.65 and an Abbe number of 66~71.

[0070] The refractive index of the third lens L3 is 1.70~1.80, and the Abbe number is 25~30;

[0071] The fourth lens L4 has a refractive index of 1.45~1.55 and an Abbe number greater than 80;

[0072] The refractive index of the fifth lens L5 is 1.65~1.75, and the Abbe number is 25~35;

[0073] The refractive index of the sixth lens L6 is 1.85~1.95, and the Abbe number is 32~37;

[0074] The refractive index of the seventh lens L7 is 1.55~1.65, and the Abbe number is 60~65;

[0075] The refractive index of the eighth lens L8 is 1.70~1.80, and the Abbe number is 25~30;

[0076] The refractive index of the tenth lens L10 is 1.95~2.05, and the Abbe number is 22~27;

[0077] The refractive index of the twelfth lens L12 is 1.55~1.65, and the Abbe number is 35~40.

[0078] All the embodiments, application examples, and technical analyses described above are intended to introduce the technical concept and features of this utility model, enabling those skilled in the art to implement the technical solution of this utility model, and do not constitute any limitation on the scope of protection of this utility model. Simple modifications and equivalent transformations to the above embodiments are all within the scope of protection of the claims of this utility model.

Claims

1. A fixed focus lens, comprising, in order along a central axis, a first lens group with positive refractive power, a second lens group with negative refractive power, and a third lens group with positive refractive power, and further comprising an aperture stop, wherein, when focusing from infinity to a close range object, the first lens group, the aperture stop, and the third lens group are fixed relative to an image plane, and the second lens group moves along the central axis toward the image plane; the first lens group comprises, in order along the central axis, a first lens configured as a positive lens with a spherical convex object side surface and a spherical concave image side surface, a second lens configured as a positive lens with a spherical convex object side surface and a spherical concave image side surface, a third lens configured as a negative lens with a spherical convex object side surface and a spherical concave image side surface, a fourth lens configured as a biconvex positive lens with a refractive index of 1.45-1.55 and an Abbe number greater than 80, a fifth lens configured as a biconcave negative lens and cemented with the fourth lens, and a sixth lens configured as a biconvex positive lens and cemented with the fifth lens; wherein the second lens group comprises only a seventh lens with a spherical convex object side surface and a spherical concave image side surface and an absolute focal length of 0.6 times an absolute focal length of the fixed focus lens; and wherein the third lens group comprises, in order along the central axis, an eighth lens configured as a biconcave negative lens, a ninth lens configured as a biconvex positive lens and cemented with the eighth lens, a tenth lens configured as a biconvex positive lens, an eleventh lens configured as a biconvex positive lens, and a twelfth lens configured as a biconcave negative lens and cemented with the eleventh lens. characterized in that The fixed focus lens has a focal length of 50-60 mm. 3.The fixed focus lens of claim 2, wherein the second lens, the ninth lens, and the eleventh lens each have a refractive index of 1.59 and an Abbe number of 68.

3. 4.The fixed focus lens of claim 3, wherein the first lens has an object side surface with a radius of curvature of +47.393 mm, an image side surface with a radius of curvature of +244.881 mm, a central axis thickness of 4.651 mm, a refractive index of 2.00, and an Abbe number of 25.4; the second lens has an object side surface with a radius of curvature of +31.581 mm, an image side surface with a radius of curvature of +51.220 mm, and a central axis thickness of 3.811 mm; the third lens has an object side surface with a radius of curvature of +138.198 mm, an image side surface with a radius of curvature of +21.779 mm, a central axis thickness of 1.000 mm, a refractive index of 1.74, and an Abbe number of 27.8; the fourth lens has an object side surface with a radius of curvature of +36.985 mm, an image side surface with a radius of curvature of -27.954 mm, a central axis thickness of 7.970 mm, a refractive index of 1.50, and an Abbe number of 81.6; the fifth lens has an object side surface with a radius of curvature of -27.954 mm, an image side surface with a radius of curvature of +21.807 mm, a central axis thickness of 1.50 mm, a refractive index of 1.67, and an Abbe number of 32.

2. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2. The fixed focus lens of claim 1, wherein ​ ​ ​ ​ ​ ​ ​ ​ ​ The sixth lens has a curvature radius of +21.807 mm on the object side, a curvature radius of 87.127 mm on the image side, a thickness of 7.405 mm at the central axis, a refractive index of 1.91, and an Abbe number of 35.

3.

5. The fixed focus lens according to claim 4, wherein, The eighth lens has a curvature radius of -18.414 mm on the object side, a curvature radius of +218.495 mm on the image side, a thickness of 1.000 mm at the central axis, a refractive index of 1.74, and an Abbe number of 27.

8. The ninth lens has a curvature radius of +218.495 mm on the object side, a curvature radius of -22.422 mm on the image side, a thickness of 6.000 mm at the central axis. The tenth lens has a curvature radius of +110.916 mm on the object side, a curvature radius of -47.839 mm on the image side, a thickness of 5.057 mm at the central axis, a refractive index of 2.00, and an Abbe number of 25.

4. The eleventh lens has a curvature radius of +50.292 mm on the object side, a curvature radius of -22.764 mm on the image side, a thickness of 10.000 mm at the central axis. The twelfth lens has a curvature radius of -22.764 mm on the object side, a curvature radius of +58.653 mm on the image side, a thickness of 1.500 mm at the central axis, a refractive index of 1.60, and an Abbe number of 38.

0.

6. The fixed focus lens according to claim 5, wherein, The seventh lens has a curvature radius of +992.649 mm on the object side, a curvature radius of +20.896 mm on the image side, a thickness of 0.800 mm at the central axis, a refractive index of 1.62, and an Abbe number of 63.

4.

7. The fixed focus lens of claim 6, wherein, The aperture stop is disposed between the second lens group and the first lens group.

Images