CN113866962A

CN113866962A - Low-distortion wide-angle lens

Info

Publication number: CN113866962A
Application number: CN202111248103.6A
Authority: CN
Inventors: 白兴安; 周小青; 陈汇东
Original assignee: Sunny Optics Zhongshan Co Ltd
Current assignee: Sunny Optics Zhongshan Co Ltd
Priority date: 2021-10-26
Filing date: 2021-10-26
Publication date: 2021-12-31

Abstract

A low-distortion wide-angle lens includes a first lens (L1), a second lens (L2), a third lens (L3), a fourth lens (L4), a stop (S), a fifth lens (L5), a sixth lens (L6), a seventh lens (L7), an eighth lens (L8), a ninth lens (L9), and a tenth lens (L10) arranged in order from an object side to an image side along an optical axis; the third lens (L3), fifth lens (L5), and seventh lens (L7) are of negative optical power, and the sixth lens (L6) and the tenth lens (L10) are of positive optical power. Through the reasonable placement and matching of the positive and negative focal power lenses and the concave and convex lenses, the optical distortion of the image is reduced, the lens can not be virtual-focused within the temperature range of minus 30 ℃ to 70 ℃, and the image pixels are further improved. Due to the reasonable arrangement of the lenses, the requirement on tolerance sensitivity between the lenses can be reduced, and meanwhile, the overall size of the lens can be reduced, the weight is reduced, and the cost is saved.

Description

Low-distortion wide-angle lens

Technical Field

The invention relates to the technical field of optical imaging, in particular to a low-distortion wide-angle lens.

Background

In a video conference, a wide-angle and low-distortion image is required, but in a common lens, the distortion is more serious with the increase of a visual angle, so that the imaging is highly distorted. The prior art discloses a lens, which has the characteristic of low distortion, can effectively reduce the distortion degree of an image, but has high cost, large volume, no temperature compensation function and no wide angle and low distortion.

Disclosure of Invention

In order to make up for the above defects, the present invention aims to provide a low distortion wide-angle lens, which has the advantages of small volume, low cost, low tolerance sensitivity, no virtual focus in the temperature range of-30 ℃ to 70 ℃, etc. while giving consideration to both wide angle and low distortion.

To achieve the above object, the present invention provides a low distortion wide-angle lens, comprising: a first lens, a second lens, a third lens, a fourth lens, a diaphragm, a fifth lens, a sixth lens, a seventh lens, an eighth lens, a ninth lens, and a tenth lens arranged in this order from an object side to an image side along an optical axis;

the third lens, the fifth lens and the seventh lens are negative focal power, and the sixth lens and the tenth lens are positive focal power.

According to an aspect of the present invention, the first lens and the second lens have negative refractive power, the fourth lens and the eighth lens have positive refractive power, and the ninth lens has positive refractive power or negative refractive power.

According to an aspect of the present invention, the first lens and the second lens are convex-concave lenses, the third lens is a convex-concave lens, the eighth lens is a biconvex lens, and a paraxial region of the tenth lens is a convex-concave lens.

According to an aspect of the present invention, the fourth lens is a convex-concave lens or a convex-convex lens, the fifth lens is a biconcave lens or a convex-concave lens, the sixth lens is a biconvex lens or a convex-concave lens, the seventh lens is a biconcave lens, a convex-concave lens or a convex-concave lens, and the paraxial region of the ninth lens is a convex-concave lens or a convex-concave lens.

According to an aspect of the invention, the radius of curvature R1 of the first lens object side surface and the radius of curvature R2 of the first lens (L1) image side surface satisfy: R1/R2 is more than or equal to 1.83 and less than or equal to 2.20.

According to an aspect of the present invention, the first lens, the fourth lens, the fifth lens, the sixth lens, the seventh lens, and the eighth lens are spherical lenses, and the second lens, the third lens, the ninth lens, and the tenth lens are aspherical lenses.

According to an aspect of the invention, the refractive index Nd1 of the first lens satisfies: nd1 is more than or equal to 1.80.

According to an aspect of the invention, an abbe number Vd6 of the sixth lens satisfies: vd6 is more than or equal to 70.0.

According to an aspect of the invention, an abbe number Vd8 of the eighth lens satisfies: vd8 is more than or equal to 70.0.

