 SOF was primarily used for news film prior to the advent of portable videotape recording, but was used until recently for documentary film recording.For Optical recording, the film only had a single perforation and the area where the other set of perforations would have been was exposed to a small bright lamp inside the camera controlled by an amplifier (usually in a separate box) that would vary the area of recording (RCA type), by means of shutters pulled back by variation in current, or variations in intensity (Western Electric type).Editing involved painting or taping over the optical track for the distance the sound led the film through the camera and projector.