From their earliest origins, human communities
have always felt the need to tell stories, whether about themselves
or about other groups of people. In our societies today, going to watch a film
together in a darkened theatre is a well established natural extension
of this ritual. Motion pictures derive their power from their ability
to evoke reality, and in particular from the illusion of motion that they create.
The best way to understand the mechanisms that let us portray reality by running film through a projector is to think
of them as having solved two distinct problems. The first was
to keep the audience from noticing the intervals of darkness
between the images, as well as the flicker (fluctuating light
intensity) produced by the alternation between images and darkness.
The second problem was to create the central illusion of the
cinemahow
to take a series of still images that differ slightly from one
another and make the audience perceive them as a single image
that is moving. Retinal persistence was long thought to play
a central role in solving both of these problems, but its role is now considered
negligible.
Because of the constraints imposed
by your computer screen, this animation can unfortunately
give only an approximation of the real effect.
The first problem
was solved by increasing the number of images projected per
second onto the screen compared with the number shot per
second by the movie camera. For older films, shot at 16 frames
per second, the number was tripled; for today's films, shot
at 24 frames per second, it is doubled. The viewer thus sees
close to 50 images per second, the threshold at which flicker
becomes too rapid to perceive (see box below). So what happens
to the intervals of darkness, which account for almost half
the projection time? Apparently, retinal persistence has
nothing to do with it. We just ignore these intervals, because
for the brain they simply constitute an absence of information.
As regards the second problem, it is now
generally agreed that what makes us perceive motion in place of
a rapid succession of still images is a psychological effect, known
as the beta effect, that has nothing to do with
retinal persistence either. The simplest example of the beta effect
occurs when two closely spaced points of light go on and off
in succession. Though there is no actual movement, our perceptual
processes subjectively link the two points into a single one that
is moving. The same principle is at work, in more complex form,
in the moving signs used on billboards and in sports arenas, where
hundreds of tiny lights going on and off in succession produce
highly realistic motion effects.
The beta effect can also create the illusion
of motion toward or away from the viewer. For example, when you
show people a series of progressively smaller images of the same
object, they will generally experience it as moving away from them.
And conversely, if the images are growing larger, then people will
experience the object as moving toward them. Similarly, if the
series of images begins with bright colours that gradually grow
duller and start to fade into the background, people will usually
say that the object has moved away from them. Thus the beta effect
is the basis not only for the illusion of motion in films in general,
but also for numerous graphic subterfuges in animated films in
particular.
The frequency at which the flicker
caused by a succession of images becomes imperceptible
to the human visual system is called the flicker
fusion threshold. This threshold is not absolute.
It depends on the degree of illumination of the images,
and is higher for brighter ones. It also depends on what
part of the retina is capturing the image. The rods respond
to light more rapidly than the cones. Hence we may sometimes
perceive flicker in our peripheral field of vision when
we cannot see it in our central field, where the image
is captured by the fovea,
which consists of cones.
On television, as
in the movies, the illusion of motion is created by a rapid
succession of still images. But instead of being projected
from a film, they are produced by a varying-intensity electronic
beam that scans the inner surface of the television's cathode-ray
screen at high speed. This surface is coated with phosphorus,
so the variations in the beam's intensity leave behind
traces of light of varying intensity that last for a few
moments.
On a conventional television receiver, the electron beam
reconstructs each image by drawing horizontal lines across
the screen, starting at the top and working down. The number
of lines per screen depends on which standard is used in
your country: 625 for the PAL/SECAM standard, or 525 for
the NTSC standard. PAL/SECAM television cameras record 25
images per second, while NTSC cameras record 30, so a television
displays a new image either 25 or 30 times per second.
In television, it is not possible to double the number of
images per second by means of a shutter, so another strategy
is used to eliminate flicker. Each image is drawn twice.
The first time, the electron beam draws the odd lines, and
the second time, it draws the even ones. Thus each image
actually consists of two fields, which means that 50 or 60
fields are displayed per second, thus eliminating the problem
of flicker.