1. A parallax detecting apparatus which calculates a parallax that occurs between optical systems, said parallax detecting apparatus comprising: a point spread function approximating unit configured to modify at least one of images obtained from the respective optical systems such that point spread functions of the optical systems are made to sufficiently approximate a point spread function of a predetermined optical system; and a parallax calculating unit configured to calculate the parallax that occurs between the optical systems, using the image modified by said point spread function approximating unit.