| 摘要 |
PROBLEM TO BE SOLVED: To make only the chromatic aberrations of magnification regulatable with substantially no change in various aberrations by putting a first lens group and a third lens group into an afocal state between the groups and moving the second lens group having substantially no powder between the groups. SOLUTION: This optical system is constituted by arranging the respective lens groups; the first lens group G1, the second lens group G2 and third lens group G3, successively from an object side and putting the luminous flux emitted from the first lens group G1 into the approximate afocal state. The second lens group G2 moves in the luminous flux put into the approximate afocal state, by which the chromatic aberrations of magnification is made variable. The second lens group G2 is formed by combining a negative lens and a positive lens in such a manner that refracting power is mostly offset. The third lens group G3 has an imagery function. Further, condition equations (1) and (2) are satisfied.|f/fG2|<0.1 (1),νp -νn >10 (2), where the focal length of the entire system defined as f, the focal length of the second lens group as fG2 , the minimum value of the Abbe number in the negative lens of the second lens group asνn and the maximum value of the Abbe number in the positive lens in the second lens group asνp .
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