Traditionally, each film frame is projected 3 times. The psycho-perceptual reason is that the phi-phenomenon (“light chaser effect”) kicks in about 12–15 Hz, so

Traditionally, each film frame is projected 3 times. The psycho-perceptual reason is that the phi-phenomenon (“light chaser effect”) kicks in about 12–15 Hz, so camera frame rate must exceed that to give a perception of smooth motion. The critical flicker frequency is around 50 Hz for most people, so projected frame rate must exceed that to avoid the appearance of flickering. Silent movies were shot 18 fps and projected 54 fps. The frame rate for talkies was boosted to allow enough bandwidth for the audio. The reason silent movies look jerky today is every third frame is projected twice. They are smooth when properly projected.