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Persistence of vision by Matt Ottewill

Persistence of vision is a theory which attempts to explain how the human eye/brain can be "fooled" into seeing continuous motion when presented with a sequence of discrete still images (film or video frames) at a rate of 10 frames per second (fps) or greater.

What is the theory?

The theory maintains that our eye/brain retains an impression of what we see for approx 0.1 seconds. Because the eye/brain can only handle so much visual information at a time, any information over and above this capacity is not perceived, or at best is perceived as a blur. The inference is that our eye/brain functions like the shutter of a camera producing a continuous sequence of "snapshots" which blur into one another.

Furthermore, television and movie technologies have exploited this effect to deceive us into perceiving continuous motion, where there is none. This is of course done by displaying a series of still images (snapshots or frames) in fast succession.

10 fps is regarded as a minimum to produce the illusion of continuous motion but a frame rate of 24 is preferable because it eradicates any distracting flicker.

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Technology Frame rate (fps)
PAL (European TV) 25
NTSC (US etc TV) 30
Film 24
Internet 8-12 (average)

Problems with the theory

It is worth saying that the theory has many detractors and critics who maintain that the theory is technically flawed, not an adequate explanation, and should no longer be perpetuated or taught.

"A typical explanation of persistence of vision went something like this: when the human eye is presented with a rapid succession of slightly different images, there is a brief period during which each image, after its disappearance, persists upon the retina, allowing that image to blend smoothly with the next image. Such an explanation might begin to account for a sense of constancy of the light source (flicker fusion), but it is, of course, a totally inadequate explanation of the illusion of motion in the cinema. The proposed fusion or blending of images could produce only the superimposition of successive views ..." "The result would be a piling up of images, or at best a static collage of superimposed still pictures, not an illusion of motion. It is the obvious inadequacy of the explanation, coupled with its recurrence in film literature for almost a century, that arouses one's curiosity about the origins of the notion (theory) and the means by which it has been perpetuated." Joseph and Barbara Anderson 1993

Such critics have elaborate and often compelling alternatives to the theory. Search the web to find them.

This writer's opinion

For what its worth, I think that any theory that attempts to explain the effect in physical terms (physiological/chemical/electrical etc) runs into problems. By ignoring the way in which light waves/particles fall onto the retina of the eye and are sent to the brain as electrical impulses in a continuous process, the theory losses credibility.

I prefer to think of it as a result of the way the human brains "comprehends" and interprets the information it receives from the optic nerve. It doesn't make sense to me to say that the brain "retains" an image for a short time because this ignores the continuous nature of physical perception. Our vision has always been selective, we cannot consciously "acknowledge" everything we see all the time, so we tend to pick out and focus on those elements which are important to what we are doing. There may also be an element of prediction (expectation) that leads us to perceive continuous motion. Perhaps a combination of expectation and selective processing produces the illusion of continuous motion.

So, if the theory is disputed by many, why study it?

Perhaps, because it offers a simple to understand explanation, the theory may still have some value as a useful "working" theory, and because it is cited so often, it's no bad thing to at least be aquainted with it.