| 摘要 |
An imaging optical system includes an aperture stop, a first lens group disposed on an object side of the aperture stop, a second lens group disposed on an image side of the aperture stop and a plate-like ND filter disposed close to a position of the aperture stop in an optical axis direction of the first and second lens groups, and the second lens group includes positive refracting power as a whole, the ND filter is configured to be retractable from the optical axis, and to attenuate a passing light amount.
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