According to an aspect of the invention, at least one of the fifth lens, the sixth lens, the seventh lens and the eighth lens includes a cemented lens.

According to an aspect of the present invention, a focal length FC of the cemented lens and an overall focal length F of the lens satisfy: FC/F is more than or equal to-93.94 and less than or equal to 20.8.

According to an aspect of the present invention, the overall focal length FA of the first to fourth lenses and the overall focal length FB of the fifth to tenth lenses satisfy: FA/FB is more than or equal to 0 and less than or equal to 2.26.

According to one aspect of the invention, the focal length F4 of the fourth lens and the overall focal length F of the lens satisfy: F4/F is more than or equal to 1.04 and less than or equal to 3.17.

According to one aspect of the invention, the image plane diameter Y of the lens and the total optical length TTL of the lens satisfy: Y/TTL is more than or equal to 0.31 and less than or equal to 0.34.

According to one aspect of the invention, the maximum aperture diameter D1 of the first lens and the total optical length TTL of the lens barrel satisfy: D1/TTL is more than or equal to 0.39 and less than or equal to 0.47.

According to an aspect of the invention, the stop is placed on an image side surface of the fourth lens or on an object side surface of the fifth lens.

According to an aspect of the invention, the second lens element, the third lens element, the ninth lens element and the tenth lens element are made of plastic material.

The low-distortion wide-angle lens reduces the optical distortion of the image through the reasonable placement and matching of the positive and negative focal power lenses and the concave and convex lenses, can ensure that the lens does not have virtual focus within the temperature range of-30 ℃ to 70 ℃, and further improves the image pixels. Due to the reasonable arrangement of the lenses, the requirement on tolerance sensitivity between the lenses can be reduced. Meanwhile, the overall size of the lens can be reduced, the weight is reduced, and the cost is saved.

According to the low-distortion wide-angle lens, the first lens can accommodate large-angle incident light rays by adjusting the bending direction and the presented shape of the first lens, so that the wide-angle function is realized.

The low-distortion wide-angle lens can improve the temperature compensation function of the lens by further limiting the ranges of the abbe numbers of the first lens L1 and the eighth lens L8 and the abbe number of the sixth lens L6, and simultaneously reduces the chromatic aberration of an optical system, improves the resolving power and ensures that the imaging has higher image quality.

In the low-distortion wide-angle lens of the present invention, at least one cemented lens is included in the fifth lens L5, the sixth lens L6, the seventh lens L7, and the eighth lens L8; and the focal length FC of the cemented lens and the focal length F of the lens meet the condition that FC/F is more than or equal to-93.94 and less than or equal to 20.8. The imaging performance of the lens is improved, the chromatic aberration of an imaging part is eliminated, and the resolving power of the optical system is improved.

Drawings

FIG. 1: an optical system schematic diagram of the low-distortion wide-angle lens of the first embodiment;

FIG. 2: the optical system of the low-distortion wide-angle lens of the second embodiment is schematically illustrated;

FIG. 3: the optical system of the low-distortion wide-angle lens of the third embodiment is schematically illustrated.

Detailed Description

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments will be briefly described below. It is obvious that the drawings in the following description are only some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive effort.

In describing embodiments of the present invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like are used in an orientation or positional relationship that is based on the orientation or positional relationship shown in the associated drawings, which is for convenience and simplicity of description only, and does not indicate or imply that the referenced device or element must have a particular orientation, be constructed and operated in a particular orientation, and thus, the above-described terms should not be construed as limiting the present invention.

The present invention is described in detail below with reference to the drawings and the specific embodiments, which are not repeated herein, but the embodiments of the present invention are not limited to the following embodiments.

As shown in fig. 1, the low-distortion wide-angle lens of the present invention sequentially includes, from the object side to the image side: a first lens L1, a second lens L2, a third lens L3, a fourth lens L4, a stop S, a fifth lens L5, a sixth lens L6, a seventh lens L7, an eighth lens L8, a ninth lens L9, and a tenth lens L10.

In the present invention, the third lens L3, the fifth lens L5, and the seventh lens L7 are of negative power, and the sixth lens L6 and the tenth lens L10 are of positive power. Further, the first lens L1 and the second lens L2 have negative optical power, the fourth lens L4 and the eighth lens L8 have positive optical power, and the ninth lens L9 has positive optical power or negative optical power.

