A system for determining photoelectrically the position of at least one focus plane of an image within an optical apparatus, wherein the object is reproduced at least at one spatial frequency filter of an optical image correlator and there is measurement and/or display of the light fluxes leaving the local frequency filter.In this system light fluxes traversing differing pupil sections of the reproducing optics are modulated in common by a spatial frequency filter and are split up geometrically or physically, or by means of additional modulation in correspondence with the pupil sections. They are consecutively or simultaneously projected on a common photoelectric receiver or on separate photoelectric receivers, the output signals of which are processed further for purposes of controlling display and/or follow-up devices.