Patent ID: 8238040

Claim:
A zoom optical system comprising, in order from an object side; a first lens group having positive refractive power; a second lens group having negative refractive power; a third lens group having positive refractive power; and a fourth lens group having positive refractive power; at least a portion of lenses within said third lens group composing a movable lens group which moves in a direction having a component perpendicular to an optical axis, upon zooming from a wide-angle end state to a telescopic end state, a distance between said first lens group and said second lens group, a distance between said second lens group and said third lens group and a distance between said third lens group and said fourth lens group being variable, and the following conditional expressions being satisfied; 0.47<φ3 /f 3<1.00 4.50 <f 1/(− f 2)<8.50 where φ3 denotes a light beam effective diameter of lens surface positioned closest to the object side of said third lens group, f1 denotes a focal length of said first lens group, f2 denotes a focal length of said second lens group, and f3 denotes a focal length of said third lens group.