JPS62143011A - Lens system for short range photographing - Google Patents

Abstract

PURPOSE:To improve the image forming performance by a simple lens constitution by increasing a spatial interval of the front group and the rear group for satisfying a specified condition and moving both the groups to an object side, in case of focusing to a short range object from an infinity object. CONSTITUTION:The titled lens system is provided with a front group G1 and a rear group G2 having a positive refractive power, respectively, and a diaphragm provided between both the groups, and by increasing a spatial interval between both the groups, in which the diaphragm has been placed, and moving together both the groups to an object side, focusing to a short range object from an infinity can be executed. Focal length of both the groups are formed so as to satisfy conditions of expressions I, II. The diaphragm is constituted so as to be movable on the optical axis with one of the front group and the rear group in unison. Also, an average break rate N1 of a lens for constituting the front group is formed so as to satisfy a condition of an expression III. Moreover, radiuses of curvature r1, r2 of faces of an object side and an image side of a positive lens of the nearest position to the object side in the front group are formed so as to satisfy a condition of an expression IV, respectively. In this way, an image forming performance can be improved for photographing extending from a short range to an infinity object by a lens of a simple constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a close-range photographing lens system capable of photographing objects from an infinite distance to an extremely close distance.

2. Description of the Related Art Hitherto, various close-range photography lenses called microlenses or microlenses have been put into practical use, but they have not yet been sufficiently developed. Although these lenses are designed to provide the best performance for objects at relatively close distances, it is inevitable that the imaging performance will deteriorate as the magnification increases. According to this rule, in order to maintain excellent focusing performance for objects at close distances where the photographing magnification is 1x, a special attachment lens for aberration correction must be attached to the lens body. 2) The F number of the conventional lens was around 3.5 at most, which was unsatisfactory even in terms of brightness as a regular lens.

The object of the invention is to provide a lens for close-range photography, which has a large aperture and always has excellent constant imaging performance in the photographing range from infinite distance to extremely close objects.

The close-range photography lens of the present invention is composed of a front group having a positive refractive power, a rear group also having a positive refractive power, and a diaphragm provided between both groups, and is capable of shooting short distances from an object at infinity. When focusing on an object, both groups are moved toward the object while increasing the air gap between the two groups.

The present invention establishes a new method of correcting aberrations in a close-range photographing state of a lens system consisting of two positive groups by employing so-called interval correction. In the present invention, when shooting at close range, the air gap between both groups, that is, the so-called diaphragm gap, increases. If the diaphragm is provided integrally with the rear group, the entrance pupil moves far away from the object and enters the lens system. Since the angle between the light beam and the optical axis becomes smaller, aberration correction becomes easier.

When the aperture is provided integrally with the front group, the exit pupil is moved away from the image, and the angle of the light beam exiting the lens system becomes small, making it easy to correct aberrations even in this case. Therefore, by appropriately balancing the distribution of refractive power between the front and rear groups, that is, the distribution of apparent brightness, it is possible to reduce aberration deterioration at close range without creating a complicated lens system. A Gaussian type lens system is typical as a lens system consisting of two lens groups with positive refractive power sandwiching an aperture, but this type of lens system has a large aperture ratio and a sufficiently long back lens. When trying to obtain focus, the refractive power of the front group tends to be significantly weaker than the refractive power of the rear group. When increasing the aperture ratio, each lens becomes thicker and the back focus becomes shorter. Therefore, in order to obtain sufficient back focus when photographing objects at infinity, the refractive power of the front group must be weakened. This means that the −72+ element flux emitted from the object is not converged much in the front group, and the closer the object is, the more the light emitted from the front group diverges. This light flux is passed on to the rear group, which requires an increase in refractive power and brightness, but it is possible to satisfactorily correct various aberrations*a! very difficult to do. For this reason, it is necessary to make the refractive power of the front group stronger than that of conventional large aperture ratio lenses of this type, and to increase the load on the front group as much as possible. In this way, the burden of aberration correction on the rear group is reduced at close range, and even at close range, the light beam emitted from the object can be used as a convergent light beam in the front group.

However, if the refractive power of the front group becomes too strong, it becomes difficult to correct aberrations in the front group, so it is possible to keep the light beam from the object at the closest distance to a slightly divergent beam that exits the front group. Desirable O Considering the characteristics of the lens system according to the present invention as described above, it is desirable that the following condition is satisfied.

1.6〈door(2,4(1) f.

1.5(-(Rei5(2)) Here, the composite focal length of the entire frri system, fl s J'2
represent the focal lengths of the front group and the rear group, respectively.

The condition of equation (1) describes the appropriate distribution of refractive power to the front group. If the lower limit of this condition is exceeded, the refractive power of the front group will become too strong and it will be difficult to make the back focus sufficiently long in infinity shooting conditions.
The variation in spherical aberration becomes significant between the shooting state of an object at infinity and the state of shooting a close-range object. In order to achieve this correction, the front group must have a complicated structure. On the other hand, if the upper limit is exceeded, the burden of refractive power on the rear group becomes relatively strong.
Spherical aberration occurs significantly at close range and becomes difficult to correct. Increasing the refractive power of the rear group is effective for increasing the aperture ratio as described above, but it is disadvantageous for objects at close range, and it is not suitable for a lens system that achieves high imaging magnification such as the present invention. It is desirable to set it within the above range.

