A device and a method for detecting the edge of a recording material, in particular, a printing plate, in an exposer for recording printing originals includes an exposer having an exposure drum holding the plate, and an exposure head moved axially along the drum and focusing exposure beams onto the plate. An optical fiber is let into the drum surface and an illuminating device, moved axially along the drum, radiates light radially into the fiber. A photodetector at the fiber receives the light radiated therein. Covering the light radiated in with the plate is used to detect the plate edge. Counting cycles of a feed drive moving the illuminating device determines an axial position of the edge. Alternatively, light of a light source is radiated axially into the fiber and the light emitted radially by the fiber is received using an optical detector moved by the feed drive.