Light quantity information of an imaging optical system is acquired according to a focus detection position in an imaging screen. Conversion is performed from the light quantity information and a first aperture value of the imaging optical system, so that the first aperture value is converted into a second aperture value according to the focus detection position. A conversion coefficient is set according to the second aperture value and an exit pupil distance. A correction value to correct output signals from an imaging unit is obtained according to the second aperture value and an exit pupil distance.