1. An image capturing apparatus comprising: an image sensor having a focus detection pixel group including 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 including 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 performs a first image processing depending on a size of an aperture of the focus detection pixels and a second image processing depending on a different size of an aperture of the focus detection pixels, wherein in the first image processing, the image processing unit generates an image signal corresponding to a position of the focus detection pixels by using a signal of the focus detection pixel at the position, and wherein in the second image processing, the image processing unit generates an image signal corresponding to a position of the focus detection pixels by using signals of neighboring image forming pixels of the focus detection pixels at the position.