JPS61129613A - Variable focal length lens - Google Patents

Abstract

PURPOSE:To obtain a variable focal length lens which is sufficiently separated from a film and a photographic paper in case of a low magnification as well as a high magnification and has an excellent focusing capacity, by constituting the lens system with a negative lens group and a positive lens group and setting them to satisfy prescribed conditions. CONSTITUTION:The lens system consists of the first lens group G1 having a negative focal length and the second lens group G2 having a positive focal length. In the variable focal length lens where the interval on the axis between both lens groups is varied to vary the focal length of the whole of the system, the first lens group G1 consists of the first negative lens L1 and the second positive lens L2 which are arranged in order from the front side, and the second lens group G2 consists of a front group Ga having the third positive lens L3, the fourth lens L4, the fifth negative lens L5, and the sixth positive lens L6 which are arranged in order from the front side and a rear group Gb having the seventh negative meniscus lens L7, and these lenses satisfy conditions of prescribed formulas.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention relates to a lens whose magnification can be changed continuously;
In particular, it relates to variable focal length lenses used at limited distances, such as in photographic enlargers.

(Background of the Invention) In order to change the enlargement magnification in a general enlarger, a lens with a fixed focal length is used and the positional relationship of the film, lens, and photographic paper is changed. In enlargers for photographic purposes, a variable focal length lens is often used to fix the position of the film and photographic paper.The lens system for this purpose is a system consisting of two lens groups. The method of changing the focal length by changing the interval is the simplest and cheapest method. By the way, in order to incorporate the exposure control, shutter mechanism, etc. around the lens in an enlarger, it is necessary to maintain a certain distance between the film and the lens, and the distance between the lens and the photographic paper must be as large as possible. This is desirable from the viewpoint of workability, but with conventional variable focal length lenses consisting of two groups consisting of a negative lens group and a positive lens group from the photographic paper side, the distance between the photographic paper and the lens becomes short at low magnification. Conversely, with a variable focal length lens consisting of a positive lens group and a negative lens group from the photographic paper side, the distance between the lens and the film tends to become extremely short at high magnification. For this reason, it has been difficult to realize a practical enlarger with good operability while maintaining excellent imaging performance using a lens system with a simple two-group configuration.

(Object of the Invention) The object of the present invention is to solve the above-mentioned drawbacks, and to provide a simple lens system consisting of two groups, a negative lens group and a positive lens group, in order from the photographic paper side, and which can be used at low magnifications as well as at high magnifications. To provide a variable focal length lens which has excellent imaging performance even when the lens is sufficiently far away from film and photographic paper.

(Summary of the Invention) As in the illustrated embodiment, the present invention comprises, in order from the front side, a first lens group G1 having a negative focal length as a whole, and a second lens group G□ having a positive focal length as a whole. In a variable focal length lens in which the focal length of the entire system can be changed by changing the axial distance between the first lens group G1 and the second lens group G, the second lens LG1 is attached in order from the front side. Consisting of a first lens 1+, which is a negative lens, and a second lens L, which is a positive lens. 4 lens L4, a 5th lens L5 which is a negative lens cemented or separated from the 4th nuclear lens, and a 6th lens L which is a positive lens made of a cemented negative lens and a positive lens.
a front group G having a lens diameter of
It is composed of a rear group G and h having a lens and a rear group G and h. Here, the front side means the enlargement side in terms of the mutually conjugate object-image relationship with respect to the lens system, and the side on which photographic paper is placed in terms of the enlarger.

In the lens configuration of the present invention, the following condition is further satisfied: 61.6<lfg/f
Al<2.6 (1) 0.5f! < D <
1.2ft (2) 0, -4 < R, /R, <
0.7' (3) 0.6 < Re/Rm <
1.1 (4) However, r^ is the second lens group G
The composite focal length of the front group in 2, f@ is the second lens group G
, the composite focal length of the rear group in "2 is the second lens group G
2, D is the distance between the rear principal point of the front group in the second lens group Gt and the front principal point of the rear group, and Ra and Rb are the front and rear lenses of the seventh lens, respectively. The radii of curvature of the surfaces, Rc and Rd, are the radius of curvature of the rear lens surface of the first lens and the radius of curvature of the front lens surface of the second lens, respectively.

