1. A parallax detecting apparatus which calculates a parallax that occurs between optical systems, the parallax detecting apparatus comprising: a point spread function approximating unit configured to modify an image obtained from a source optical system, the point spread function approximating unit comprising: a frequency domain transforming unit configured to transform the image from a spatial domain into a frequency domain; a point spread function transforming unit configured to modify the transformed image by applying modification data to the transformed image, such that a point spread function of the source optical system is made to sufficiently approximate the point spread function of a predetermined optical system; and a spatial domain transforming unit configured to inversely transform the modified image from the frequency domain to the spatial domain, the parallax detecting apparatus further comprising a parallax calculating unit configured to calculate a parallax using at least the inversely transformed image, wherein the modification data comprises data obtained by dividing a first optical transfer function by a second optical transfer function, the second optical transfer function being that of the source optical system.