1. An image capturing apparatus comprising: an image sensor having a focus detection pixel group comprising a plurality of focus detection pixels that receive light passing through a part of a pupil area of an imaging lens for forming an object image and an image forming pixel group comprising a plurality of image forming pixels that receive light passing through the whole of the pupil area of the imaging lens; and an image processing unit that generates an image signal corresponding to a position of each of the focus detection pixels by using at least either of a signal of the focus detection pixel at the position and signals of neighboring image forming pixels of the focus detection pixel at the position, depending on a size of an aperture of the focus detection pixels.