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Why does film in a motion-picture camera stop briefly for each frame?

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Diagram showing the negative-positive photographic process: a scene is captured on film in a camera, developed into a negative, projected onto paper with an enlarger, and developed again to produce a positive print. — technology of photography Figure 1: Sequence of negative–positive process, from the photographing of the original scene to enlarged print (see text).

In a motion-picture camera, the film must stop briefly for each frame to ensure that a sharp, clear image is recorded during the exposure.12

The camera’s film-transport system uses a mechanical claw or pin-registering mechanism to pull the film into position behind the lens and hold it perfectly still within the exposure gate. While the film is stationary, a synchronized rotary shutter opens to allow light to strike the film, capturing a single, high-quality still photograph. Once the exposure is complete, the shutter closes to block the light, and the claw advances the film to the next frame, repeating the process.13

This intermittent movement is essential because, unlike continuous-motion devices that may use very brief stroboscopic exposures, a bright, high-quality image requires the film to be held motionless while the shutter is open. To manage the tension caused by this stop-and-go motion, cameras incorporate a slack-forming loop—often called the Latham loop—which allows the film to move continuously from the supply and take-up reels while remaining intermittent at the aperture.2