Movie film stock is made of acetate, transparent celluloid, or polyester base, coated with an emulsion containing chemicals sensitive to light. Cellulose nitrate was originally the film base used to record motion pictures, however because of its flammability was subsequently replaced by safer materials. Standard widths and the film format for on the reel images have had a opulent history, although until recently many large commercial films were still shot on (and distributed to movie theaters) as 35 mm prints.
Moving picture film was initially shot and projected at various speeds using cameras and projectors cranked by hand; using 1000 frames per minute (16⅔ frame/s) is generally quoted as a standard silent speed, research suggests most films were shot using 16 frame/s to 23 frame/s and projected using 18 frame/s on up (reels typically included instructions on the speed that each scene should be shown).
When sound film was made available in the late 1920s, a constant speed became necessary for the sound head. 24 fps was selected because it was the slowest (and thus least expensive) speed which allowed for sufficient sound quality. Improvements after the late 19th century include the automation of cameras – letting them record at a consistent speed, silent camera engineering – letting sound recorded on-set to be used without requiring huge "blimps" encasing the camera, the development of more sophisticated lenses and filmstock, allowing directors to film in increased dim environments, and the advent of synchronized sound, permitting sound to be recorded at precisely the same speed as its synonymous action. Although a soundtrack can be recorded independently from shooting the film, however, for live-action movies many parts of the soundtrack are most often recorded simultaneously.