Patent ID: 6847784

Claim:
A focus detecting optical system detecting a focus position of a photographing optical system from a positional relationship between at least one pair of secondary object images, wherein the focus detecting optical system comprises: a condenser lens placed in the proximity of a preset imaging plane equivalent to an imaging plane of a photographic lens; a pair of aperture stops that are placed on an exit side of the condenser lens and that divide a pupil of the photogrphic lens into two areas; and a pair of re-imaging lenses for forming two secondary object images corresponding to the aperture stops, and satisfies the following conditions: 0.45 <|mg |<0.75 0.75 <|R 1 /R 2 |<1.25 | R 3 /R 4 |≦0.02 where mg is an imaging magnification of the focus detecting optical system, R 1 is a radius of curvature of an entrance surface of the condenser lens, R 2 is a radius of curvature of an exit surface of the condenser lens, R 3 is a radius of curvature of an entrance surface of each of the re-imaging lenses, and R 4 is a radius of curvature of an exit surface of each of the re-imaging lenses.