Patent ID: 7817912

Claim:
An optical system of a focus detection apparatus, comprising: a condenser lens positioned behind an expected focal plane of a photographing lens; an auxiliary lens group positioned behind said condenser lens to be coaxial therewith; a pair of separator lenses positioned behind said auxiliary lens group in close vicinity thereof and integrally molded from resin, wherein an object image formed on said expected focal plane is divided into two images by said pair of separator lenses to be reformed on a pair of areas on a sensor, respectively, and wherein said auxiliary lens group includes a negative lens element made of resin, and a positive lens element made of glass, and a separator mask which is positioned between said auxiliary lens group and said pair of separator lenses and includes a pair of apertures facing said pair of separator lenses, respectively, wherein said resin-made auxiliary lens element, said glass-made auxiliary lens element, said pair of separator lenses and said separator mask satisfy the following condition (1): 0.2< t s ·f n /( F IN ·m 0 ·q n ·P )<1.1 (1), wherein f n designates a focal length of said resin-made auxiliary lens group; P designates a distance between optical axes of said pair of separator lenses; q n designates a rate of focal length change of said resin-made auxiliary lens element due to a temperature change thereof which corresponds to the sum of a variation in shape and a variation in refractive index according to a linear expansion coefficient; t s designates a linear expansion coefficient of said pair of separator lenses due to a temperature change thereof; F IN designates an f-number of said optical system of said focus detection apparatus that is determined by said pair of apertures of said separator lens mask; and m 0 designates a magnification of said optical system of said focus detection apparatus.