A focus detecting apparatus is provided with an optical assembly having a plurality of components disposed near the predetermined imaging plane of an objective lens and performing the actions of different field lenses, a re-imaging lens unit for receiving a light beam from the optical assembly and forming light distributions whose relative position changes in conformity with the focus adjusted state of the objective lens, and a photoelectric converting device having a plurality of sensor arrangements for receiving the light distributions and converting the light distributions into electrical signals. Predetermined one of the sensor arrangements are adapted to receive the light beam from predetermined ones of the portions of the optical assembly.