JP2005141017A - Lens system and camera - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small-sized and low-cost lens system suitable for a digital camera using an imaging device such as a CCD. <P>SOLUTION: In the lens system 1 constituted of, in order from an object side, a 1st lens group G1 with negative refractive power, a 2nd lens group G2 with positive refractive power, and a 3rd lens group G3 with positive refractive power, the 2nd lens group G2 is constituted of, from the object side, a biconvex positive lens L21, a biconcave negative lens L22, and a negative meniscus lens L23 of which the concave surface is faced to the object side 8. Thus, various aberrations can be corrected favorably, and cost reduction is achieved. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a lens system suitable for forming a subject image on an image sensor such as a CCD.

As a lens system for a digital camera using an image pickup device such as a CCD, a retrofocus type which is easy to make the image side telecentric is suitable. The negative and positive three-group zoom lens can reduce the size of the entire lens system with a simple configuration, and has been proposed as shown in Patent Documents 1 to 6 below.
Japanese Patent Laid-Open No. 6-94996 JP-A-10-170826 JP-A-10-133115 JP-A-11-174322 JP 2002-267930 A Japanese Patent Laid-Open No. 11-287953

  The lens systems described in JP-A-6-94996, JP-A-10-170826, JP-A-10-133115, JP-A-11-174322, and JP-A-2002-267930 are disclosed on the object side ( In order from the object side) to the image side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power are disposed. Although the whole structure is a simple structure of three groups, the second lens group that moves most when zooming is composed of four or more lenses, and the size of the entire lens system can be sufficiently reduced. In addition, there is a problem that the cost is high.

  The zoom lens described in Japanese Patent Laid-Open No. 11-287953 has a configuration in which the middle two groups have three lenses, and can provide a compact and low-cost lens system for the above lens system. Two groups of zoom lenses described in Japanese Patent Laid-Open No. 11-287953 include a biconvex positive lens or a positive meniscus lens having a convex surface facing the object side, a biconcave negative lens, and a biconvex positive lens. Thus, it is easy to correct various aberrations such as chromatic aberration, field curvature and astigmatism with a small number of lenses. This configuration is basically similar to a configuration called a triplet, and has a strong negative power for correcting the Petzval sum for flattening the image plane by adopting two positive lenses. A negative lens having a surface can be used.

  However, on the other hand, since the power of the negative lens of the second group is strong, there is a tendency to correct aberrations excessively. In particular, it is difficult to correct the aberration when the second group is moved by zooming between the first group and the third group. It has a configuration. For example, when the power of each lens fluctuates due to a temperature change, there is a concern that the performance of the entire lens system is deteriorated. In particular, the biconcave negative lens needs to secure a distance from the convex lens to the negative surface from the point of correcting the field curvature aberration, and further to form a surface with a large curvature, so the lens becomes thicker and larger. Because of its large curvature, it is easily affected by temperature changes. Therefore, it is difficult to construct a low-cost and lightweight plastic lens.

  Therefore, an object of the present invention is to provide a lens system that can be further reduced in weight and cost. It is another object of the present invention to provide a low-cost lens system with a simple configuration capable of exhibiting more stable performance against zooming and temperature changes.

  For this reason, in the present invention, the second lens group includes, in order from the object side, a biconvex positive lens, a biconcave negative lens, and a negative meniscus lens having a concave surface facing the object side. That is, the lens system of the present invention includes a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power in order from the object side to the image side. The second lens group includes, in order from the object side, a biconvex positive lens, a biconcave negative lens, and a negative meniscus lens having a concave surface facing the object side. It is configured.

  The lens group that can correct the Petzval sum typified by a triplet has a positive-negative-positive configuration. However, considering the configuration in units of surfaces, a positive power surface convex toward the object side and an image side By placing a negative power surface between the convex positive power surface, substantially the same effect can be obtained. Therefore, in the present invention, a biconvex positive lens on the object side is arranged to form a convex positive power surface on the object side, and a concave surface on the object side, that is, a convex surface on the image side is directed to the image side. By arranging a negative meniscus lens, a convex positive power surface is formed on the object side. Then, the negative refraction of the lens itself is used while using the concave surface on the object side of the negative meniscus lens as a negative power surface with a large curvature arranged away from the convex positive power surface on the object side. The power is suppressed. Therefore, a lens group capable of satisfactorily correcting the Petzval sum with a weak meniscus lens having a weak power can be formed.

