Patent ID: 8681433

Claim:
A variable magnification optical system comprising: a first lens group having positive refractive power; a second lens group having negative refractive power; a stop; a third lens group having positive refractive power; and a fourth lens group having positive refractive power, which are in this order from an object side, wherein the first lens group, the stop and the third lens group are fixed relative to an image plane, but the second lens group and the fourth lens group move in an optical axis direction during magnification change, and wherein the fourth lens group has a focusing function, and wherein the third lens group includes at least an aspherical surface, and consists of, in the following order from the object side, a positive lens and a cemented lens of a positive lens and a double-concave lens cemented together, and wherein the following formulas (1) through (4) are satisfied: 3.5 <f 3 /fw< 6 (1); 0.4 <f 31 p/f 3<1.2 (2); −0.8 <f 3 /f 3 C< 0.1 (3); and 45<ν3 Cn<ν 3 Cp (4), where f3: a focal length of the third lens group, fw: a focal length of the entire system at a wide-angle end, f31p: a focal length of the positive lens on the most object side in the third lens group, f3C: a focal length of the cemented lens, ν3Cn: an Abbe number of the double-concave lens constituting the cemented lens for d-line, and ν3Cp: an Abbe number of the positive lens constituting the cemented lens for d-line.