In the present invention, the first lens L1 and the second lens L2 are convex-concave lenses, the third lens L3 is a convex-concave lens, the eighth lens L8 is a double convex lens, and the paraxial region of the tenth lens L10 is a convex-concave lens. Further, the fourth lens L4 is a convex-concave lens or a convex-convex lens, the fifth lens L5 is a biconcave lens or a convex-concave lens, the sixth lens L6 is a biconvex lens or a convex-concave lens, the seventh lens L7 is a biconcave lens, a concave-convex lens or a convex-concave lens, and the paraxial region of the ninth lens L9 is a convex-concave lens or a concave-convex lens.

According to the concept of the invention, through the reasonable placement and matching of the positive and negative focal power lenses and the concave and convex lenses, the optical distortion of imaging is reduced, the lens can not be virtual focused within the temperature range of minus 30 ℃ to 70 ℃, and the imaging pixels are further improved. Due to the reasonable arrangement of the lenses, the requirement on tolerance sensitivity between the lenses can be reduced, the overall size of the lens can be reduced, the weight is reduced, and the cost is saved.

In the present invention, the curvature radius of the object-side surface of the first lens L1 is R1, and the curvature radius of the image-side surface of the first lens L1 is R2. Both satisfy the relation: R1/R2 is more than or equal to 1.83 and less than or equal to 2.20. When the curvature radius R2 of the image side surface of the first lens and the curvature radius R1 of the object side surface of the first lens satisfy the above relationship, the first lens can accommodate incident light rays with large angles by adjusting the bending direction and the presented shape of the first lens, and the wide-angle function is realized.

In the present invention, the first lens L1, the fourth lens L4, the fifth lens L5, the sixth lens L6, the seventh lens L7, and the eighth lens L8 are spherical lenses, and the second lens L2, the third lens L3, the ninth lens L9, and the tenth lens L10 are aspherical lenses. Through the arrangement, the temperature compensation function of the lens is improved, and the condition that virtual focus does not occur in the temperature range of minus 30-70 ℃ of the lens is further ensured.

In the present invention, the refractive index Nd1 of the first lens L1 satisfies the relation: nd1 is not less than 1.80, and the Abbe number Vd6 of the sixth lens L6 satisfies: 70.0 is not less than Vd6, and the Abbe number Vd8 of the eighth lens L8 satisfies the relation: vd8 is more than or equal to 70.0. The ranges of the refractive index of the first lens L1 and the abbe numbers of the eighth lens L8 and the sixth lens L6 are further limited. According to the concept of the present invention, the lens can be selected from the above range, and can be used with the power and the shape of other lenses in the present invention. The temperature compensation function of the lens is improved, meanwhile, the chromatic aberration of the optical system is reduced, the resolution power is improved, and the imaging quality is higher.

In the present invention, at least one cemented lens is included in the fifth lens L5, the sixth lens L6, the seventh lens L7, and the eighth lens L8. Further, in the present invention, the focal length FC of the cemented lens and the lens focal length F satisfy: FC/F is more than or equal to-93.94 and less than or equal to 20.8. By adopting the cemented lens, the imaging performance of the lens can be improved, and the chromatic aberration of the imaging part can be eliminated. According to the concept of the present invention, the resolution of the optical system can be further improved by determining the ratio of the focal length of the cemented lens to the focal length of the lens within the above range.

In the present invention, the overall focal length FA of the first through fourth lenses L1-L4 and the focal length FB of the fifth through tenth lenses L5-L10 satisfy the relation: FA/FB is more than or equal to 0 and less than or equal to 2.26. Further, the focal length F4 of the fourth lens (L4) and the overall focal length F of the lens satisfy the relation: F4/F is more than or equal to 1.04 and less than or equal to 3.17. By such arrangement, the tolerance sensitivity of the whole optical system can be reduced, the assembly sensitivity is low, and the production yield is improved.