The condition of equation (2) determines the ratio of the focal lengths of the front group and the rear group, that is, the distribution of refractive power. Together with the condition of equation (1), it provides brightness, a short close-up distance, and a high imaging magnification. That's what it is. Also, this condition expresses the distance between the front and rear groups based on the condition of equation (1). If the lower limit is exceeded, the refractive power of the front group becomes too strong and the amount of various aberrations in the front group increases. To compensate for this, it is possible to increase the number of lenses in the front group and make the lens thicker, but the image-side principal point of the front group is embedded inside the lens. The lenses in the group will interfere with each other, making it difficult to provide sufficient aperture space. If the upper limit is exceeded, aberrations can be corrected satisfactorily in infinity photography conditions even if the lens is fairly bright, but in close-up photography conditions, various aberrations occur and good correction cannot be maintained.

In the FL configuration consisting of the front group and the rear group as described above,
Specifically, a so-called modified Gauss type lens system was adopted as the basic configuration. That is, as shown in FIG. 1, which shows the entire cross-sectional view of the optical system of the first embodiment, the front group Gxvi includes, in order from the object side, the first positive lens L1, the positive meniscus lens h1 having a convex surface on the object side, and the positive meniscus lens h1, which also has a convex surface on the object side. The convex surface is composed of a negative meniscus lens L, and the rear group is composed of a negative lens and a positive lens, and the concave surface on the object side is composed of a companion meniscus lens L4 and a second positive lens L8. Configure it. In the conventional method of photographing objects at close range without changing the aperture interval in such a lens configuration,
Spherical aberrations were overcorrected, astigmatism was large, field curvature became significant, and excessive coma aberration also occurred, but by changing the aperture spacing according to the present invention, these worsening aberrations can be reduced. can be corrected very well.

This unique construction satisfies the achromatic conditions for both the front and rear groups, so there is little image deterioration due to chromatic aberration even when the aperture distance is changed and each group is moved over a wide range. . Therefore, with a relatively simple configuration, various aberrations can be sufficiently corrected even for objects at extremely close distances. Here, the average refractive index N! of each lens constituting the front group! 1.68 < Nt < 1.78 (3
), and the first positive lens L1 located closest to the object side
It is desirable to set the radius of curvature of the object-side surface and the image-side surface to rl and 2, respectively.

When the refractive index of the front group decreases beyond the lower limit of the condition in equation (3), the curvature of each lens surface increases to bear the refractive power of the front group determined by equations (1) and (2). Become,
Various aberrations, especially higher-order spherical aberrations, occur significantly, making it difficult to correct them at close range. On the other hand, if the upper limit of this condition is exceeded, the refractive index of the negative lens that performs achromatization in the front group tends to increase, the Petzval sum becomes positively excessive, and the curvature of field increases. vinegar. The condition of equation (4) is intended to satisfactorily correct high-order spherical aberration at the closest distance, and the larger the aperture ratio of the lens system, the more significant negative distortion occurs at the closest distance. Correct aberrations
It's a special thing. In this lens system with the above configuration, it is possible to suppress the astigmatism difference and maintain the flatness of the image plane by increasing the aperture distance for objects at close distances, but on the other hand, Negative distortion tends to become significant. It is desirable to allow a certain degree of positive distortion for this narrowed object at infinity, and the shape of the first positive lens L1, which plays the largest role in correcting distortion aberration, is
) is determined using the so-called shape factor. If the lower limit of this condition is exceeded, the occurrence of comatic aberration increases, and if the upper limit is exceeded, negative distortion aberration increases, making it difficult to correct other factors satisfactorily.

An embodiment of a lens system for close-range photography according to the present invention will be described below. As shown in FIG. It's nicely structured. FIG. 1(α) shows the state when photographing an object at infinity in this embodiment, and FIG. 1(b) shows the state when photographing an object at the closest distance. Here, the aperture is configured to move together with the rear group. The specifications of this example are shown in Table 1, and various aberration diagrams are shown in Fig. 2. Second
Figure (α) shows the object distance (distance from the frontmost lens surface of the lens system to the object when n4 = ω, ψ in Figure 2) #: object distance d = 164,194, photographing magnification β - 1, Various aberrations in the case of 0 are shown. 4 = ■ and β = -1 in Madashi 2 Figure 0
, 0, the chromatic aberration of spherical aberration and the chromatic aberration of magnification are shown. Although it is a fairly bright lens system with an F number of 2.8, it can be seen that even at the same magnification, an extremely good correction of 4 Vc is maintained for not only chromatic aberration but also for each station difference.

