Disclosed is a focus detecting apparatus detecting an image offset of an optical system. The apparatus comprises a micro lens array arranged with a plurality of micro lenses, a photo-detector having a plurality of detecting elements provided so as to correspond to each of the plurality of micro lenses and receiving an optical beam from the optical system via the micro lenses, and a calculator that selects a pair of detecting elements from the plurality of detecting elements in accordance with a plurality of aperture values of the optical system and calculates an image offset of the optical system based on outputs from the pair of detecting elements. The pair of detecting elements receive respectively a pair of optical beams passing through different areas of the optical system.