| 摘要 |
PROBLEM TO BE SOLVED: To provide an inner focus type optical system which excellently corrects variation in aberration at the time of focusing and possesses an excellent optical performance over the whole object distance. SOLUTION: In this inner focus type optical system which has, sequentially from the object side, two lens groups of a 1st group L1 of a positive refracting power and a 2nd group L2 of a negative refracting power and focalizes by moving the 2nd group on the optic axis, the 1st group L1 consists of a 1st a group L1a of a positive refracting power, a 1st b group L1b of negative refracting power, and a 1st c group L1c of positive refracting power with a wide lens space between the 1st and 2nd groups, and when the focal length of the entire system is expressed by F, the focal length of the 1st a group by F1a, and the distance between the 1st a group and the 1st b group by D1ab, the system is arranged so as to satisfy the conditions: 0.4<F1a/F<1.1, 0.035<D1ab/F<0.15.
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