JPH08152557A - Photographic lens - Google Patents

Abstract

PURPOSE: To obtain a high optical performance over the overall object distance and the entire screen and easily obtain specific-length back focus while maintaining the advantages of a rear focus system by providing five lenses on the whole and properly setting various optical constants of the respective lenses. CONSTITUTION: A front group LF consists of two lenses, that is, a 1st negative meniscus lens which has a convex surface on the object side and a 2nd positive lens which has a convex surface on the image plane side in order from the object side. A rear group LR consists of three lenses, of a 3rd positive biconvex lens, a 4th biconcave lens, and a 5th biconvex lens. The lens is put in focus by moving the rear group LR on the optical axis and a condition 0.5<fF/fR<15 is satisfied where fF and fR are the focal lengths of the front group LF and rear group LR. Under this condition, the specific-length back focus is secured and telecentric characteristics on the image side can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear focus type photographing lens suitable for a photographic camera, a video camera, etc., and particularly to an electrophotographic camera or a video camera having a long back focus and using a solid-state image pickup device as an image pickup means. Suitable for shooting angle of view 40 ° to 50 °, F number 2 to 2.8
It is about the taking lens.

[0002]

2. Description of the Related Art In recent years, video cameras, electrophotographic cameras,
Various so-called still video cameras (SV cameras) have been proposed. The taking lens used in these cameras is required to have a relatively long back focus in order to arrange various glass materials such as a low pass filter and a color filter between the final lens surface and the image pickup means.

JP-A-57-163212 discloses that the number of lenses is 5 and the back focus is relatively long at 35 m.
Wide-angle lenses for m-film have been proposed.

A photographing lens using a color solid-state image pickup device as an image pickup means is required to have good telecentricity on the image side in order to prevent color shading.

In Japanese Unexamined Patent Publication No. 64-61714 and Japanese Unexamined Patent Publication No. 2-85816, the back focus of five groups of negative, positive, negative, positive, and positive lenses is relatively long, and the telecentric on the image side. A photographic lens with good performance has been proposed.

In recent video cameras, a part of the lens group in the lens system, for example, an intermediate lens group or a rear lens group is moved in the optical axis direction for focusing, that is, a so-called inner focus type or rear type. Focus-type taking lenses are often used.

Generally, in the inner focus type and the rear focus type, the front lens is fixed when the lens system is extended as compared with the system in which the entire lens system is extended, so that it is possible to focus on a closer object, and the focusing lens group is relatively small and lightweight. Therefore, focusing can be performed with a small driving force, which is suitable for a camera or the like having an automatic focus detection device. In addition, these focusing methods have an advantage that the entire length of the lens is always constant even when focusing is performed, so that the photographing apparatus can be easily held and the camera shake is unlikely to occur.

A photographing lens using such an inner focus type or a rear focus type is disclosed in, for example, Japanese Patent Laid-Open No. 55-55.
No. 147607 and Japanese Patent Laid-Open No. 61-140910.

[0009]

Generally, in order to secure a long back focus, a so-called reverse telephoto type lens system in which the front lens unit has a strong negative refractive power and the rear lens unit has a positive refractive power is used. It is desirable to construct a lens system. However, as the negative refracting power of the front lens group is strengthened, it becomes difficult to correct distortion and coma aberration, and when trying to satisfactorily correct these various aberrations, the lens configuration becomes complicated,
The shooting system tends to be large.

The present invention has five lenses as a whole, and maintains the advantages of the rear focus type by appropriately setting the optical constants of each lens, and at the same time, from an object at infinity to a near object. An object of the present invention is to provide a photographic lens which has high optical performance over the entire distance and the entire screen, and which is suitable when a solid-state image pickup means or the like is used as an image pickup means which can easily obtain a back focus of a predetermined length. .

[0011]

The taking lens of the present invention comprises:
A front group having a positive refracting power, an aperture stop, and a rear group having a positive refracting power are arranged in this order from the object side, and the rear group is moved along the optical axis for focusing, and the focal lengths of the front group and the rear group are set. It is characterized by satisfying the condition of 0.5 <fF / fR <15 (1) when fF and fR respectively.

