Frame Rate - Background

Background

The human eye and its brain interface, the human visual system, can process 10 to 12 separate images per second, perceiving them individually, however the threshold of perception is more complex, with different stimuli having different thresholds: the average shortest noticeable dark period, such as the flicker of a cathode ray tube monitor or fluorescent lamp, is 16 milliseconds, while single-millisecond visual stimulus may have a perceived duration between 100ms and 400ms due to persistence of vision in the visual cortex. This may cause images perceived in this duration to appear as one stimulus, such as a 10ms green flash of light immediately followed by a 10ms red flash of light perceived as a single yellow flash of light. Persistence of vision may also create an illusion of continuity, allowing a sequence of still images to give the impression of motion. Early silent films had a frame rate from 14 to 24 FPS which was enough for the sense of motion, but it was perceived as jerky motion. By using projectors with dual- and triple-blade shutters the rate was multiplied two or three times as seen by the audience. Thomas Edison said that 46 frames per second was the minimum: "anything less will strain the eye." In the mid- to late-1920s, the frame rate for silent films increased to about 20 to 26 FPS.

When sound film was introduced in 1926, variations in film speed were no longer tolerated as the human ear is more sensitive to changes in audio frequency. From 1927 to 1930, the rate of 24 FPS became standard for 35 mm sound film; a speed of 456 millimetres (18.0 in) per second. This allowed for simple two-blade shutters to give a projected series of images at 48 per second. Many modern 35 mm film projectors use three-blade shutters to give 72 images per second—each frame flashed on screen three times.