The circular rotating shutters spins open and closed one time per each frame of film. Each frame is pulled into position (along the raceway behind the camera aperture) by the pull-down claw while the shutter is closed. Then a registration pin engages a perforation in the film to hold the film in place while the shutter spins open to make the exposure. This sequence is repeated for each frame of film.
The amount of exposure time for each frame is a consequence of both the frame rate (frames per second or f.p.s.) and the angle of the shutter opening. With a normal opening of 180 degrees, and a normal frame rate of 24 f.p.s., our shutter speed is 1/48 second per frame, according to the following equation.