Abstract: In a continuous film cartridge sound motion picture projector employing a closed loop film which is twisted to cause it to turn over with each passage through the projector, optical sound tracks on the emulsion side of the film are used to modulate a narrow beam of light, and this beam is prefocused on the center of the film while it is pressed against a film positioning element carried by the projector and the modulated beam of light is picked up by a photosensitive element which is carried by the projector and moved into the cartridge upon insertion thereof. This system is made practical by employing a beam of light which has a thickness of from 0.3 - 0.6 mil and which is produced using a single cylindrical lens element to provide a beam having a cone angle which is not more than 25.degree. to provide an adequate depth of focus to accommodate the alternate presentation of the opposite faces of the film when the film is turned over.