Persistence of Vision

What is persistence of vision?

Persistence of vision is a claim about the eye: an image supposedly lingers on the retina for a fraction of a second after the light is gone, so the next picture overlaps it and the two fuse. For most of the 20th century it was the standard answer to "why do movies move?"

The observation underneath is real: a sparkler swung in the dark leaves a ring of light. What doesn't hold up is the jump from "images linger" to "lingering images create motion".

The idea grew up with the optical toys. Peter Mark Roget described in 1824 how wheel spokes look curved through the gaps of a fence; Joseph Plateau's phenakistiscope (1832) made drawings on a slotted spinning disc appear to move, and the zoetrope put them in a slotted drum. Muybridge's galloping-horse photographs (1878) and the first film projectors inherited the explanation along with the machines. The full arc is in the history of film.

Why is persistence of vision considered a myth?

Think through what a lingering image would do. If each frame stayed on the retina until the next arrived, you would see frames stacked on each other: a horse with eight legs, a double exposure. An afterimage also stays where it landed on the retina; it doesn't travel. Persistence can explain blur and ghosting, not a figure moving from one spot to the next.

Film scholars Joseph and Barbara Anderson made the case in "The Myth of Persistence of Vision" (1978), pointing out that film studies kept repeating the idea long after perception research had moved on. The current view: the brain's motion system does the work, reading a sampled series of positions much as it tracks a real object.

For a filmmaker the accurate version is more useful: the illusion has rules you can shoot for or against. It's one of the foundational ideas of film history and theory.

Phi phenomenon vs. beta movement

Both names come from Gestalt psychology. In 1912 Max Wertheimer published experiments in which two lines flashed one after the other and, at the right interval, observers saw movement between them. Researchers group these effects as apparent motion.

• Beta movement is what cinema relies on: two positions of a figure shown in quick succession read as one object moving between them. Flipbooks, chasing marquee bulbs and film frames all work this way, best when the jumps are small and the timing steady.
• The phi phenomenon, in Wertheimer's strict sense of "pure phi", is motion sensed across the gap without any object seeming to travel through it.

In writing about film, "phi phenomenon" is often used loosely for both; strictly, the moving figure on screen is beta movement.

Flicker fusion: the part the old theory got half right

A separate illusion runs through every screening. A film projector's shutter blocks the light while the next frame moves into the gate, so the screen goes dark dozens of times a second. You don't notice because above the flicker fusion threshold, pulsing light looks steady.

That's why projectors flash each frame more than once: a two-bladed shutter turns 24 fps into 48 flashes a second, and three-bladed shutters did the same for the slower footage of silent film. The frame rate carries the motion; the flash rate hides the flicker. Flicker fusion keeps the screen lit, apparent motion makes the figures move.

What apparent motion means on set and in AI video

The brain builds motion from small, regular steps, so anything that makes the steps big or irregular weakens the illusion:

• Fast pans strobe. At 24 fps a quick pan jumps the background so far between frames that it judders. Slow down, track a subject, or let motion blur bridge the gap.
• Shutter angle links frames. A 180° shutter at 24 fps leaves enough blur to connect each frame to the next, the logic of the 180-degree shutter rule. A narrow shutter gives the staccato look of the Omaha Beach landing in Saving Private Ryan (1998).
• Break it on purpose. Step printing and stop-motion on twos make the steps visible.
• AI video. When a face or prop changes too much between frames, viewers see morphing instead of movement.

In FlashBoards, keep reference clips of the motion you want next to your generated takes and play them side by side.

The illusion happens in the viewer's head, so no model can draw it, but you can generate the devices built to produce it and set one spinning. Keep all three takes on one FlashBoards board beside your frame-rate notes.

Common mistakes

• Decorative strip. Without "successive poses" the model paints a random frieze.
• Closed drum. Slits merge into a solid band; state "narrow vertical slits" and keep the camera at table height.
• The gallop doesn't read. The model animates the object, not your perception. Frame tighter on two or three slits and ask for a slower spin.
• Wrong-era projector. Say "35mm film projector" and "film feeding from a reel".

Further reading

Joseph Anderson and Barbara Anderson, "The Myth of Persistence of Vision" (Journal of the University Film Association, 1978) and "The Myth of Persistence of Vision Revisited" (Journal of Film and Video, 1993). Max Wertheimer, "Experimentelle Studien über das Sehen von Bewegung" (1912).