| 摘要 |
A stop is disposed forwardly of the lens system which comprises three individual lenses. A first lens located nearest the stop represents a positive meniscus lens, followed by a second and a third individual lens, both of which represent negative meniscus lenses. Each of the first to the third lens has its concave surface directed toward the stop. A small air gap exists between the second and the first lens. Representing the focal length of the first lens by f1, the composite focal length of the first and the second lens by f1,2, the composite focal length of the first to the third lens by f, the radius of curvature of a second refractive surface in the lens system by r3, the radius of curvature of a third refractive surface by r4, the refractive index of the first lens by n1, and the Abbe number of the first and the second lens by nu 1, nu 2, respectively, these parameters satisfy the following inequalities. (1) 5.32<f/f1<5.52 (2) 0.9<f/f1,2<1.1 (3) 1.60<n1 (4) 1.02<r3/r4<1.05 (5) 56+/-n< nu 1<56+/-3n where n represents an arbitrary real number and nu 2=41+/-0.5n. A pair of lens systems, each constructed as defined above, are disposed on the opposite side of the stop in a perfectly symmetrical manner, thereby providing a wide angle copying lens with high aperture efficiency including six individual lenses.
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