Film stock consists of transparent celluloid, acetate, or polyester base coated with an emulsion containing light-sensitive chemicals. Cellulose nitrate was the first type of film base used to record motion pictures, but due to its flammability was eventually replaced by safer materials. Stock widths and the film format for images on the reel have had a rich history, though most large commercial films are still shot on (and distributed to theaters) as 35 mm prints.
Originally moving picture film was shot and projected at various speeds using hand-cranked cameras and projectors; though 1000 frames per minute (16⅔ frame/s) is generally cited as a standard silent speed, research indicates most films were shot between 16 frame/s and 23 frame/s and projected from 18 frame/s on up (often reels included instructions on how fast each scene should be shown).^[5] When sound film was introduced in the late 1920s, a constant speed was required for the sound head. 24 frames per second was chosen because it was the slowest (and thus cheapest) speed which allowed for sufficient sound quality. Improvements since the late 19th century include the mechanization of cameras – allowing them to record at a consistent speed, quiet camera design – allowing sound recorded on-set to be usable without requiring large "blimps" to encase the camera, the invention of more sophisticated filmstocks and lenses, allowing directors to film in increasingly dim conditions, and the development of synchronized sound, allowing sound to be recorded at exactly the same speed as its corresponding action. The soundtrack can be recorded separately from shooting the film, but for live-action pictures many parts of the soundtrack are usually recorded simultaneously.
As a medium, film is not limited to motion pictures, since the technology developed as the basis for photography. It can be used to present a progressive sequence of still images in the form of a slideshow. Film has also been incorporated into multimedia presentations, and often has importance as primary historical documentation. However, historic films have problems in terms of preservation and storage, and the motion picture industry is exploring many alternatives. Most movies on cellulose nitrate base have been copied onto modern safety films. Some studios save color films through the use of separation masters: three B&W negatives each exposed through red, green, or blue filters (essentially a reverse of the Technicolor process). Digital methods have also been used to restore films, although their continued obsolescence cycle makes them (as of 2006) a poor choice for long-term preservation. Film preservation of decaying film stock is a matter of concern to both film historians and archivists, and to companies interested in preserving their existing products in order to make them available to future generations (and thereby increase revenue). Preservation is generally a higher concern for nitrate and single-strip color films, due to their high decay rates; black-and-white films on safety bases and color films preserved on Technicolor imbibition prints tend to keep up much better, assuming proper handling and storage.
Some films in recent decades have been recorded using analog video technology similar to that used in television production. Modern digital video cameras and digital projectors are gaining ground as well. These approaches are preferred by some moviemakers, especially because footage shot with digital cinema can be evaluated and edited with non-linear editing systems (NLE) without waiting for the film stock to be processed. Yet the migration is gradual, and as of 2005 most major motion pictures are still shot on film.