Persistence of Vision, by Stephen Herbert

"Virtually every ... account of the perception of movement in film texts [is] wrong. The impression of movement is not due to persistence of vision. The very persistence with which this 'explanation' has been recited says more about the hermetic and impressionistic world of some film scholarship than it does about the actual mechanisms involved. "
Bill Nichols and Susan J. Lederman

More than fifteen years ago, a number of writers pointed out, in detailed studies, that the long-quoted mechanism for motion perception in cinema, 'persistence of vision', an effect usually attributed to a 'defect' of the eye, (or in some accounts the 'eye-brain combination'), was an archaic concept long left behind by psychologists and physiologists specialising in perception. Most film writers were (and still are) unaware of, or unconvinced by, these writings, and continue to write of persistence of vision as being the mechanism by which we see motion pictures. They argue, when challenged, that the term is 'simple to understand', 'elegant' even 'poetic'. They are either unaware or unconcerned that it is incorrect.

The word 'persistence' has a meaning, and it seems that we do not perceive motion pictures through any kind of persistence, either in the eye or in the brain. So let's take a look at what those researchers wrote back around 1980, as their work is still valid (though still largely ignored) today.

"The notion of 'persistence of vision' seems to have been appropriated from psychology in the first decade of the century, the period during which cinema came into being. But while most film scholars accepted 'persistence of vision' as the perceptual basis of the medium and proceeded to theorise about the nature, meaning and functioning of cinema from that base, perceptual psychologists continued to question the mechanisms involved in motion perception; and they have achieved insights that necessitate the re-thinking of many conclusions reached by film scholars during the past 50 years."
Joseph and Barbera Anderson

"To begin with, 'persistence of vision' is itself an imprecise term. We can only guess that film writers are referring to what psychologists call 'positive after images'. When a person stares at a light, he or she can still see it after the light has been turned off. Positive after-images retain the colour and brightness relations of the original stimulus. Common sense would suggest that the positive after-image is a plausible explanation of motion perception in film since it allows one image-frame to 'bleed' into another, despite the fact that the beam of light projecting the film-frame itself is intermittent. But this fusion occurs regardless of whether motion is perceived or not. The appearance of a continuously visible series of images, in other words, is a phenomenon distinct from the appearance of motion. "
Bill Nichols and Susan J. Lederman

It may help to understand the phenomenon of apparent motion in motion pictures therefore, if we think of it as two things:

1) Firstly, it is necessary to ensure an apparently continuous, reasonably flicker-free image on the screen. Early experiments showed that a minimum of about 10 separate frames must be projected every second to give the illusion of movement. But this is not enough; the image will flicker very badly if a projector with only a single blade (to cover the period of film pulldown) is used. A simple experiment will show that the flicker rate must be of the order of 50 per second for it not to be obvious. For much of the 'silent' period, films were shot at roughly 16 frames per second and shown on a projector with a three-bladed shutter. Each individual frame was shown three times, so around 48 screen images were projected every second, (close enough to fifty to give a reasonably flicker-free result). Taking and projecting speeds gradually increased so that by the time 'talkies' arrived a standard speed of 24fps was decided upon, which meant that the shutter could be reduced to two blades to achieve the same effect of 48 screen images (each frame shown twice) per second.

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