JPH0556485B2 - - Google Patents

Description

[Detailed description of the invention]

OBJECT OF THE INVENTION (INDUSTRIAL APPLICATION FIELD) This invention relates to a symmetric lens for copying. (Prior Art) In recent years, with the miniaturization of copying machines, there has been a strong demand for low-cost copying lenses that have a wide angle of view, a full lens length, and a small lens outer diameter. In order to make lenses more compact and lower in cost, attempts have been made to reduce the number of lens components and to arrange the lens system symmetrically around the aperture. It is well known that distortion is eliminated when the lens system is arranged symmetrically with respect to the aperture.
Moreover, this makes it possible to reduce costs. Such a symmetrical lens having four elements in four groups can be found in Japanese Patent Application Laid-Open Nos. 55-81316 and 1982-161823. However, although the former has an F value of 5, which is a little bright, the angle of view is small at 15° to 18°, which is insufficient to realize the miniaturization of copying machines. The latter has a large angle of view of 22.6°, but has a dark F value of 8. (Problems to be Solved by the Invention) This invention is a symmetrical lens system with four elements in four groups centered around an aperture with an F value of 5.6 and a half angle of view of 18.8°.
The objective is to obtain a copying lens that is compact, inexpensive, and has sufficient aberration correction. Configuration of the Invention (Means for Solving the Problem) As shown in FIG. Yes, when looking at each lens component from the object side, the first lens is a biconvex positive lens, the second lens is a biconcave negative lens, the third lens is the same biconcave negative lens as the second lens, and the fourth lens is a biconcave negative lens. Consists of the same biconvex positive lens as the first lens: 0.024<N ₁ −N ₂ <0.061 ...(1) 2.00<R ₂ /R ₃ <2.39 ...(2) 0.30<f ₁ /f<0.42 ... …(3) −0.50＜f ₂ /f＜−0.30 …(4) However, N ₁ : Refractive index of the first lens N ₂ : Refractive index of the second lens R ₂ : Radius of curvature of the image side surface of the first lens R ₃ : radius of curvature of the object side of the second lens f : focal length of the entire lens system f ₁ : focal length of the first lens f ₂ : focal length of the second lens. However, a symmetrical arrangement with respect to the aperture does not only mean complete symmetry, but also a case where the ratio of form factors, thickness ratio, and difference in refractive index of lenses corresponding to each other with respect to the aperture are 0.9<q _i /q _i ′<1.1. 0.7<t _i /t _i ′<1.3 |N _i −N _i ′|<0.1 Where, q _i , t _i , and N _i are the form factor, thickness, refractive index, and q of the i-th lens on the object side than the aperture. _i ′、t _i ′、N _i ′
etc., those with ' are located on the image side of the aperture and indicate the i'-th lens and surface from the image side, and the form factor is q _i = R _2i where R _i is the radius of curvature of the i-th surface. +R _2i-1 R _2i -R _2i-1 is within the range of symmetry in the lens of the present invention. However, the examples show only completely symmetrical cases. (Function) Condition (1) relates to the difference in refractive index between a biconvex lens and a biconcave lens, and is a condition for properly correcting various aberrations over the entire wide angle of view and obtaining well-balanced performance. If the refractive index of the positive lens increases beyond the upper limit, the Petzval sum becomes too small and the astigmatism difference becomes large, which not only deteriorates the lens performance but also increases the cost of the glass material. If the lower limit is exceeded and the refractive index of the positive lens becomes small, the meridional image plane in the middle of the screen will be overcorrected. Condition (2) relates to the ratio of the radius of curvature of the adjacent surfaces of the positive lens and the negative lens, and is intended to satisfactorily correct comatic aberration, spherical aberration, and specific point aberration over a wide angle of view. If the lower limit is exceeded, spherical aberration will be insufficiently corrected, and coma flare will become excessive, making it difficult to correct it in the entire lens system. On the other hand, when the upper limit is exceeded, the astigmatism difference becomes large, making it difficult to obtain well-balanced performance over the entire screen. Conditions (3) and (4) are conditions for properly correcting field curvature and astigmatism under conditions (1) and (2) and making it possible to widen the angle of view. If the upper limit of condition (3) and the lower limit of condition (4) are exceeded, the curvature of field becomes large, making it difficult to obtain uniformly good performance over the entire screen.
Furthermore, when the lower limit of condition (3) and the upper limit of condition (4) are exceeded, astigmatism becomes large, making it difficult to obtain well-balanced performance over the entire screen. moreover,
Aberration fluctuations due to processing errors in the intervals between the first lens, the second lens, the third lens, and the fourth lens become significant, and processing tolerances must be strictly controlled, leading to increased costs. (Example) Examples of the present invention will be shown below. The symbols in the table are
R is the radius of curvature of the lens surface, D is the distance between lens surfaces, N
is the refractive index of the lens material, ν is the Abbe number of the lens material, F is the F number, ω is the half angle of view, and the focal length f was 100 mm.

【table】

【table】

[Table] Effects of the Invention The lens of this invention has a bright F number of 5.6 and a wide half angle of view of 18.8°, as seen in the above embodiment and the aberration diagrams in Figures 2 to 4. However, it is compact and provides well-balanced aberration correction across the entire screen. This makes it possible to obtain a copying lens that enables high image quality and miniaturization of copying machines.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the structure of the copying lens of the present invention, FIG. 2 is an aberration curve diagram of Example 1, FIG. 3 is an aberration curve diagram of Example 2, and FIG. 4 is an aberration curve diagram of Example 3. It is a curve diagram.

Claims (1)

[Claims] 1. The lens system has a four-element structure arranged symmetrically with respect to an aperture located at the center of the lens system, and when looking at each lens component from the object side, the first lens is a biconvex positive lens. , the second lens is a biconcave negative lens, the third lens is a biconcave negative lens that is the same as the second lens, and the fourth lens is a biconvex positive lens that is the same as the first lens, and 0.024<N ₁ −N ₂ <0.061 2.00<R ₂ /R ₃ <2.39 0.30<f ₁ /f<0.42 −0.50<f ₂ /f<−0.30 where N ₁ : refractive index of the first lens N ₂ : refractive index of the second lens R ₂ : Radius of curvature of the image side of the first lens R ₃ : Radius of curvature of the object side of the second lens f : Focal length of the entire lens system f ₁ : Focal length of the first lens f ₂ : Each of the focal lengths of the second lens A copying lens that satisfies the following conditions.