In contrast to digital cameras, film cameras are designed to allow the loading and unloading of light-sensitive film without exposing it to light. Traditionally, this is achieved by placing individual film sheets in a light-proof holder, a method still prevalent in large format cameras.
For these cameras, a double-sided film holder slides into the rear, displacing the focusing screen, and opens one side to face the lens. Smaller cameras, however, use film rolls, allowing multiple exposures in a single loading.
In these systems, the film moves from the supply to the take-up compartment, guided behind a metal chassis and held flat by a spring-loaded pressure plate. This design ensures the film remains protected from light during loading and unloading, as it is contained in a cartridge or cassette with a velvet-lined feed slot that prevents light leakage.
Film rolls are wound onto reels with an opaque backing paper to shield them from light between exposures. In the case of 35mm film, it is wound through the camera on a fixed spool and must be rewound into its light-proof cassette before the camera is opened for film removal.
Roll film, on the other hand, winds completely onto a second reel and is protected by the backing paper until removal. Once inside the camera, the cartridge opens automatically to allow the film to advance, and after the last exposure, it rewinds back into the cartridge, which then seals itself for removal.
Sheet film holders offer the flexibility to switch between different film types without wasting exposures. For 35mm or roll film, achieving the same flexibility typically requires either multiple camera bodies with lens swaps or cameras equipped with interchangeable film backs. Many cameras with magazine backs also support instant film packs.
Film advance mechanisms in these cameras are either powered by an electric motor triggered after each shutter release or manually operated by a wind lever. The winding, shutter, and exposure release mechanisms are interlocked to prevent accidental double exposures or missed frames. Some cameras include a button to bypass this interlock for special overlay effects.
Older medium format cameras do not automate film advance; users must manually advance the film after each shot, often guided by a frame counter window on the back. Additionally, users may need to reinsert a dark slide between exposures to shield the film from light.
Cameras equipped with motorized drives typically rewind the film into its cassette automatically at the end of the roll, readying it for removal.