The present invention basically achieves this by making the second lens group a telephoto type with a small telephoto ratio (the ratio of the distance from the apex of the foremost lens surface of the lens system to the image plane to the focal length of the entire system). , the above objectives were achieved. Each conditional expression according to the present invention will be explained below.

Equation (1) is a necessary condition to ensure a sufficient distance between the lens system and photographic paper and to maintain excellent imaging performance. (1) If the upper limit of 2 is exceeded, the telephoto ratio of the second lens group Gt will become large, making it difficult to maintain a long distance between the photographic paper and the lens at low magnification. If this happens, the distance between the first lens group G1 and the second lens group aX should be widened (in order to ensure the zoom ratio of the lens without having to widen the distance between the first lens group G1 and the second lens group aX, Focal length #fB has to be made smaller, which makes it difficult to correct aberrations, and furthermore, the telephoto ratio of the second lens group G8 becomes too small, making it impossible to secure a sufficient distance between the film and the lens required at high magnification. (It becomes.

In formula (2), if the upper limit is exceeded, the total length of the lens becomes too long and the sixth lens L becomes too large, which loses the advantage of a variable focal length lens that requires small size and light weight. , the distance from the rear end of the seventh lens L7 to the photographic paper (image plane) becomes too short on the high magnification side. On the other hand, when the lower limit is exceeded, in order to secure the necessary telephoto ratio in the second lens group G, the focal length fB of all groups must be
and f.

The absolute values of both must be made small, making aberration correction difficult.

(3) In the second case, when the upper limit is exceeded, a large positive distortion occurs, and the inner coma becomes too negative in the low magnification range. If the lower limit is exceeded, the bulge of the spherical aberration becomes too large, and the spherical aberration on the short-length side becomes thick (overcorrected).And, if the upper limit of the condition of equation (4) is exceeded, On the high magnification side, the image plane becomes too negative compared to the on-axis axis, and on the low magnification side, it becomes too positive, and this cannot be corrected by other lens parts.If the lower limit is exceeded, the image plane on the high magnification side becomes too negative. It becomes positive, and becomes negative on the low multiplier side.

In the configuration of the present invention as described above, it is desirable that the following conditions are further satisfied.

First, when the focal length of the first lens group G1 is 1, and the distance between the first lens and the second lens is d□, 0.061 fB
It is desirable to satisfy the condition l' d, < 0.161fBl (5). Under these conditions, if the upper limit is exceeded, the entire lens system will become larger, and
The sine condition becomes too negative on the low multiplier side. Moreover, when the lower limit is exceeded, comatic aberration occurs positively on both the outer and inner sides on the low magnification side, and lateral chromatic aberration on the high magnification side becomes large. Also, 0.5 < f+/ fh < 1.3 (
It is also desirable to satisfy the condition 6). If the upper limit of this condition is exceeded, the change in the air distance between the first lens group G and the second lens group G2 to ensure the necessary magnification range becomes too large, which is undesirable as it increases the size of the entire lens system. , if the lower limit is exceeded, the refractive power of each lens group becomes too strong, making it difficult to correct aberrations, although the size may be reduced.

(Example) Examples according to the present invention will be described below.

All three embodiments have a variable power ratio of about 2. In the first embodiment shown in FIG. 1, the first lens L is on the front side (
It is a positive meniscus lens with the convex surface facing the photographic paper side.
The second lens L2 is also a negative meniscus lens with a convex surface facing toward the rear. The third lens group in the front group G of the second lens group Gt
The lens 2 is a biconvex positive lens, which is constructed by separating the fourth lens L4 and the fifth lens L, and a biconvex air lens is formed between them. 6th
The lens L6 has a biconvex shape in which a negative lens with its convex surface facing forward and a biconvex positive lens are cemented together.

