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. 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. 2) Secondly, if the screen image is composed of a series of slightly differing pictures, as in a cine film, the image will appear to move. Those proposing this explanation at least recognise that the important part of the phenomenon, that which causes a series of static images to apparently move, is a psychological effect that does not require ‘persistence of vision’ to work, and they suggest that ‘persistence of vision’ is only necessary to ensure that the image which we see on the screen appears continuous and does not flicker. However, it has in fact long been determined that the so-called ‘persistence of vision’ is also probably irrelevant to the effect of (1), a continuous, flickerless image. So where did this ‘persistence’ (or after-image) notion come from? Even the suggestion that ‘persistence’ ensures that we do not see flicker in a conventional projection system is in fact not the case, for there is another problem with this ‘persistence’ or after-image theory. What actually happens after a bright image is removed is that we see a succession of after-images, some positive and some negative. Obviously, the negative after-images are irrelevant; we do not perceive negative images when we look at motion pictures. More importantly, the first after-image, which is positive, appears some 50 milliseconds after the image ceases. In that period of time, not one but at least two screen images (at 48 flashes per second; that is, 24 frames per second with a two-bladed shutter) would have been viewed. Put simply, the first after-image of a film frame is not actually perceived until after at least one following frame has been projected. So ‘persistence of vision’ is not even the mechanism by which we are made unaware of the blanks between images. So why don’t we see those blanks? Quite simply because they are dark periods of nothing, and are sufficiently short that they do not register in our brain.
Persistence of Vision and Motion Picture
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