In the invention, the lens image surface diameter Y and the total optical length TTL of the lens satisfy the relational expression: Y/TTL is more than or equal to 0.31 and less than or equal to 0.34. Further, the maximum aperture diameter D1 of the first lens element L1 and the total optical length TTL of the lens barrel satisfy the following relation: D1/TTL is more than or equal to 0.39 and less than or equal to 0.47. By determining the value ranges of the lens image surface diameter Y, the total optical length TTL of the lens and the maximum caliber diameter D1 of the first lens L1, the whole lens has smaller size and volume and is convenient to carry.

In the present invention, a stop S is provided on the image-side surface of the fourth lens L4 or the object-side surface of the fifth lens L5. The form of the optical framework can be optimized through the position adjustment of the diaphragm S, and the imaging performance is improved.

In the invention, the second lens L2, the third lens L3, the ninth lens L9 and the tenth lens L10 are made of plastic materials, so that the overall weight of the lens is further reduced, and the cost is saved.

Three groups of embodiments are given below for the above arrangement according to the present invention to specifically describe the low distortion wide-angle lens according to the present invention. Since the low distortion lens according to the invention has a total of 10 lenses, each lens has two optical surfaces, plus the stop S, the imaging plane IMA of the lens and the plane of the parallel plate between the imaging plane IMA and the lens. In the present invention, when one or more doublet lenses are present among the 10 lenses, there are a maximum of 22 optical surfaces. For convenience of description, the respective face numbers are designated as S1 to S22.

Specific parameters for three sets of embodiments are shown in table 1 below:

	first embodiment	Second embodiment	Third embodiment
				Y	16	16.02	16.47
TTL	48.489	50.024	49.998
				F4	15.727	20.006	10.998
F	7.314	7.316	7.503
				Nd1	1.85	1.86	1.85
Vd6	81.6	81.6	81.6
				Vd8	81.6	90.2	90.2
FA	14.809	33.124	7.937
				FB	18.336	19.195	48.891
FC1 (cemented lens)	-22.643	30.09	48.457
				FC2	/	-582.283	/
R1	22.13	21.5	18.33
				R2	10.47	10.14	9.61
D1	21.95	21.34	20.31

Table 1 specific parameters of three groups of embodiments specifically satisfying the above conditional expressions are shown in table 2 below:

TABLE 2

The first embodiment:

fig. 1 is a view schematically showing the construction of a low distortion wide-angle lens according to a first embodiment of the present invention.

In the present embodiment, the fourth lens L4 is a convex-concave lens, the fifth lens L5 is a concave-concave lens, the sixth lens L6 is a convex-convex lens, the seventh lens L7 is a concave-convex lens, the ninth lens L9 is a negative power lens, and the paraxial region of the ninth lens L9 is a convex-concave lens; the fifth lens L5, the sixth lens L6, and the seventh lens L7 constitute a first cemented lens.

Table 3 lists relevant parameters of each lens in the present embodiment, including surface type, radius of curvature, thickness, refractive index of material, abbe number:

table 3 in the present embodiment, the aspherical surface formula is as follows:

the aspheric coefficients of each aspheric lens in the lens include a conic constant K of the surface, a fourth order aspheric coefficient a4, a sixth order aspheric coefficient a6, an eighth order aspheric coefficient A8, a tenth order aspheric coefficient a10, and a twelfth order aspheric coefficient a12, as shown in table 4 below:

number of noodles

Value of K

A4

A6

A8

A10

A12

3

9.58

9.16E-04

-1.94E-05

2.33E-07

-1.51E-09

3.27E-12

4

-6.74

2.43E-03

-4.74E-05

4.22E-07

4.67E-09

-9.19E-11

5

-2.91

-3.72E-04

-8.31E-07

1.62E-07

-3.83E-09

2.66E-11

6

-4.72

-4.41E-04

5.51E-06

-5.10E-08

-1.21E-11

3.13E-12

15

-9.16

1.35E-04

1.12E-05

-6.96E-07

1.38E-08

-1.04E-10

16

-10.47

-6.24E-04

3.87E-05

-1.407E-06

2.28E-08

-1.41E-10

17

-6.81

-1.04E-03

-7.29E-06

4.12E-07

-1.17E-08

9.45E-11

18

-15.56

-4.33E-04

-1.57E-05

3.77E-07

-5.08E-09

2.50E-11

TABLE 4

As shown in fig. 1, the stop S of the present embodiment is provided on the object side surface of the fifth lens L5. According to the concept of the invention, the lens can not be defocused in a high-temperature and low-temperature environment of-30-70 ℃, and the temperature drift amount is less than 0.006 mm. In the present embodiment, the fifth lens L5, the sixth lens L6, and the seventh lens L7 are cemented together to form a cemented lens, which can achieve both low distortion and wide-angle performance and satisfy high imaging quality. The lens also has the advantages of low cost, small volume, low assembly sensitivity and the like, and can be suitable for more scenes with different conditions.