Table 1 (First Example) Focal length f = ioo, o F number 8 Angle of view 2ω =
42.92°ct, :When variable object distance d6=cc+, cL6=13.301 object distance do" 164.194, that is, photographing magnification β=-1,
0 Throat d6 = 29,773 The diaphragm is at the front 5.818 position f of the frontmost lens surface of the rear group G, = 208,824 fx = 115.797 2nd Example i-j Figure 3 (α), As shown in (6), a positive meniscus lens having a relatively weak refractive power and having a convex surface facing the object side between the positive meniscus lens L2 and the negative meniscus lens L3 in the front group in the configuration of the first embodiment described above. Lzs f provided.

This allows better correction of aberrations in the front group. Figure 3 (α) fl-tcoo = cry, Figure 3 (h) F
The respective states of i cLo =+209 and 4β=-0.7143 are shown.

The specifications of this example are shown in Table 2, and do=■ and do''2
Various aberrations when β=09.438 and β=−0,7143 are shown in FIG. 4 @) and FIG. 4(b), respectively.

It can be seen that although the lens has an F number of 2.0 and a large aperture ratio, various aberrations are well corrected even at the closest distance.

Table 2 (Second Example) Focal length 7=100. OF number 2.0 angle of view 2ω = 3
2.13゜4 = Variable object distance do = When oo, d = 13,710 object distance do'' 209,438, that is, photographing magnification β = -0,7
143 (tg=32,377 aperture i-, j rear group G
1 front lens surface n1 square 5,333 so 111rf
.

fI = 2 0 4, 0 0 0 fz = 1 0 7, 6 9 2 As shown in Fig. 5 @) and (b)), the third embodiment is a positive meniscus lens L2 in the configuration of the second embodiment as described above.
In place of m, a negative lens L23 consisting of a negative lens and a positive lens is provided to achieve excellent achromatization in the front group and to improve the ability to correct other aberrations. As in the second embodiment, an F number of 2.0 is possible. 7toWJ5 Figure (z) u do = ω, Figure 5C
h) indicates the respective states of do=205.696 and β=-0,7143. The specifications of this example are shown in Table 3, and d
Various aberrations when o=ω and β=−0,7143 are shown in FIG. 6(α) and FIG. 6(A), respectively.

Table 3 (Third fruiting example) Focal length f-=100. OF number 2.0 angle of view 2ω =
32.13°4=When variable object distance d6=co 4=11,086 object distance do=205,696, that is, photographing magnification β=-0,71
43, 4 = 29,753 The aperture is located 5,333 in front of the frontmost lens surface of the rear group Gl fl = 204,000 fz = 107,692 As described above, according to the present invention, the auxiliary lens is We hope to have achieved a lens system that maintains good aberration correction even when photographing objects at extremely close distances with a large aperture ratio, even though it is a simple lens without any additions.

[Brief explanation of drawings]

FIG. 1 is a sectional view of an optical system according to a first embodiment of the present invention, in which (α) shows the state when photographing an object at an infinite distance, and (h) shows the state when photographing an object at the closest distance. Figure 2 is a diagram of various aberrations in Figure 1, and @) is the object distance do =
<b>n object distance do = 164.194, photographing magnification β
=-i, o, (C) u CLo =■, β=-1,0
The various aberrations in the case of - Figure 3 is a sectional view of the optical system of the second embodiment of the present invention (L:L
) rtz 4= Co, (h) l't d-o= 20
9,438 and β=-0,7143. Figure 4 d In the aberration diagram in Figure 3, (α) shows the various aberrations when 4 = ω, and (h) shows the various aberrations when β = -0,7143% (Lo = 209.438. In the cross-sectional view of the optical system of Example 3, (α) ke cL6
=oo, (lI) is d−o= 205,696, β=−
The state is 0,7143. Figure 6 is a diagram of various aberrations in Figure 5, (a) tid-o=ω,
(h) shows various aberrations when do=205,696 and β=-0,7143. [Explanation of symbols of main parts] Fig. 1 (A9 Fig. 2 (0-) σ., ~ Spherical aberration '4L Mulberry aberration Distorted paternity 'Aberration (
8) 〆ri=-iO Fig. 2 (C) Ryo ■ 硅 color gradation 0 ρq Matara 4 Fig. (σ) Spherical convergence 4 ■ i Mata dust Distortion aberration Normal convergence 2 digits)
/S'=-0,741L5 ,,+ Toro procedural amendment December 15, 1985

Claims (1)

[Claims] 1. Consisting of a front group having a positive refractive power, a rear group also having a positive refractive power, and a diaphragm provided between the two groups, it is possible to move from an object at infinity to an object at a short distance. 1. A lens system for close-up photography, characterized in that when focusing on a subject, both groups are moved toward the object side while increasing an air gap between the two groups. 2. In the lens for short-distance photography according to claim 1, when the composite focal length of the entire system is f, and the focal lengths of the front group and the rear group are f_1 and f_2, respectively, 1.6<f_1/ It is characterized by satisfying the following conditions: f<2.4 1.5<f_1/f_2<2.5.