Cinema in your head

Copyright: Tetra Images / Tetra Images / Getty Images
Kino im Kopf
Author: Ulrich Kraft

Movements on the cinema screen seem as real as life. Yet this only becomes possible through the clever exploitation of the quirks and flaws of human visual perception. To see what is really rolling, let’s take a peek behind the curtain…

Scientific support: Prof. Werner Sommer, Prof. Dr. Michael Bach

Published: 19.09.2017

At a glance
  • Movies use special features of the human visual system to suggest a flow of movement that isn’t there at all – since cinemas basically just show static images in swift succession, as in a slide-show.
  • The brain uses the variation between single images to calculate a trajectory and connect what is seen in a logical sequence.
  • This is caused by the stroboscope effect: the specific means by which the visual system perceives movement.
  • The black gaps between the pictures are blanked out and don’t reach our consciousness.

… straight into the screening room. Whether the film is Ben Hur, an arthouse movie or a current blockbuster, it's here that movies wait for their appearance, coiled on big rolls. Each consists of a seemingly endless strip with a series of photographs that are separated by thin black lines. 24 ‘frames’ a minute, which is what movie people call those frozen images, are shot by standard movie cameras, which means it takes 130,000 frames to make a 90-minute feature.  The camera takes a 10-second scene depicting a strolling pair of lovers and breaks it into 240 single images. In the process, single frames are created, between which each person and object that moves during in the scene will change its position just a little.

Black screen – and nobody notices

Unlike what many people may think, a movie isn't displayed under continuous illumination from a projector. If that were the case, we would see pictures with a black border between them that rolls from the top to the bottom of the screen. Instead, each frame is fixed between the light and the lens of the projector for a fraction of a second. This lighting phase is followed by a transport phase. In front of the light, an aperture closes, the light is extinguished and in the cover of darkness, the projector moves the film to the next frame. So for a good part of the time, the audience is not watching Batman or James Bond, but a black screen! Still, this won't lead to a refund of your money – nobody notices the dark intervals or the flickering that originates from the switching between light and dark phases. Why is this? And how can it be that we perceive a succession of frozen images as a unified moving picture?

The Phi-phenomenon makes lettering move

From the invention of the flicker book, the forebear of cinema and TV, we have known how that flowing movement is an easily created illusion.  The question of the processes of perception behind this has nevertheless stimulated a lot of discussion among experts. For a long time, a phenomenon called "visual persistence" was held mainly responsible. It describes the fact that the retina and the visual processing centers of the brain retain a visual impression for fractions of a second even after the underlying visual stimulus has vanished. So that means that what one has just seen continues to have an effect on our perception for a moment.

On the one hand, such afterimages do fill the dark images between the frames. And since each new image superimposes a persisting visual impression on the subsequent one – one frame effectively dissolving into the next – visual persistence in cinema helps cause the illusion of continuous movement. Yet on the other hand, even though this is still the model given in a lot of movie textbooks, the theory has a drawback: about 50 milliseconds separate the disappearance of the "original picture" and the appearance of the after-image. During this time, the movie projector has already thrown at least two more frames onto the screen. That means that the earliest the audience member can perceive an after-image is when the next frame has already been projected. This in turn precludes the creation of a perception of movement on the grounds that the after-image of a frame superimposes the next fixed image onto it. 

 

“Experimental Studies on the perception of movement” (“Experimentelle Studien über das Sehen von Bewegung“) was the title of a paper that Max Wertheimer (1880-1943) wrote in 1912 to qualify as a professor at the University of Frankfurt. Here the Prague-born psychologist describes, among other things, the Phi-phenomenon that appears in its simplest form when two stationary dots of light flash one after another. As long as a certain spatial and temporal distance is kept, the viewer receives an impression of a dot of light that moves from the first to the second position. The huge electronic moving fonts and advertising boards that are installed in places like soccer stadiums work on this principle. Even though these cases provide very realistic effects of movement, they are created by hundreds of fixed lights that turn on and off in a certain order. This is an illusion of movement created by our brain.
 

Flicker-free using a cheap trick

Yet one thing is certain: without the stroboscopic effect, which is another name for the perception of movement caused by positional change, there would be no television, no Youtube, no cinema. Provided that single images only vary slightly from one another, 12 to 15 frames per second are enough to evoke a semblance of motion. Still, the problem of flickering remains. At least 45 images per second are required for an audience to watch a flicker-free movie. This, by the way, has absolutely nothing to do with the phenomenon of retinal persistence. Rather, such very short dark intervals simply represent a temporary lack of information. That is why they aren't consciously perceived.

Recording 45 single images per second may be technically possible, but is cumbersome and expensive due to the costs of film material. Instead, a clever gimmick is used. Each frame is cast onto the screen not just once, but two or three times – with the help of rotating apertures. A two-wing aperture turns the 24 single images recorded by the camera into 48 projected images. Three-wing apertures force the rate up to 72 images per second. So it's thanks to a "cheap trick" (in the truest sense of the phrase) that we can watch Hollywood’s heroes without constant flickering... And what difference does it make, as long as it works…? After all, cinema has always been a grand illusion.

Veröffentlichung: am 03.11.2010
Aktualisierung: am 19.09.2017

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