An automatically focusing optical system for copiers, duplicators and other reproduction equipment having a variable magnification capability. The auto-focusing mechanism comprises a camming means for moving a mirror or other component of the optical system to maintain focus throughout the magnification range of the system. The camming means is additionally designed to be adjustable in such a way that it can accommodate imaging lens systems which differ in focal length within the normal manufacturing tolerance band, and thus replace the family of cams utilized in the prior art to accommodate lenses of different focal length.