Patent ID: 7339748

Claim:
A variable magnification optical system for forming an optical image of an object on a light-receiving surface of an image sensor, said variable magnification optical system includes: from an object side, at least a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having a positive optical power, wherein, in magnification variation from a wide angle end to a telephoto end, at least the first lens unit moves, wherein the variable magnification optical system has a five-unit construction including a fifth lens unit which has a positive optical power and which is arranged to an image side of the fourth lens unit, wherein, in variable magnification from the wide angle end to the telephoto end, the fifth lens unit is kept in fixed position, and wherein conditional formulae (1) and (2) below are fulfilled, 6.0 <fl/fw <20.0 (1) 1.83<Nd ave (2) where fl represents a focal length of the first lens unit, fw represents a focal length of the entire system at the wide angle end, and Nd ave represents an average refractive index for a d-line among all lens elements included in the second lens unit.