Image pickup apparatus, focus detecting method, and storage medium

An image pickup apparatus comprising:

• an image sensor configured to capture an object image formed by an imaging optical system, and to acquire a first pair of signals generated by light beams that have passed areas in an exit pupil that are different from each other and arranged in a first direction and a second pair of signals generated by light beams that have passed areas in the exit pupil that are different each other and arranged in a second direction different from the first direction;
• and
• a determining unit configured to determine a defocus amount to be used for focus control of the imaging optical system by preferentially using one of the first pair of signals and the second pair of signals according to a type of the imaging optical system,
• wherein, in a case where the imaging optical system is of a type having two optical systems arranged in parallel, the determining unit preferentially selects a focus detecting direction in which the first direction is orthogonal to a direction of an optical axis shift.