The second embodiment:

fig. 2 is a view schematically showing the construction of a low distortion wide-angle lens according to a second embodiment of the present invention.

In the present embodiment, the fourth lens L4, the fifth lens L5, the sixth lens L6, and the seventh lens L7 are convex-concave lenses, the ninth lens L9 is a positive power lens, and the paraxial region of the ninth lens L9 is a concave-convex lens. Wherein the fifth lens L5 and the sixth lens L6 constitute a first cemented lens by cementing, and the seventh lens L7 and the eighth lens L8 constitute a second cemented lens by cementing.

Relevant parameters of each lens in the present embodiment, including surface type, radius of curvature, thickness, refractive index of the material, abbe number, are listed in table 5 below:

table 5 in the present embodiment, the aspherical surface formula is as follows:

the aspheric coefficients of each aspheric lens in the lens include a conic constant K of the surface, a fourth order aspheric coefficient a4, a sixth order aspheric coefficient a6, an eighth order aspheric coefficient A8, a tenth order aspheric coefficient a10, and a twelfth order aspheric coefficient a12, as shown in table 6 below:

number of noodles

Value of K

A4

A6

A8

A10

A12

3

9.5

7.82E-04

-1.57E-05

2.01E-07

-1.53E-09

5.24E-12

4

-5.24

1.88E-03

-3.23E-05

3.28E-07

4.17E-10

-2.35E-11

5

-2.77

-3.18E-04

-7.49E-07

1.10E-07

-3.22E-09

2.29E-11

6

-6.11

-3.16E-04

4.26E-06

-5.10E-08

2.11E-10

1.33E-12

16

30

-1.27E-03

-1.42E-06

-1.06E-06

-5.62E-09

-6.82E-10

17

23.38

-2.14E-03

3.60E-05

-1.39E-06

2.57E-08

-3.60E-10

18

-22.06

-1.20E-03

-2.63E-05

5.85E-07

-5.51E-09

-5.46E-12

19

7.38

-7.67E-04

-1.40E-05

3.36E-07

-3.71E-09

5.36E-12

TABLE 6

As shown in fig. 2, the stop S of the present embodiment is disposed between the fourth lens L4 and the fifth lens L5. According to the concept of the invention, the lens in the embodiment does not defocus under the high and low temperature environment of-30-70 ℃, the temperature drift amount is less than 0.0015mm, and the low distortion and wide-angle performance are considered, so that the high imaging quality is realized. In the embodiment, the lens also has the advantages of low cost, small volume, low assembly sensitivity and the like, and can be applied to more scenes with different conditions.

Third embodiment:

fig. 3 is a view schematically showing the construction of a low distortion wide-angle lens according to a third embodiment of the present invention.

In the present embodiment, the fifth lens L5 and the seventh lens L7 are biconcave lenses, the fourth lens L4 and the sixth lens L6 are biconvex lenses, the ninth lens L9 is a negative power lens, and the paraxial region of the ninth lens L9 is a convex-concave lens. Among them, the sixth lens L6, the seventh lens L7, and the eighth lens L8 constitute a first cemented lens.

Relevant parameters of each lens in the present embodiment, including surface type, radius of curvature, thickness, refractive index of the material, abbe number, are listed in table 7 below:

table 7 in the present embodiment, the aspherical surface formula is as follows:

the aspheric coefficients of each aspheric lens in the lens include a conic constant K of the surface, a fourth order aspheric coefficient a4, a sixth order aspheric coefficient a6, an eighth order aspheric coefficient A8, a tenth order aspheric coefficient a10, and a twelfth order aspheric coefficient a12, as shown in table 8 below:

