Abstract: A camera metering mechanism discriminates between 1) adjacent perforations in a set, which trigger said metering mechanism to interrupt the film advance, and 2) adjacent perforations between sets, which do not trigger the interruption. The mechanism uses a difference in spacing between the perforations, which alternates between a first distance within a set and a second longer distance between sets. The mechanism is triggered to interrupt film advance when the space between detected perforations is less than a predetermined distance. If the space exceeds the predetermined distance, then film advance continues until the next successive perforation. According to more specific features, the perforation detector initiates electrical transitions at the leading and trailing edges of the perforations, and film distances are a determined from the time difference between selected transitions.