1. An image pickup apparatus comprising: a plurality of pixels each having a photoelectric converter that converts an optical image formed by an image pickup optical system into an electrical signal; a plurality of color filters provided on the plurality of pixels respectively; and an image pickup element in which the plurality of pixels are arranged two-dimensionally, wherein at least some of the pixels on which color filters that give the largest weight to a brightness signal or color filters that have the highest transmittance among the plurality of color filters are provided are pixels for focus detection that are designed in such away that the direction of incidence of light beams incident thereon is restricted, the pixels other than the pixels for focus detection are pixels for image picking-up that are designed in such a way that the degree of restriction placed on light beams incident thereon is smaller than that placed on light beams incident on the pixels for focus detection, the pixels for focus detection output at least a signal for ranging, the pixels for image picking-up output at least a signal for an image, and there are at least three pixels for focus detection in an area defined between a circle having its center at the center of any one of the pixels for focus detection and having a radius equal to 3.1 times the pixel pitch and a circle having its center at the center of the aforementioned any one of the pixels for focus detection and having a radius equal to 5.9 times the pixel pitch.