Focus control apparatus and method for performing focus control by phase difference detection, and image capturing apparatus

A focus detection apparatus comprises: an image sensor configured to receive a plurality of beams having passed through different exit pupil regions of an imaging optical system and output a pair of image signals corresponding to the different exit pupil regions; a calculation unit configured to obtain, from the pair of image signals, a spatial frequency at which a phase difference between a pair of images is minimized, and obtain an image shift amount between the pair of image signals from the spatial frequency; and a focus control unit configured to perform focus control by a phase difference detection method based on the image shift amount obtained by the calculation unit.