Persistence of Vision - Page 2
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.
It is sometimes said that to ensure a flickerless, apparently continuous image, as explained in (1), persistence of vision is necessary, and that apparent motion (2) is a psychological effect of the brain.
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?
Roget, in describing the phenomenon of spokes of a wheel appearing to be curved when viewed through a series of vertical slits, also mentions the
"illusion that occurs when a bright object is wheeled rapidly round in a circle, giving rise to the appearance of a line of light throughout the whole circumference; namely, that an impression made by a pencil of rays on the retina, if sufficiently vivid, will remain for a certain time after the cause has ceased."
As Joseph and Barbera Anderson have pointed out, there is no logic in ascribing how we see motion pictures to this effect:
"The illusion he describes is not an illusion of motion, nor does he claim that it is. Roget has described a case in which a series of moving points results in a static image, while in cinema a series of static images results in the illusion of motion."
Later in their article, they point out that:
"There is no motion on the screen, just a succession of still images. If there were persistence of these images in the eye of the viewer, figures on the screen would pile up, one on top of the other, resulting in a kind of chronophotographic display."
As previously mentioned, it has been suggested that, although perhaps not the mechanism by which successive still pictures are perceived as a moving image, 'persistence' (i.e. the after-image) is necessary in the cinema, in order to ensure that we are unaware of the blank (dark) screen which occurs between the projection of each image; and that if it were not for this persistence,the result would appear to flicker. Firstly, motion picture systems exist which do not have a blank screen between images; e.g. Steenbeck viewing machines, in which a prism block 'mixes' one image into the next. So even if 'persistence' were necessary for conventional projection with an intermittent shutter projector, the existence of systems that do not have blanking periods means that it can hardly be considered important to our understanding of how we perceive apparent motion in motion pictures.
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.