Early silent films had stated frame rates anywhere from 16 to 24 frames per second (FPS), but since the cameras were hand-cranked, the rate often changed during the scene to fit the mood. Projectionists could also change the frame rate in the theater by adjusting a rheostat controlling the voltage powering the film-carrying mechanism in the projector.Silent films were often intended to be shown at higher frame rates than those used during filming
These frame rates were enough for the sense of motion, but it was perceived as jerky motion. To minimize the perceived flicker, projectors employed dual- and triple-blade shutters, so each frame was displayed two or three times, increasing the flicker rate to 48 or 72 Hertz and reducing eye strain. Thomas Edison said that 46 frames per second was the minimum needed by the visual cortex: “Anything less will strain the eye.”
In the mid to late 1920s, the frame rate for silent films increased to between 20 and 26 FPS.