[0012]

1 to 4 are numerical examples 1 to 4 of the present invention.
3 is a lens cross-sectional view of FIG. 5 to 8 are various aberration diagrams of Numerical Examples 1 to 4 of the present invention. In the lens cross-sectional view, LF is a front group having a positive refractive power, SP is a diaphragm, and LR is a rear group having a positive refractive power. G is a glass block such as a low-pass filter or a color filter, and IP is an image plane.

In this embodiment, in order from the object side, the front lens group LF is made up of two lenses: a negative meniscus first lens having a convex surface facing the object side and a second positive lens having a convex surface facing the image side. ing. The shapes and refractive powers of the first lens and the second lens are appropriately set, and astigmatism, field curvature, distortion, etc. are mainly corrected in a well-balanced manner. The rear lens group LR includes three lenses, a positive third lens whose both lens surfaces are convex surfaces, a fourth lens whose both lens surfaces are concave surfaces, and a fifth lens whose both lens surfaces are convex surfaces. The shapes of the three lenses in the rear group are appropriately set as described above so that variation in aberration when focusing is performed in the rear group is reduced.

In this embodiment, it is composed of 5 elements in 5 groups having the above-mentioned refractive power and lens shape, and by satisfying the conditional expression (1), a back focus of a predetermined length is secured,
Also, the telecentric characteristics on the image side are good, and the rear lens group LR
Is composed of three lenses having a predetermined refractive power and a lens shape, and by using a rear focus type in which the rear lens group LR is moved on the optical axis for focusing, the entire object distance from an infinite object to a short-distance object can be obtained. A photographic lens having a photographic field angle of 40 ° to 50 ° and an F number of 2 to 2.8 and having high optical performance with little aberration variation is obtained.

Next, the technical meaning of the conditional expression (1) will be described.

Conditional expression (1) defines the ratio of the focal length of the front lens group to the focal length of the rear lens group, and is a condition for obtaining good aberration correction mainly over the entire screen. When the value goes below the lower limit of conditional expression (1), the refractive power of the front group becomes relatively strong, so that it becomes difficult to correct various aberrations generated in the front group by the rear group. In particular, the occurrence of coma becomes remarkable, and other aberrations become large when trying to correct it. Also, if the upper limit is exceeded, the combined refractive power of the rear group will become strong, spherical aberration will be undercorrected, and the meridional image surface will be greatly curved and the astigmatic difference in the peripheral area of the screen will be large. It becomes difficult to obtain good optical performance.

The objective lens of the present invention can be achieved by setting each element as described above, but in order to obtain high optical performance over the entire screen and over the entire object distance, one of the following conditions is required. It is good to satisfy at least one.

(1-1) When the front group has a negative first lens and a positive second lens, and the focal lengths of the first lens and the entire system are f1 and f, respectively-2.2 <F1 / f <−0.9 ... (2) It is to satisfy the condition. The conditional expression (2) is also related to the conditional expression (1) and represents the ratio of the focal length of the divergent component (first lens) in the front group to the focal length of the entire system. This is for ensuring a sufficient length of back focus and performing good aberration correction.

If the lower limit of conditional expression (2) is exceeded, it becomes necessary to increase the distance between the first lens and the second lens in order to obtain a back focus of a sufficient length. Is longer, and the diameter of the front lens also becomes larger, which is not good. On the other hand, if the value exceeds the upper limit, the negative refractive power of the divergent component in the front group becomes too strong, and astigmatism and negative distortion occur in large numbers, which is not preferable.

(1-2) In order from the object side, the front group has a negative meniscus first lens having a convex surface directed toward the object side, and a positive second lens having a convex surface directed toward the image side. The rear lens group is a positive third lens having a convex surface directed toward the object side, a negative fourth lens having both concave lens surfaces, and a positive fifth lens having both convex lens surfaces.
It has a lens. By satisfying the above-mentioned configuration, a photographic lens having a good imaging performance with a photographic field angle of about 40 ° to 50 ° is obtained.

(1-3) In order from the object side, the front group has a negative meniscus first lens having a convex surface directed toward the object side, and a positive second lens having a convex surface directed toward the image surface side. The rear lens group is a positive third lens having a convex surface directed toward the object side, a negative fourth lens having both concave lens surfaces, and a positive fifth lens having both convex lens surfaces.
The i-th lens when it has a lens and the aperture is counted as one surface
When the radius of curvature of the th lens surface is Ri, the i-th lens thickness or the air gap is Di, and the total lens length is D, 0.1 <D3 / (D2 + D3) <0.6 (3) 15 < D / D7 <400 (4) | R8 | <| R7 | (5).

