Patent ID: 7633686

Claim:
A zoom optical system comprising in the order from an object side: a first lens group having a negative optical power; a second lens group having a positive optical power; a third lens group having a negative optical power; and a fourth lens group having a positive optical power, wherein in zooming from a wide angle end to a telephoto end, at least the second lens group and the third lens group are moved in a state that a moving range of the second lens group is partially overlapped with a moving range of the third lens group, the first lens group is constituted of a negative lens element and a positive lens element, the second lens group is constituted of two or more lens elements, the third lens group is constituted of a negative lens element, and the zoom optical system satisfies the following conditional expressions (1), (2), and (3): 0<( r 1+ r 2)/( r 1− r 2)<1.0 (1) −0.9 <f 3/√{square root over ( )}( fw×ft )<−0.4 (2) 1.6<nd2p (3) where r 1 is a curvature radius of an object-side lens surface of the negative lens element in the first lens group, r 2 is a curvature radius of an image-side lens surface of the negative lens element in the first lens group, f 3 is a focal length of the third lens group, fw is a focal length of an entirety of the optical system at the wide angle end, ft is a focal length of the entirety of the optical system at the telephoto end, and nd 2 p is an average refractive index of a positive lens element in the second lens group.