Patent ID: 7158314

Claim:
An imaging-optical system comprising a negative first lens group, a positive second lens group and a positive third lens group, in this order from an object, wherein a zooming operation is performed by moving said positive second lens group and said positive third lens group while an object distance to said object is being varied without varying a distance from the most object-side surface of said negative first lens group to the imaging plane; wherein an in-focus state of said imaging-optical system is maintained while the focal length of said imaging-optical system is being varied through said zooming operation; wherein said negative first lens group comprises cemented lens elements, and said cemented lens elements are positioned at the most object-side of said negative first lens group; and wherein said cemented lens elements satisfy the following condition: 0.9 <Ra/fw< 2.0 wherein Ra designates the radius of curvature of a bonding surface of said cemented lens elements of said negative first lens group; and fw designates the focal length of said imaging-optical system at the short focal length extremity.