An image pickup element has a plurality of pixels which are arranged two-dimensionally. The plurality of pixels form at least two pixel sets having a first pixel set and a second pixel set. An exit pupil is assumed, and four divided areas which are separated by two straight lines which are mutually orthogonal in a plane which is orthogonal to an optical axis thereof, are set in the exit pupil. Pixels in the first pixel set are arranged to receive a light beam from a first pupil area for which, a center of gravity of area is in one of the four divided areas. For the optical axis, a center of gravity of area of a second pupil area is in a divided area which is symmetrical to a divided area in which the center of gravity of the first pupil area lies. Pixels in the second pixel set are arranged to receive a light beam having a center of gravity of area in the second pupil area. A signal from the first pixel set and a signal from the second pixel set are compared in at least one of directions of the two straight lines, and defocus information is acquired.