Patent ID: 7551366

Claim:
A zoom lens comprising: in order from an object side, a first lens group having a positive refractive power and fixed in an optical axis direction; a second lens group having a negative refractive power and moving along an optical axis to zoom; a third lens group having a positive refractive power and fixed in the optical axis direction; and a fourth lens group having a positive refractive power and moving in a non straight line along the optical axis to compensate variations of an image formation position and compensate change of the image formation position caused by changes of an object distance, wherein; the first lens group includes a cemented lens of a first lens of a concave meniscus lens whose convex surface faces towards the object side and a second lens of a convex lens, and a third lens of a convex meniscus lens whose convex surface faces towards the object side, the lenses being located in order from the object side; the second lens group includes a fourth lens of a concave meniscus lens whose convex surface faces towards the object side, and a cemented lens of a fifth lens of a biconcave lens and a sixth lens of a convex lens, the lenses being located in order from the object side; the third lens group includes a seventh lens of a convex lens; the fourth lens group includes at least two or more cemented lenses; the third lens group and fourth lens group include at least one surface formed of an aspheric surface; and the zoom lens satisfies following conditional expressions (1), (2), (3), and (4): 1.9 <f 3 /f 4<5.1 (1) 1.5 <|f 2/ fw|< 2.5 (2) 7.5 <dz/fw< 12.5 (3) 1.3 <Lz/Lf< 2.2, (4) where f2: combined focal distance of the second lens group, f3: combined focal distance of the third lens group, f4: combined focal distance of the fourth lens group, dz: amount of movements of the second lens group when zooming, fw: focal distance of whole lens system in wide angle end state, Lz: distance between a surface closest to the object side and a surface closest to the image plane side in the second lens group in whole lens system at telephoto end, and Lf: distance between a surface closest to the image plane side and the image plane of whole lens system in the third lens group, wherein: a following conditional expression (5) is satisfied; 0.18<|β2 w |<0.21, (5) where β2w is image forming power ratio of the second lens group in wide angle end state.