Patent ID: 7800844

Claim:
A focus detection optical system, comprising: a condenser lens located near a predetermined imaging plane of a taking lens, a pupil division optical system that is located on an exit side of said condenser lens and divides a pupil of said taking lens into two pupil areas to form a pair of secondary object images with light beams passing through said pupil areas, and a light receptor element array that is located on a side of said pair of secondary object images of said pupil division optical system, so that a relative position relation between said pair of secondary object images received at said light receptor element array is detected to detect a focus position of said taking lens, wherein: said pupil division optical system comprises an aperture stop having a pair of openings with an optical axis of said taking lens held between them, and a pair of re-imaging lenses, each consisting of a double-convex lens having a convex entrance-side surface and a convex exit-side surface, wherein: when, on a plane passing through the optical axis of said taking lens and centers of gravity of the openings in said aperture stop, a surface apex is defined by a position of said convex entrance-side surface, and said convex exit-side surface, which position is extended most in a direction parallel with the optical axis of said taking lens, surface apexes of said entrance-side surface and said exit-side surface of said double-convex lens are positioned away from the optical axis of said taking lens in the same direction, and a distance of the surface apex of said entrance-side surface from the optical axis of said taking lens is larger than a distance of the surface apex of said exit-side surface from the optical axis of said taking lens.