JPH0412453B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a copying lens used at around 1:1 magnification, and particularly to a copying lens that compensates for temperature changes due to a plastic lens. (Prior art) Copying lenses used at around 100% magnification have a narrow aperture and are configured symmetrically to reduce coma and
Since distortion can be eliminated, it is common to use a symmetrical four- or five-element lens system.
Optical glass has been used as a lens material. In recent years, there has been an increasing demand for smaller and lighter copying machines, and compact copying lenses with four or five lenses have come into widespread use. Examples of this type of lens include, for example, JP-A-57-
Examples include those described in Publication No. 147611.
However, the lens material is optical glass, and the first and fifth lenses are made of so-called lanthanum glass, which has a refractive index of 1.65 or more and a specific gravity of about 4, so it is still reasonably cheap and lightweight. It is difficult to say. If this can be replaced with a plastic lens with a specific gravity of about 1.2, a lighter and cheaper copying lens can be provided. However, plastic has a large change in refractive index due to temperature, which is more than 10 times the change in refractive index of optical glass, and when it is attached to a device such as a copying machine where the temperature rises due to the light source, the focal length of the lens system becomes large. There were drawbacks such as changes. By the way, if a copying lens with 5 elements in 5 groups as shown in Figure 1 is designed using only plastic lenses, the results will be as shown in Table 1, and when the temperature is 20°C, f =
230mm, but when the temperature reaches 60℃, f=
It becomes 233.6mm, making the focal length approximately 3.6mm longer.

[Table] When this is used for 1-magnification imaging, the distance between the object and the image is 920 mm at 20°C and 934 mm at 60°C, and the difference between the two is 14 mm, which is the amount of movement of the focus position. in general,
Copying lenses have a large F number and require low resolving power, so they have a deep depth of focus.
Such a large shift in focus position cannot be covered. Furthermore, there are only a few types of plastic materials that are transparent, homogeneous, easy to process, and have a suitable hardness that can be used in lenses, and they generally have a low refractive index.
Since it is low at 1.65 or less, the degree of freedom in design is limited. (Objective of the Invention) The object of the present invention is to use a plastic material having the above-mentioned drawbacks, to have an F number of about 8, a half angle of view of about 20°, various aberrations are good, and the temperature of the lens system changes. In this case, the objective is to obtain a copying optical system in which the change in focal length caused by changes in the refractive index is small. (Structure of the Invention) The lens system of the present invention, as shown in its cross section in FIG. 2, has a basic structure of five lenses similar to the lens shown in FIG.
If the power of the fourth lens is suppressed to be 10 times or more or -10 times or less the combined focal length of the entire system, changes in converging power and diverging power due to temperature changes will be almost negligible. Therefore, it is only necessary to consider the influence of the first and fifth positive lenses and the third negative lens as to the influence on the change in focal length of the entire system due to temperature change. If these three lenses are made of glass material, the effect of temperature changes will be small, but it will also reduce costs.
Needless to say, it is desirable to make more of the lenses made of plastic in order to reduce weight. This invention is characterized in that either the first lens or the fifth lens and the third lens are made of plastic. That is, if only one of the first positive lens and the fifth positive lens is made of plastic, the influence of temperature changes is halved compared to the case where both are made of plastic. In addition, the third lens has negative power, and the influence of temperature change acts in a direction that cancels out the influence of the positive lens, so that the influence as a whole can be made extremely small. Specifically, the copying lens of the present invention includes, in order from the object side, a biconvex first lens, a meniscus second lens with a convex surface facing the object side, and a biconcave third lens.
The lens consists of 5 elements in 5 groups: a meniscus fourth lens with a convex surface facing the image side, and a biconvex fifth lens.
Either the first lens or the fifth lens is made of optical glass, and the other four lenses are made of plastic, -1/10f<1/f ₂ (=1/f ₄ )<1/10f ... (1 ) where f: synthetic focal length of the entire system fi: configured as a lens that satisfies the conditions of the focal length of the i-th lens from the object side. Furthermore, with this configuration, F number 8,
In order to perform good aberration correction up to a half angle of view of about 20°, by increasing the positive refractive power of the third and eighth surfaces, the biconvex first lens is made of a low refractive index material.
Compensate for the lack of positive refractive power of the lens and the fifth lens,
Reduces aberrations caused by biconvex lenses, and
It is desirable that the negative refractive power of the biconcave third lens be shared between the fourth and seventh surfaces to prevent excessive aberration correction by the third lens. The conditions for this are as follows. 0.5f<f ₁ +f ₅ /2<0.6f ...(2) 0.1f<r ₃ (=-r ₈ )<0.2f ...(3) where r ₁ : curvature of the i-th refractive surface from the object side Radius If the lower limit of the above conditions (1), the upper limit of (2), and the lower limit of (3) are exceeded, spherical aberration will be over-corrected, but the meridional image surface at the periphery will be under-corrected, so the image surface will be flattened. Can not do it. When the upper limit of condition (1) and the upper limit of condition (2) are exceeded, the positive refractive power of the plastic lens exceeds the positive refractive power of the optical glass, so as the temperature rises, the focal length becomes longer. When the upper limit of condition (3) is exceeded, the refractive power borne by the third surface (eighth surface) becomes insufficient, resulting in insufficient correction of spherical aberration. Examples of this invention will be shown below. In both examples, the first lens is a glass lens and the other lenses are plastic lenses, and the refractive index n and focal length f are 20°C and 60°C.
The figure for the d-line is shown. r, d, and ν respectively represent the radius of curvature of the surface, the distance between the surfaces, and the Abbe number of the lens material, and y represents the image height.

【table】

[Table] 2
_f2 = _f4 =-26f
_r3 = _−r8 =0.16f

[Table] (Effects of the invention) Since the multiple lenses of this invention have the above configuration, they use four plastic lenses with a specific gravity of about 1.2 and only one glass lens with a specific gravity of about 3, so they are sufficiently lightweight. We were able to create a low-cost lens using a plastic lens that is large and does not require polishing. By using one glass lens and optimizing the refractive power distribution, compared to the case where all plastic lenses are used, the focal length due to temperature change can be suppressed to less than 1, and almost temperature compensation can be achieved. did it. By appropriately distributing the refractive index to each surface, we were able to achieve good aberration correction using a low refractive index material. It has remarkable effects such as

[Brief explanation of drawings]

FIG. 1 is a sectional view of a conventional lens, FIG. 2 is a sectional view of a copying lens of the present invention, and FIGS. 3 and 4 are aberration diagrams of Examples 1 and 2.

Claims (1)

[Claims] 1. In order from the object side, a biconvex first lens, a meniscus second lens with a convex surface facing the object side, a biconcave third lens, a meniscus fourth lens with a convex surface facing the image side, and Consists of 5 lenses in 5 groups of a biconvex 5th lens, either the 1st or 5th lens is made of optical glass, the other 4 lenses are made of plastic, -1/10f<1/f ₂ (=1/f ₄ )<1/10f where f: synthetic focal length of the entire system fi: a copying lens that satisfies the conditions of the focal length of the i-th lens from the object side.