Conditional expression (3) expresses the relationship between the air gap between the first lens and the second lens and the wall thickness of the second lens (lens thickness), and is mainly a condition for obtaining good optical performance. Is. When the value goes below the lower limit of conditional expression (3), it becomes difficult to favorably correct the coma aberration generated in the negative first lens, and the light ray incident on the first lens passes through a position away from the optical axis. Therefore, it becomes difficult to satisfactorily correct the distortion. Further, if the upper limit is exceeded, it is advantageous for correction of distortion, but it is necessary to strengthen the refracting power of the negative first lens in order to obtain a sufficient back focus, so many other aberrations occur. So not good.

Conditional expression (4) is defined by the positive third lens and the negative fourth lens.
It defines the ratio of the total lens length to the air distance of the lens, and relates to ensuring sufficient back focus and good aberration correction in a photographing lens having a short total lens length. If the lower limit of conditional expression (4) is exceeded, it will be difficult to achieve both good optical performance and sufficient back focus. On the other hand, if the air gap between the third lens and the fourth lens becomes smaller than the upper limit, the manufacturing error of the lens and the variation of the aberration with respect to the assembling accuracy increase, which is not preferable.

Conditional expression (5) compares the radius of curvature of the image side lens surface of the third lens with the radius of curvature of the object side lens surface of the fourth lens. The two adjacent lens surfaces play a particularly effective role in correcting spherical aberration, and by satisfying the conditional expression (5), insufficient correction of spherical aberration is prevented.

(1-4) At least one lens surface, for example, the lens surface on the image side of the first lens or the lens surface on the image surface side of the fifth lens is provided with an aspherical surface by distortion and other various aberrations. It is preferable to correct the balance in a good balance.

Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air gap from the object side, and Ni and νi are respectively from the object side of the i-th lens. The refractive index of glass and the Abbe number. In the numerical examples, the last two lens surfaces are glass blocks such as face plates and filters. Table 1 shows the relationship between the above-mentioned conditional expressions and various numerical values in the numerical examples. When the aspherical shape is the X axis in the optical axis direction, the H axis in the direction perpendicular to the optical axis, the traveling direction of light is positive, R is the paraxial radius of curvature, and B, C, and D are aspherical coefficients, respectively.

[0027]

[Equation 1] It is expressed by the formula.

<Numerical Example 1> f = 6.8 FNo = 2.5 2ω = 47.5 ° i RD N ν 1 20.517 0.7 1.744 44.79 2 3.857 4.0 1 3 50.219 6.0 1.603 38.01 4 -6.756 2.0 1 5 (Aperture) 2.0 1 6 9.696 1.9 1.804 46.58 7 -15.069 0.3 1 8 -8.905 2.5 1.847 23.78 9 7.695 1.2 1 10 79.192 2.1 1.697 55.53 11 -6.957 5.0 1 12 ∞ 5.0 1.516 64.15 13 ∞ <Numerical Example 2> f = 8.0 FNo = 2.5 2ω = 41.1 ° i RD N ν 1 13.394 1.0 1.697 55.53 2 5.997 12.5 1 3 -18.275 2.0 1.762 26.52 4 -9.467 2.0 1 5 (aperture) 2.0 1 6 -7.332 1.8 1.772 49.60 7 -69.050 0.4 1 8 -16.171 2.4 1.847 23.78 9 7.153 1.2 1 10 26.669 1.6 1.697 55.53 11 -9.202 5.0 1 12 ∞ 5.0 1.516 64.15 13 ∞ <Numerical Example 3> f = 7.0 FNo = 2.0 2ω = 46.7 ° i R D N ν 1 20.144 0.8 1.713 53.84 2 6.058 10.3 1 3 -24.787 2.1 1.699 30.12 4 -10.100 2.0 1 5 (Aperture) 2.0 1 6 7.916 2.0 1.603 60.70 7 -24.925 2.0 1 8 -8.231 2.0 1.847 23.78 9 10.838 1.3 1 10 21.819 2.4 1 55.53 11- 6.513 5.0 1 12 ∞ 5.0 1.516 64.15 13 ∞ Aspherical surface coefficient (11th surface) B ＝ 4.79 × 10 ^-4 C ＝ 6.57 × 10
^-6 D = -4.23 × 10 ^-8 <Numerical Example 4> f = 7.0 FNo = 2.8 2 ω = 46.4 ° i R D N ν 1 13.983 1.0 1.720 50.25 2 4.510 13.0 1 3 20.164 5.0 1.603 38.01 4 -13.710 2.0 1 5 (Aperture) 2.0 1 6 9.078 2.5 1.720 50.25 7 -21.559 0.1 1 8 -14.165 2.0 1.847 23.78 9 7.463 1.5 1 10 19.554 2.1 1.697 55.53 11 -14.556 5.0 1 12 ∞ 5.0 1.516 64.15 13 ∞ Aspheric coefficient (2nd surface) ) B = -5.98 x 10 ^-4 C = 3.36 x 1
0 ^-6 D = -2.67 x 10 ^-6

