Patent ID: 7982968

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; and a third lens group with a negative optical power; and a fourth lens group with a positive optical power, wherein an interval between the first lens group and the second lens group decreases when a power of the variable-power optical system varies from a wide-angle end to a telephoto end, the first lens group comprises a negative lens and a positive lens, the second lens group consists of a positive lens and a negative lens, and includes at least one aspheric surface, and the variable-power optical system satisfies the following expressions (1) through (4): TLw/TLt< 1.2, (1) Δ N 2>0.15, (2) Δν2>20, and (3) Lb/fw≦ 1.2, (4) where TLw and TLt are distances on an optical axis from a tip of a lens surface to an image plane, at a wide-angle end (w) and a telephoto end (t) respectively, when an object is placed at an infinite distance, the lens surface being arranged at a closest position to the object side in the variable-power optical system, ΔN 2 is a difference in refractive indexes of lenses in the second lens group, Δν 2 is a difference in Abbe numbers of lenses in the second lens group, Lb is a distance on the optical axis from a tip of a lens surface with a power to the image plane at the telephoto end, the lens surface with a power being arranged at a closest position to an image side in the variable-power optical system, wherein the distance on the optical axis is calculated on the assumption that air is present as a medium in an area between the lens surface with a power and the image plane, and fw is a composite focal length of a total optical system of the variable-power optical system at the wide-angle end, wherein the variable-power optical system adjusts a focal point from an object at an infinite distance to an object to a near distance by moving the third lens group toward the image side, and variable-power optical system satisfies the expression (6): 1.5 <|f 3 /fw|< 6.0, (6) where f 3 is a composite focal length of the third lens group, and wherein the second lens group satisfies the expression (5): 0.6 <f 2 /fw< 2.0, (5) where f 2 is a composite focal length of the second lens group.