Patent ID: 7859764

Claim:
A variable-power optical system comprising, in order from an object side thereof: a first lens group with a negative optical power; a second lens group with a positive optical power, moving when a power of the variable-power optical system varies; and a third lens group with a positive or negative optical power, wherein the first lens group comprises two or more lenses including one positive lens and one negative lens which include at least two or more lenses made of a plastic material, and the variable-power optical system satisfies the following expressions (1) through (3): 0.5 <|f 1 /f 2|<1.4, (1) 0.5 <f 2 /fw< 2.0, (2) −3.2<β2 t<− 1.4, (3) where f 1 is a focal length of the first lens, f 2 is a focal length of the second lens, fw is a composite focal length of a total optical system of the variable-power optical system, at a wide-angle end, and β 2 t is a image-forming magnification of the second lens group at a telephoto end.