1. A focus detecting apparatus using an area sensor comprising two-dimensionally arranged photoelectric conversion elements, said apparatus comprising: (a) a designating circuit for designating a predetermined area in the sensor corresponding to a focus detection area of a scene, said circuit designating an area on the sensor corresponding to the focus detection area of the scene in consideration of a difference between the focus detection area of the scene and the area on the sensor corresponding to the focus detection area of the scene, the difference being determined by an optical property of an optical system; and (b) a calculation circuit for performing a focus detection calculation based on an output from a plurality of photo-electric conversion elements in the area designated by said designating circuit.