  Furthermore, it is possible to correct chromatic aberration satisfactorily by using a biconvex positive lens and a biconcave negative lens. For this reason, when the lens system of the present invention is a zoom lens in which the second lens group moves, the moving second lens group can improve various aberrations such as chromatic aberration, field curvature, and astigmatism. It becomes possible to correct. In addition, since the negative power in the second lens group can be within an appropriate range, the range of generation and correction of various aberrations due to the negative power is also appropriate in relation to the first and third lens groups. It is possible to provide a lens system that can fall within the range and has good aberration correction over the entire range from the wide-angle end to the telephoto end. In zooming, when the first lens group and / or the second lens group is zoomed from the wide-angle end to the telephoto end, the air space between the first and second lens groups is decreased, and the second and third lens groups are reduced. This is done by increasing the air spacing of the lens group.

  The simplest configuration example of a biconvex positive lens and a biconcave negative lens is bonding. By using a double convex positive lens and a double concave negative lens as a doublet, the configuration of the second lens group is further simplified.

  In the lens system of the present invention, the power of the negative lens, particularly the negative meniscus lens, constituting the second lens group is small. Also, the negative meniscus lens can be made thin. Therefore, the present invention can provide a lens system that has a configuration that is hardly affected by temperature and that has stable imaging performance against changes in the environment. In addition, since it is difficult to be affected by temperature changes, the negative meniscus lens can be a plastic lens, and the configuration is suitable for introducing an aspherical surface. Therefore, it is possible to provide a lightweight lens system with stable imaging performance at low cost.

  The lens system of the present invention is a retro-focus type lens system of negative-positive-positive and is easy to make the image side telecentric. Therefore, the lens system is suitable for a digital camera using an image pickup device such as a CCD. Therefore, the camera having the lens system of the present invention and the image sensor disposed on the image side of the lens system is lightweight, low-cost, and has stable performance against environmental changes such as temperature. It can be demonstrated.

  As described above, the lens system of the present invention is a lens system having a negative-positive-positive configuration from the object side and having a good telecentricity for a digital camera using an image sensor such as a CCD. is there. Further, the second lens group includes a biconvex positive lens, a biconcave negative lens, and a negative meniscus lens having a concave surface directed toward the object side, without significantly increasing the negative power component. The configuration can correct various aberrations. Therefore, when the second lens group is moved as a zoom lens, the lens system can obtain stable performance from the wide-angle end to the telephoto end.

(Example 1)
FIG. 1 shows a digital camera 10 equipped with the lens system 1 of the present invention, and FIG. 2 shows a specific configuration of the lens system 1. The digital camera 10 includes a CCD 11 disposed on the image side of the lens system 1, converts image information of an object imaged on the CCD 11 by the lens system 1 into a digital signal, and records it on an appropriate recording medium. Can be provided via a computer network such as the Internet.

  The lens system 1 of this example functions as a zoom lens. FIG. 2 shows the lens arrangement at the wide-angle end (short focal length end), and the arrow indicates the lens movement direction at the telephoto end (long focal length end). is there. The lens system 1 of this example is composed of seven lenses grouped in order into three lens groups G1, G2 and G3 from the object side 8 toward the image side 9 where the focal plane of the CCD 11 is arranged. Yes.

  The first lens group G1 located closest to the object side 8 has a negative refractive power as a whole, and in order from the object side 8, a negative meniscus L11 convex to the object side 8 and a biconcave negative lens. 12 and a biconvex positive lens L13. The negative lens L12 and the positive lens L13 are configured as a cemented cemented lens.

  The second lens group G2 has a positive refractive power as a whole, and in order from the object side 8, it is concave on the biconvex positive lens L21, the biconcave negative lens L22, and the object side (subject side) 8. Negative meniscus lens L23. The negative meniscus lens L23 is a plastic lens, and both surfaces are aspherical to further improve the aberration. A stop S that moves together with the second lens group G2 is provided on the object side 8 of the second lens group G2.

  The third lens group G3 closest to the image side 9 has a positive refractive power as a whole, and is composed of one biconvex positive lens L31. The third lens group G3 does not move during zooming from the wide-angle end to the telephoto end. On the other hand, during zooming from the wide-angle end to the telephoto end, the first lens group G1 and the second lens group G2 move so that their distance d1 decreases, and the second lens group is the third lens group. The distance d2 between the lens groups is increased.

  In the lens data shown below, No. Is the number of the lens surface arranged in order from the object side 8, Ri is the radius of curvature (mm) of each lens arranged in order from the object side, Di is the distance between each lens surface arranged in order from the object side (mm) ), Nd represents the refractive index (d line) of each lens arranged in order from the object side, and νd represents the Abbe number (d line) of each lens arranged in order from the object side. The distance d1 indicates the distance (mm) between the first lens group G1 and the second lens group G2, and the distance d2 indicates the distance (mm) between the second lens group G2 and the third lens group G3. ). Flat indicates a flat surface. The same applies to the following embodiments.

