a set the action of the combined field lens will now be described but for the sake of comparison a description will first be given of the optical action of the construction of fig7 in which a field lens 101 and a single aperture field mask 100 are provided instead of the combined field lens and the multi aperture field stop according to the embodiment of the present invention what is called light here may be not only visible light but also infrared and near infrared light both of a point p on the optic axis in the focus detecting area and a point q in the focus detecting area outside the optic axis are on the predetermined imaging plane and the state of the light beam when these points are projected onto the photoelectric converting device 55 through an opening 100a in the field mask is as shown in fig8 in fig8 the reference numerals 112 and 113 designate light rays emitted from the points p and q respectively and their imaging points are a point r on the photoelectric converting element and a point s in the concave lens 54 where this focus detecting apparatus is applied to a single lens reflex camera the distance between the point p and the point q is of the order of 6 15 mm and further the distance from the point p to the photoelectric converting element is also limited to the order of 20 35 mm and therefore even if the concave lens 54 for correcting curvature of image field is used such deviation of the secondary image surface by the focus detecting area cannot be eliminated as a.