| 摘要 |
PROBLEM TO BE SOLVED: To provide a simply constituted single focal point lens suitable as a compact, small-sized photographic lens for an imaging element which has high performance in spite of having a three-lens-element constitution of a positive lens, a stop, and negative and positive lenses in order from the object side, is inexpensive and simple by using aspherical lenses and properly setting the composite focal length on the image plane side of the stop. SOLUTION: The simply constituted single focal point lens is constituted by arraying a 1st lens L1 composed of a positive meniscus lens which has aspherical surfaces as both surfaces and is convex to the image plane side, the stop S, a 2nd lens L2 composed of a biconcave lens which has a large- curvature surface on the object side, and a 3rd lens L3 composed of a biconvex lens which has an aspherical surface as the image-plane side surface and a large-curvature surface on the image plane side. Here, (1) 0.3<f'/f'23 <0.8 holds for the composite focal length of the 2nd lens L2 and 3rd lend L3 and specified conditional expressions are satisfied as to the radii of curvature of both the surfaces of the 1st lens L1 and the glass materials of the 1st lens L1 and 3rd lens L3 .
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