FOCUS DETECTING APPARATUS

In a focus detecting apparatus wherein an imaging optical system is disposed rearwardly of the predetermined imaging plane of an objective lens to thereby form a plurality of object images on the basis of lights passed through different portions of the pupil of the objective lens and each of the object images is detected by a line sensor comprising a plurality of photoelectric conversion elements to thereby detect the focus condition of the objective lens from the amount of deviation of each object image, a field limitation plate having a plurality of openings and an optical filter for causing the characteristics of the light passed through the openings of the field limitation plate to differ from one another are provided near the predetermined imaging plane of the objective lens. Thus, the apparatus is adaptable for different focus detection conditions and is very simple in construction.