An image sensing apparatus including: an image sensor including a plurality of focus detection pixel pairs that perform photoelectric conversion on each pair of light beams that have passed through different regions of a photographing lens and output an image signal pair; a flash memory that stores shift information on relative shift between an optical axis of the photographing lens and a central axis of the focus detection pixel pairs; a correction unit that corrects a signal level of the image signal pair based on the shift information and exit pupil information of the photographing lens so as to compensate for an unbalanced amount of light that enters each of the focus detection pixel pairs; and a focus detection unit that detects a focus of the photographing lens using the image signal pair corrected by the correction unit.

The purpose of the present invention is to reduce the size of an image pick-up lens unit. A part of a bundle of rays representing subject optical images is deflected vertically downward by a polarization prism, and is further deflected forwards by a total reflection mirror. A bundle of rays totally reflected by the total reflection mirror is split in three directions by a tri-directional splitting prism. The bundle of rays, which is split in three directions, is incident on a first optical-path-length-difference image pick-up element, a second optical-path-length-difference image pick-up element, and a phase-difference image pick-up element included in a phase-difference AF optical system. Auto focus (AF) is performed on the basis of the optical path length difference from signals obtained from the optical-path-length-difference image pick-up elements, and AF is performed on the basis of the phase difference from a signal obtained from the phase-difference image pick-up element. Since the phase-difference AF optical system is disposed so as to be parallel to the axis of the light forming the subject optical images, the size of the image pick-up lens unit can be reduced.