| 摘要 |
PROBLEM TO BE SOLVED: To obtain an objective optical system whose entire length is short and whose lens outside diameter is small through it can realize enlarging observation by moving a 2nd group on an optical axis so that variable power action may be performed and making it satisfy specified conditions. SOLUTION: This optical system is constituted of a 1st group G1 having positive refractive power, a 2nd group G2 having negative refractive power and a 3rd group G3 having positive refractive power in order from an object side. By moving the 2nd group G2 on the optical axis, the variable power action is performed, and the system satisfies the conditional expressions: 0.1<|1/ f2 (DW-DT)}|<2, 1<|(fW.fT)<1/2> /f1|<2. Provided that f1 and f2 are the focal distances of the 1st and the 2nd groups G1 and G2, DW and DT are intervals between the 1st and the 2nd groups G1 and G2 at a wide angle end and a telephoto end, fW and fT are the focal distances of the entire system at the wide angle end and the telephoto end, respectively. Thus, the occurrence of aberration in the 2nd group G2 is restrained and the aberration is kept excellent, and the refractive power of the 2nd group G2 is made strong while keeping the aberration excellent.
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