Lens data (No. 1)
No. Ri Di nd vd
1 17.937 1.200 1.74300 49.20 Lens L11
2 4.477 3.001
3 -25.031 1.000 1.55371 42.21 Lens L12
4 6.494 2.798 1.83969 29.34 Lens L13
5 139.923 d1 (variable)
6 Flat 0.500 Aperture S
7 6.968 3.335 1.77250 49.60 Lens L21
8 -8.458 1.572 1.83487 24.44 Lens L22
9 55.249 2.309
10 -10.214 1.280 1.49757 68.58 Lens L23
11 -11.420 d2 (variable)
12 22.269 1.959 1.48700 70.40 Lens L31
13 -37.918 1.200
14 Flat 1.500 1.51633 64.14
15 Flat

The second, tenth, and eleventh surfaces are aspheric surfaces, and the aspheric coefficients thereof are as follows. However, the aspherical formula is as follows.
x = (1 / R) Y ² / [1+ {1− (1 + K) (1 / R) ² Y ² } ^1/2 ]
+ AY ⁴ + BY ⁶ + CY ⁸ + DY ¹⁰
Second surface R = 4.478
K = 0.00000
A = −3.9858 × 10 ⁻⁴ , B = −5.0959 × 10 ⁻⁵
C = 4.0995 × 10 ⁻⁶ , D = −2.8190 × 10 ⁻⁷
10th surface R = -10.214
K = 0.00000
A = -2.6092 × 10 ⁻³ , B = 1.5458 × 10 ⁻⁴
C = −1.3292 × 10 ⁻⁵ , D = 1.1044 × 10 ⁻⁶
11th surface R = -11.421
K = 0.00000
A = −6.3996 × 10 ⁻⁴ , B = 1.3270 × 10 ⁻⁴
C = −2.6669 × 10 ⁻⁶ , D = 3.1305 × 10 ⁻⁷

Various numerical values during zooming of the projection zoom lens system of this example are as follows.
Wide-angle end Intermediate Telephoto end combined focal length 4.7200 8.1800 14.1497
d1 14.85667 6.37741 1.50000
d2 3.63774 8.23486 16.16153

  FIGS. 3A and 3B show spherical aberration, astigmatism, and distortion at the wide-angle end and the telephoto end of the lens system 1, respectively. The spherical aberration indicates values at wavelengths of 656.28 nm (broken line), 587.56 nm (solid line), and 486.13 nm (dashed line). As can be seen from these figures, the spherical aberration and astigmatism of the lens system 1 of this example are well within ± 0.1 mm or less from the wide-angle end to the telephoto end, and distortion is also small. Various aberrations are sufficiently corrected at −5% or less at the high angle end. Therefore, the lens system has sufficient imaging performance as a zoom lens with a zoom ratio of 3 times.

  As can be seen from the above lens data, the power of the negative lens L22 and the negative meniscus lens L23 constituting the second lens group G2 is kept small. In particular, the negative meniscus lens L23 having a concave surface directed toward the object side is thin and has a weak lens power. For this reason, the temperature change of the performance of the meniscus lens L23 is also small, and the influence of the change in performance due to the temperature change of the meniscus lens L23 on the aberration performance of the lens system 1 is very small. Therefore, this lens system 1 has a configuration capable of satisfactorily correcting various aberrations in the operating temperature range even if a plastic lens is adopted as the lens L23, is suitable for weight reduction, and can be supplied at low cost. Furthermore, it is easy to introduce an aspheric surface by using a plastic lens, and the cost increase when an aspheric surface is introduced can be suppressed. Therefore, in this respect as well, the lens system 1 can provide a lens system with good imaging performance at low cost.

  As described above, the lens system 1 of the present example is a lens system having a high imaging performance while having a simple configuration of seven in total. Further, this lens system 1 is a retrofocus type zoom lens in which the first, second and third lens groups have negative-positive-positive powers, respectively, and the image side is telecentric, and the CCD 1 or the like is imaged. Suitable for imaging with respect to the element. For this reason, by adopting this lens system 1 in a digital camera, a compact and high-resolution digital camera 10 can be provided at a low cost.