Further, in the second embodiment shown in FIG. 2, the first lens group G1
The first lens L in the middle is composed of a single negative lens, and the fourth lens L and the fifth lens Ls in the front group of the second lens group G2 are cemented together, and the whole has a convex surface facing the front side. It is configured to have a meniscus shape.

In the third embodiment shown in FIG. 3, the first lens group G
The second lens L2 as a negative lens in G!1 is composed of a cemented positive lens and a negative lens, and the second lens group G! The fourth lens L4 and the fifth lens L in the front group are configured separately.

The table below shows the specifications of each example. In the table, the leftmost number represents the order from the front side (photographic paper side), and β represents the magnification on the film surface when ray tracing is performed from the front side (photographic paper side). In addition, the refractive index and the number are respectively d
The value is for the line (λ-587, 6n+w), do represents the distance between the photographic paper and the foremost lens surface, and Bf represents the distance between the final lens surface and the film surface.

Table 1 (First Example) Table 2 (Second Example) Image Circle 87φ Table 3 (Third Example) Various aberration diagrams for the first, second and IK3 examples above are shown in Figures 4 and 5, respectively. As shown in FIG.

Each aberration diagram shows each aberration state in the three magnification states shown in the specification table above.

According to each aberration diagram, it can be seen that all examples maintain good imaging performance over the entire magnification range. Moreover,
As shown in each specification table, the distance from the lens to the photographic paper @d and the distance Bf to the film are sufficiently secured in both high and low magnification conditions, making it suitable as a variable focal length lens for enlargers. It is also clear that this is a practical lens system.

(Effects of the Invention) As described above, according to the present invention, although it is a simple lens system consisting of two groups, a negative lens group and a positive lens group in order from the photographic paper side, the lens can be used both at low magnification and at high magnification. is sufficiently far away from the film and paper to achieve a variable focal length lens with excellent imaging performance.

[Brief explanation of the drawing]

FIG. 1 is a lens configuration diagram of a first embodiment of a variable focal length lens according to the present invention, FIG. 2 is a lens configuration diagram of a second embodiment of the present invention, and FIG. 3 is a lens configuration diagram of a third embodiment of a variable focal length lens according to the present invention. The configuration diagram, FIG. 4, FIG. 5, and FIG. 6 are various aberration diagrams of the first embodiment, the second embodiment, and the third embodiment, respectively. [Explanation of symbols of main parts] G1...first lens group G having a negative focal length!・・・
・Second lens group G having a positive focal length,...front group G of the second lens group,...rear group of the second lens group

Claims (1)

[Claims] Consisting of, in order from the front, a first lens group G_1 having a negative focal length as a whole, and a second lens group G_2 having a positive focal length as a whole, the first lens group G_1 and the second lens group G_2 having a positive focal length as a whole. In a variable focal length lens that can change the focal length of the entire system by changing the axial distance between it and the second lens group G_2, the first lens group G_1 is made of a bonded or a single negative lens in order from the front side. The second lens group G_2 has a first lens and a second lens that is a positive lens, and the second lens group G_2 includes, in order from the front side, a third lens that is a positive lens, a fourth lens that is a positive lens,
A front group having a fifth lens which is a negative lens cemented with or separated from the fourth lens, a sixth lens which is a positive lens which is made of a cementation of a negative lens and a positive lens, and a rear group which is made of a cementation of a negative lens and a positive lens. and a rear group having a seventh lens which is a negative meniscus lens with a convex surface facing the side, and is characterized by satisfying the following conditions: 1.6<|f_B/f_A|<2.6 (1) 0.5f_
2<D<1.2f_2(2) 0.4<R_a/R_b<0.7(3) 0.6<R_c/R_d<1.1(4) However, f_A is the The composite focal length of the front group, f_B is the composite focal length of the rear group in the second lens group G_2, f_2 is the composite focal length of the second lens group G_2, and D is the composite focal length of the front group in the second lens group G_2. The distance between the rear principal point and the front principal point of the rear group, R_a and R_b are the radii of curvature of the front and rear lens surfaces of the seventh lens, respectively, and R_c and R_d are the rear lens surfaces of the first lens, respectively. and the radius of curvature of the front lens surface of the second lens.