1. An image sensor comprising: a plurality of phase difference detection pixels, each phase difference detection pixel including: one on-chip lens; and a plurality of photoelectric conversion layers formed below the on-chip lens, wherein each of at least two of the plurality of photoelectric conversion layers is split, partially formed, or partially shielded from light with respect to a light-receiving surface, wherein the phase difference detection pixels are for performing auto focus (AF) by phase difference detection; and a plurality of imaging pixels for generating an image, wherein the phase difference detection pixels are arranged to be distributed among the plurality of imaging pixels arranged two-dimensionally in a matrix form, and wherein a difference in output between at least one of the photoelectric conversation layers in at least a first one of the phase difference detection pixels and at least a first one of the imaging pixels placed around the first phase difference detection pixel is used for the phase difference detection.