| 摘要 |
PROBLEM TO BE SOLVED: To reduce the effective diameter of a first lens group, miniaturize the whole lens system in its turn, and keep a high optical performance over the whole zoom area by specifying the structure and focal distance of the first lens group. SOLUTION: The whole body is formed of four lens groups, and in order to suppress the increase in effective diameter of a first lens group, an inner focusing system in which the first lens group is divided into three groups of concave lens, convex lens and convex lens, and the concave lens group 1a on the side closest to an object and the convex lens group 1c on the side closest to an image are fixed is adapted. The focal distance fw of the whole system in wide angle end, the photographing effective screen size I, the composed focal distance f1 in focusing to an infinite-end object of the first lens group, and the focal distance f1a of the 1a lens group are set so as to satisfy conditional expressions (1) 0.41<= fw/I and (2) 1.0<|f1a /f1 <1.5. Thus, the optical performance of the whole screen extending over the whole variable power area can be satisfactorily kept while extending the angle and making the system compact.
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