[0029]

[Table 1]

[0030]

As described above, according to the present invention, five lenses are provided as a whole, and by appropriately setting the optical constants of each lens, the advantages of the rear focus system can be maintained while maintaining the infinity. Suitable when using a solid-state image pickup device or the like as an image pickup device that has high optical performance over the entire object distance from the object to the short-distance object and over the entire screen, and that can easily obtain a back focus of a predetermined length. A shooting lens can be achieved.

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view of Numerical Example 1 of the present invention.

FIG. 2 is a lens cross-sectional view of Numerical Example 2 of the present invention.

FIG. 3 is a lens sectional view of Numerical Example 3 of the present invention.

FIG. 4 is a lens cross-sectional view of Numerical Example 4 of the present invention.

FIG. 5 is a diagram of various types of aberration in Numerical example 1 of the present invention.

FIG. 6 is a diagram of various types of aberration in Numerical example 2 of the present invention.

FIG. 7 is a diagram of various types of aberration in Numerical example 3 of the present invention.

FIG. 8 is a diagram of various types of aberration in Numerical example 4 of the present invention.

[Explanation of symbols]

 LF front group LR rear group SP aperture IP image plane d d line g g line S sagittal image plane M meridional image plane

Claims (5)

[Claims] 1. A front group having a positive refracting power, an aperture stop, and a rear group having a positive refracting power are arranged in this order from the object side, and the rear group is moved along the optical axis for focusing, and the front group and the rear group are arranged. A photographing lens characterized by satisfying a condition of 0.5 <fF / fR <15, where fF and fR are focal lengths of the group, respectively. 2. The front group has a negative first lens and a positive second lens, and the focal lengths of the first lens and the entire system are f, respectively.
The photographing lens according to claim 1, wherein a condition of -2.2 <f1 / f <-0.9 is satisfied when 1 and f are satisfied. 3. The front group, in order from the object side, has a negative meniscus first lens having a convex surface directed toward the object side, and a positive second lens having a convex surface directed toward at least the image side, and the rear group. Has at least a positive third lens having a convex surface directed toward the object side, a negative fourth lens having both concave lens surfaces, and a positive fifth lens having both convex lens surfaces. The shooting lens of item 1. 4. The front group includes, in order from the object side, a negative meniscus lens having a convex surface directed toward the object side, and a positive second lens having a convex surface directed toward at least the image side, and the rear group. Has at least a positive third lens with a convex surface facing the object side, a negative fourth lens with both lens surfaces concave, and a positive fifth lens with both lens surfaces convex, and the first lens and the entire system 2. The taking lens according to claim 1, wherein the following conditions are satisfied: 2.2 <f1 / f <-0.9, where f1 and f are focal lengths of f1 and f2, respectively. 5. The front group has, in order from the object side, a negative meniscus lens having a convex surface directed toward the object side, and a positive second lens having a convex surface directed toward at least the image surface side, and the rear group. Has at least a positive third lens with a convex surface facing the object side, a negative fourth lens with both lens surfaces concave, and a positive fifth lens with both lens surfaces convex, counting the aperture as one surface When the radius of curvature of the ith lens surface is Ri, the ith lens thickness or air gap is Di, and the total lens length is D, 0.1 <D3 / (D2 + D3) <0.6 15 <D The taking lens according to claim 1, wherein a condition of / D7 <400 | R8 | <| R7 | is satisfied.