Patent ID: 7532408

Claim:
A variable magnification optical system comprising a plurality of lens groups for imaging light from an object side on an image surface of an image sensor, wherein the plurality of lens groups includes at least: in order from the object side to an image side, the first lens group having a negative optical power, the second lens group having a positive optical power, the third lens group having a negative optical power, and the fourth lens group having a positive optical power; wherein, in a case of magnification variation from a wide angle end to a telephoto end, in an optical axis direction, the third lens group is immobile with respect to the image surface, while at least the second lens group and the fourth lens group move, whereby: an interval between the first lens group and the second lens group decreases, an interval between the second lens group and the third lens group increases, and an interval between the third lens group and the fourth lens group decreases, and on the other hand, the third lens group moves in an in-surface direction vertical to the optical axis direction to thereby correct shake resulting from imaging on the image surface; wherein conditional formulae (1) and (2) below are fulfilled: 0.7≦ f 2/√( fw×ft )≦1.3 Conditional formula (1), 0.4≦β3 t/β 3 w≦ 2.2 Conditional formula (2), where f2: denotes a focal length of the second lens group; fw: denotes a focal length of the entire variable magnification optical system at the wide angle end; ft: denotes a focal length of the entire variable magnification optical system at the telephoto end; βt: denotes a lateral magnification ratio of the third lens group at the telephoto end; and βw: denotes a lateral magnification ratio of the third lens group at the wide angle end.