JPS6327688B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses an asymmetric lens type, but the distortion aberration is well corrected, and even if the object-to-image distance is short, the overall length of the lens is reduced so that the lens system does not become extremely small. This invention relates to a low-magnification copying lens whose size is approximately one-fourth of the object-image distance.

Symmetrical lenses, which are commonly used in optical systems for copying machines, have the advantage of having almost no distortion and being easy to simplify the lens configuration. In addition to having little freedom in design, there is an essential drawback in that as the magnification is gradually changed from the same magnification to the enlargement or reduction system, the distortion aberration actually increases. In addition, if we try to apply the conventional symmetrical lens, which is often used in optical systems for copying machines, to a copying optical system where the effective screen size is small and the object-to-image distance is very short, on the order of several tens of millimeters. , the lens system must be made considerably smaller, and miniaturization tends to produce surfaces with a very small radius of curvature.
This not only significantly increases manufacturing costs but also often makes manufacturing difficult in practice.

An object of the present invention is to solve the above-mentioned drawbacks, and to solve the problem when the effective screen size is small, for example, when a small film image is enlarged or reduced to be copied onto a TV camera tube or the like.
In addition, as a lens system that realizes a low-magnification copying optical system with a short object-to-image distance, the lens system uses an asymmetric type, has a small number of elements, 5 elements in 5 groups, and has an object-to-image distance of several tens of millimeters. Despite the fact that the lens is very short, the overall length of the lens is kept at about one-quarter of the object-to-image distance, resulting in very small curvature that increases cost and makes manufacturing difficult. Avoiding the occurrence of surfaces with radii,
It is an object of the present invention to provide a low-magnification copying lens that satisfactorily corrects various aberrations and particularly suppresses the occurrence of distortion.

The present invention is characterized by a lens system consisting of five lenses in five groups, with a concave lens in the center and two convex lenses in front and behind it. a first lens of the lenses;
The second convex meniscus lens with the convex surface facing the object side.
a third lens that is a biconcave lens, and a fourth lens that is a convex meniscus lens with a convex surface facing the image side;
The fifth lens is a biconvex lens.

The present invention will be explained below with reference to illustrated embodiments. The low magnification copying lens shown in Figure 1 is a lens system consisting of 5 lenses in 5 groups, with a concave lens in the center and 2 convex lenses in front and behind it. The first lens _L1 is a convex meniscus lens with its convex surface facing toward the object side, the second lens _L2 is a convex meniscus lens with its convex surface facing the object side, and the third lens _L3 is a biconcave lens with its convex surface facing the image side. The fourth lens _L4 is a convex meniscus lens, and the fifth lens _L5 is a biconvex lens.

The symbols attached to the lenses in FIG. 1 above are as follows.

ri is the radius of curvature of the i-th lens surface when viewed from the object side, and di is the axial spacing between the i-th lens surface and the (i+1)-th lens surface.

The numerical values of each lens in FIG. 1 above are as follows. However, the sign is Ni; d-line refractive index νi of the i-th lens; dispersion value of the i-th lens r ₁ = −0.9250 d ₁ = 0.4033 N ₁ = 1.54814 ν ₁ = 45.8 r ₂ = −0.6109 d ₂ ＝0.0078 r ₃ ＝0.5264 d ₃ ＝0.3060 N ₂ ＝1.62041 ν ₂ ＝60.4 r ₄ ＝0.8559 d ₄ ＝0.0965 r ₅ ＝−0.3326 d ₅ ＝0.0643 N ₃ ＝1.805518 ν ₃ ＝25.5 r ₆ = 11.7682 d ₆ = 0.0596 r ₇ = −0.9218 d ₇ = 0.1553 N ₄ = 1.62041 ν ₄ = 60.4 r ₈ = −0.4142 d ₈ = 0.0078 r ₉ = 3.1068 d ₉ = 0.1302 N ₅ = 1.67790 ν ₅ = 55.5 r ₁₀ = −0.9451 FNo. =5.4 f=1.0 2ω=21゜ m=-2.0 TD=
1.2310 o/I=4.8571 r ₁ / r ₂ = 1.514, f ₁ = 2.256 r ₃ / r ₄ = 0.615, f ₂ = 1.626 However, F No.; F number f; Focal length 2ω; Angle of view m; Imaging magnification TD: total lens length o/I: object-to-image distance.

The low magnification copying lens of the present invention satisfies the following conditional expression.

