How do still pictures become a moving film?

A film is a long series of still pictures, called frames, shown so quickly one after another that your brain sees motion instead of a slideshow. Show animation frames at 16 or more per second and the movement looks continuous. The motion is not on the screen: it is constructed in your head.

For a long time, the standard explanation was persistence of vision, the idea that each image lingers on the retina for a moment and blends into the next, the way a glowing coal whirled in the dark leaves a fiery trail. That story is still repeated, including in many film-theory books. But psychologists and physiologists have rejected it as the explanation for watching films. As early as 1868, William Carpenter suggested the effect was more mental than retinal.

The decisive test came in 1912, when the psychologist Max Wertheimer flashed two similar shapes in two nearby places. At the right speed, people saw a single shape moving, yet they saw nothing in the gap between the two positions. Nothing was lingering; the brain was inferring motion. His study, run on his own colleagues, led to the founding of Gestalt psychology.

Today's view is that the eye is not a camera and does not see in frames per second. In film, each frame differs only slightly from the last, and our visual system cannot tell this kind of apparent motion from real movement. Persistence of vision survives in a narrower role: when flashes come too fast to tell apart, they fuse into steady light, which is why we don't notice the flicker between frames.