number of noodles

Value of K

A4

A6

A8

A10

A12

3

11.88

7.48E-04

-1.34E-05

1.82E-07

-1.60E-09

7.19E-12

4

-5.28

1.87E-03

-2.69E-05

3.30E-07

-1.07E-09

1.83E-11

5

-2.53

-2.43E-04

8.46E-07

7.41E-08

-3.46E-09

3.62E-11

6

-7.8

-2.13E-04

4.46E-06

-7.10E-08

4.48E-10

4.81E-12

15

-30

-1.55E-03

1.98E-05

-2.17E-06

3.65E-08

-1.36E-09

16

-30

-2.89E-03

4.99E-05

-1.40E-06

2.13E-08

-2.21E-10

17

-8.49

-1.29E-03

-2.40E-05

7.42E-07

-9.04E-09

3.19E-11

18

7.11

-5.48E-04

-1.31E-05

2.95E-07

-3.91E-09

1.47E-11

TABLE 8

As shown in fig. 3, the stop S of the present embodiment is provided on the image side surface of the fourth lens L4. According to the concept of the invention, the lens can not be defocused in a high-temperature and low-temperature environment of-30-70 ℃, and the temperature drift amount is less than 0.0015 mm. The lens can also enable light rays to smoothly enter the optical system, so that a wide-angle function is realized, imaging distortion is small, and imaging quality is high. Meanwhile, the lens has the advantages of low cost, small volume, low assembly sensitivity and the like, and is suitable for scenes with more different conditions.

The above description is only one embodiment of the present invention, and is not intended to limit the present invention, and it is apparent to those skilled in the art that various modifications and variations can be made in the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A low-distortion wide-angle lens includes a first lens (L1), a second lens (L2), a third lens (L3), a fourth lens (L4), a stop (S), a fifth lens (L5), a sixth lens (L6), a seventh lens (L7), an eighth lens (L8), a ninth lens (L9), and a tenth lens (L10) arranged in order from an object side to an image side along an optical axis; characterized in that the third lens (L3), the fifth lens (L5) and the seventh lens (L7) have negative optical power, and the sixth lens (L6) and the tenth lens (L10) have positive optical power.

2. The lens barrel according to claim 1, wherein the first lens (L1) and the second lens (L2) are of negative optical power, the fourth lens (L4) and the eighth lens (L8) are of positive optical power, and the ninth lens (L9) is of positive or negative optical power.

3. The lens barrel according to claim 1 or 2, wherein the first lens (L1) and the second lens (L2) are convex-concave lenses, the third lens (L3) is a convex-concave lens, the eighth lens (L8) is a double convex lens, and a paraxial region of the tenth lens (L10) is a convex-concave lens.

4. The lens barrel according to claim 3, wherein the fourth lens (L4) is a convex-concave or convex-convex lens, the fifth lens (L5) is a biconcave or convex-concave lens, the sixth lens (L6) is a biconvex or convex-concave lens, the seventh lens (L7) is a biconcave, concave-convex or convex-concave lens, and the paraxial region of the ninth lens (L9) is a convex-concave or concave-convex lens.

5. A lens barrel according to claim 1, wherein a radius of curvature R1 of an object side surface of the first lens (L1) and a radius of curvature R2 of an image side surface of the first lens (L1) satisfy: R1/R2 is more than or equal to 1.83 and less than or equal to 2.20.

6. The lens barrel according to claim 1, wherein the first lens (L1), the fourth lens (L4), the fifth lens (L5), the sixth lens (L6), the seventh lens (L7), and the eighth lens (L8) are spherical lenses, and the second lens (L2), the third lens (L3), the ninth lens (L9), and the tenth lens (L10) are aspherical lenses.

7. A lens barrel according to claim 1, characterized in that the refractive index Nd1 of the first lens (L1) satisfies: nd1 is more than or equal to 1.80.

8. A lens barrel according to claim 7, characterized in that the Abbe number Vd6 of the sixth lens (L6) satisfies: vd6 is more than or equal to 70.0.

9. Lens barrel according to claim 8, characterized in that the abbe number Vd8 of the eighth lens (L8) satisfies: vd8 is more than or equal to 70.0.

10. The lens barrel as claimed in claim 1, wherein at least one of the fifth lens (L5), the sixth lens (L6), the seventh lens (L7) and the eighth lens (L8) comprises a cemented lens.