(1) 1.3＜r ₁ ／r ₂ ＜1.7 (2) 0.1f＜d ₁ ＜0.7f (3) 2.0f＜f ₁ ＜2.5f (4) 0.3＜r ₃ ／r ₄ ＜0.9 (5) 0.1 f<d ₃ <0.5f (6) 1.4f<f ₂ <1.8f However, ri is the radius of curvature of the i-th lens surface from the object side, and di is the radius of curvature of the i-th lens surface from the object side and the (i+
1) The axial spacing f(9) of the th lens surface is the focal length of the i-th lens from the object side, and f represents the focal length of the entire system.

If the lower limit of the above condition (1), the lower limit of condition (2), and the upper limit of condition (3) are exceeded, the power of the first lens L ₁ becomes weaker, resulting in overcorrection of distortion, and the above condition (1) is exceeded. If the upper limit of condition (2) and the lower limit of condition (3) are exceeded, the power of the first lens _L1 becomes too strong, resulting in insufficient correction of distortion.

If the lower limit of condition (4), the lower limit of condition (5), and the lower limit of condition (6) are exceeded, the power of the second lens _L2 becomes too strong, resulting in insufficient correction of spherical aberration.
Coma aberration is undercorrected.

If the upper limit of condition (4), the upper limit of condition (5), and the upper limit of condition (6) are exceeded, the power of the second lens _L2 becomes too weak, resulting in overcorrection of both salvage aberration and coma aberration.

The above conditions (2) and (5) can be achieved by making the first lens L ₁ and the second lens L ₂ appropriate thicknesses.
This maintains the Petzval sum of the entire system at an appropriate value, satisfactorily corrects field curvature, and also contributes to maintaining the overall lens length to approximately one-fourth of the object-image distance.

In other words, if the upper limits of conditions (2) and (5) above are exceeded,
When the total lens length becomes too long, the Petzval sum becomes over-corrected, and when the lower limit is exceeded, the total lens length becomes too short and at the same time the Petzval sum becomes under-corrected.

Since the present invention is constructed as described above, the structure is simple and the manufacturing cost is low because the concave lens is placed in the center and the front group and the rear group are each made up of two convex lenses before and after the concave lens. This lens has the advantage that the distance between objects and images is low, and the total length of the lens is as long as about one-fourth of the distance between objects and images, so even if the distance between objects and images is short, the total length of the lenses does not become extremely small, making it possible to realize cost reductions. In addition, the distortion aberration is extremely small and various aberrations are well corrected, so the spatial transfer function MTF is flat from the center of the screen to the periphery, resulting in excellent resolution characteristics and high performance as a low-magnification copying lens. It is. Furthermore, by changing the orientation of the lens, it is possible to provide a low-magnification copying lens that has excellent effects such as being able to obtain good enlarged or reduced images.

[Brief explanation of the drawing]

Fig. 1 is a lens configuration diagram showing an embodiment of the low magnification copying lens of the present invention, Fig. 2 is an aberration diagram of the same, and Fig. 3 is a spatial frequency of 30/30 when the image height is normalized to 5.5 mm. It is an MTF (spatial transfer function) curve diagram of mm. 1...object, 2...image, L ₁ ...first lens,
L ₂ ... second lens, L ₃ ... third lens, L ₄ ...
4th lens, L ₅ ... 5th lens, r ₁ , r ₂ ~ _{r 10} ...
Radius of curvature of each lens, d ₁ , d ₂ to _{d 9} . . . distance between axial surfaces of each lens, f . . . focal length of the entire system.

Claims (1)

[Claims] 1. A lens system consisting of 5 lenses in 5 groups, in which a concave lens is arranged in the center and two convex lenses are arranged in front and behind it, with the convex surface facing the image side in order from the object side. A first lens of a meniscus lens, a second lens of a convex meniscus lens with a convex surface facing the object side, a third lens of a biconcave lens, a fourth lens of a convex meniscus lens with a convex surface facing the image side, and a biconvex lens. A fifth lens for low magnification copying, which satisfies the following conditional expression. (1) 1.3＜r ₁ ／r ₂ ＜1.7 (2) 0.1f＜d ₁ ＜0.7f (3) 2.0f＜f ₁ ＜2.5f (4) 0.3＜r ₃ ／r ₄ ＜0.9 (5) 0.1 f<d ₃ <0.5f (6) 1.4f<f ₂ <1.8f However, ri is the radius of curvature of the i-th lens surface from the object side, and di is the radius of curvature of the i-th lens surface from the object side and the (i+
1) The axial spacing of the th lens surface. fi is the focal length of the ith lens from the object side. f is the focal length of the entire system.