(Example 2)
FIG. 4 shows a lens arrangement at the wide-angle end of a further different lens system 1. The lens system 1 of this example also includes a first lens group G1 having a negative refractive power from the object side 8 toward the image side 9, a second lens group G2 having a positive refractive power, and a third lens group G3. It is a lined lens system. Further, the configuration of the lenses constituting each lens group is substantially the same as that of the first embodiment, but in the first lens group G1 of this example, the object side 8 of the negative meniscus lens L11 is placed on the object side 8. A positive meniscus lens L10 with a convex surface facing the side 8 is arranged. For this reason, the entire lens system 1 is composed of eight lenses. The lens data of Example 2 is shown below.

Lens data (No. 2)
No. Ri Di nd vd
1 -169.607 1.901 1.83475 36.00 Lens L10
2 -41.559 0.300
3 23.316 1.000 1.74300 49.20 Lens L11
4 4.778 2.619
5 -21.965 1.000 1.49190 66.71 Lens L12
6 5.943 2.678 1.83400 37.30 Lens L13
7 28.653 d1 (variable)
8 Flat 0.500 Aperture S
9 6.769 3.346 1.77250 49.60 Lens L21
10 -7.911 1.666 1.82686 24.69 Lens L22
11 69.578 2.199
12 -14.035 1.796 1.52760 64.22 Lens L23
13 -16.082 d2 (variable)
14 38.427 1.986 1.48700 70.40 Lens L31
15 -20.015 1.200
16 Flat 1.500 1.51633 64.14
17 Flat 2.530
18 Flat -0.030

Also in this lens system 1, the image side lens of the second lens group G2 is a negative meniscus lens convex to the image side, and the twelfth and thirteenth surfaces of both surfaces of the lens L23 are aspherical surfaces. . The aspherical coefficients are as follows. The aspherical formula is as described above.
Twelfth surface R = −14.036
K = 0.00000
A = −3.1932 × 10 ⁻³ , B = 6.0813 × 10 ⁻⁵
C = −1.6215 × 10 ⁻⁵ , D = 1.5620 × 10 ⁻⁶
13th surface R = −16.082
K = 0.00000
A = −1.0954 × 10 ⁻³ , B = 6.2601 × 10 ⁻⁵
C = -1.7481 × 10 ⁻⁶ , D = 3.1642 × 10 ⁻⁷

Further, the values during zooming are as follows.
Wide-angle end Intermediate Telephoto end composite focal length 4.9801 8.6301 14.9437
d1 13.30322 5.81778 1.50000
d2 2.80000 7.39098 15.30377

  FIGS. 5A and 5B show spherical aberration, astigmatism, and distortion at the wide-angle end and the telephoto end of the lens system 1, respectively. As can be seen from these drawings, also in the lens system 1 of this example, various aberrations are well corrected from the wide-angle end to the telephoto end, and the lens system has high imaging performance.

  In the above description, as an example of the present invention, a lens system having a seven-lens configuration and an eight-lens configuration as a whole is shown, but the present invention is not limited to these. In addition, although the zoom lens is shown above, it has sufficient imaging performance as a short focus lens system at each focal length within the zoom range, and the present invention includes a single focus lens system. Further, the lens system according to the present invention is not limited to digital cameras, but can be applied to products in various fields including cameras incorporated in PDAs and mobile phones, cameras for visualizing in automatic control devices such as robots, and other optical products. Can be applied.

It is a figure which shows schematic structure of the digital camera using the lens system of this invention. It is a figure which shows the structure of the lens system in the wide angle end of this invention. FIG. 3 is a longitudinal aberration diagram of the lens system of FIG. 2, illustrating spherical aberration, astigmatism, and distortion at the wide-angle end (a) and the telephoto end (b), respectively. It is a figure which shows the example from which the lens system of this invention differs, and is a figure which shows the structure of the lens system in a wide angle end. FIG. 5 is a longitudinal aberration diagram of the lens system of FIG. 4, illustrating spherical aberration, astigmatism, and distortion at the wide-angle end (a) and the telephoto end (b), respectively.

Explanation of symbols

1 Zoom lens 8 Object side 9 Image side

Claims (5)

The lens system includes a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power in order from the object side to the image side. And
The second lens group includes, in order from the object side, a biconvex positive lens, a biconcave negative lens, and a negative meniscus lens having a concave surface facing the object side.   2. The first lens group and / or the second lens group according to claim 1, wherein the first lens group and / or the second lens group are movable for zooming, and the first and second lens groups are movable when zooming from a wide angle end to a telephoto end. A lens system in which an air space between the lens groups is decreased and an air space between the second and third lens groups is increased.   2. The lens system according to claim 1, wherein the biconvex positive lens and the biconcave negative lens are bonded together.   2. The lens system according to claim 1, wherein the negative meniscus lens is made of plastic. A lens system according to claim 1;
A camera having an image sensor disposed on the image side of the lens system.

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