Patent ID: 7301709

Claim:
An imaging optical system having a focus function, which comprises a lens system, the lens system comprising: a lens group remaining fixed during focusing; a focusing cemented lens including a first fluid material and a second fluid material as lens materials in this order from an object, wherein the first and second fluid materials have different refractive indices, and a shape of an interface between the first and second fluid materials is variable; and a control unit that electrically controls the shape of the interface between the first and second fluid materials, depending on a distance from the object, wherein the focusing cemented lens has a power as a whole and satisfies conditions (1) to (3) over an entire focusing region: 0.1 <|Nd 1- Nd 2|<0.5 (1) 3 <|RA|/f <20 (2) | fa/f |>1.0, and, (3) wherein Nd 1 represents a refractive index to a d line of the first fluid material, Nd 2 represents a refractive index to a d line of the second fluid material, RA represents a minimum value in use of a radius of curvature of the interface between the first and second fluid materials, fa represents a focal length of the focusing cemented lens, and f represents a focal length of the lens system.