| 摘要 |
PROBLEM TO BE SOLVED: To obtain a compact and thin finder optical system though it has a high variable power ratio larger than 3.5 by performing variable power from a wide-angle end to a telephoto end by fixing a 4th lens group and moving 2nd and 3rd lens groups on an optical axis, and making it satisfy specified conditions. SOLUTION: The 1st lens group L1 to the 3rd lens group L3 are constituted of one single lens, respectively. The variable power from the wide-angle end to the telephoto end is performed by fixing the 4th lens group L4 and moving the 2nd and the 3rd lens groups L2 and L3 on the optical axis. Then, the finder optical system satisfies the respective conditions 3.5<ft/fw<6.0, 1.1<f3/fw<2.1 and 0.5<ft/fr<1.5. In the expressions, (fw) means the focal distance of an objective optical system at the wide-angle end, (ft) means the focal distance of the objective optical system at the telephoto end, f3 means the focal distance of the 3rd lens group L3 of the objective optical system and (fr) means a synthetic focal distance from the 4th lens group L4 of the objective optical system to a field lens group P.
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