Patent ID: 8213785

Claim:
A focus detection optical system comprising: a condenser lens positioned behind an expected image plane of a photographing lens; an auxiliary lens positioned behind said condenser lens in a direction along an optical axis of said condenser lens; and a separator lens including at least one pair of lenses positioned behind said auxiliary lens, wherein an object image formed on said expected image plane is divided into two images by said pair of lenses of said separator lens to be reformed on a pair of areas on a sensor, respectively, wherein said auxiliary lens includes a plastic lens having a negative refractive power, wherein said separator lens includes one of a glass lens and a hybrid lens that is configured of a glass substrate and a plastic lens having a curved surface which is layered onto said glass substrate, and wherein said focus detection optical system satisfies the following condition (1): 0.068 <m 1 /m 2 <0.090 (1), wherein m 1 designates a magnification of a first surface of said auxiliary lens, and m 2 designates a magnification of a second surface of said auxiliary lens.