| 摘要 |
An imaging-optical system includes a negative first lens group, a positive second lens group and a positive third lens group. A zooming operation is performed by moving the second and third lens groups while an object distance to the object is being varied without varying a distance from the most object-side surface of the first lens group to the imaging plane. An in-focus state is maintained while the focal length of the imaging-optical system is being varied through the zooming operation. The first lens group includes cemented lens elements, and the cemented lens elements are positioned at the most object-side of the first lens group. The cemented lens elements satisfy the following condition: <?in-line-formulae description="In-line Formulae" end="lead"?>0.9<RA/FW<2.0 (1)<?in-line-formulae description="In-line Formulae" end="tail"?> wherein Ra designates the radius of curvature of a bonding surface of the cemented lens elements of the first lens group; and fw designates the focal length of the imaging-optical system at the short focal length extremity.
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