For Science!
6 Frame Rate Facts You Didn't Know

CJ Miozzi | 12 Nov 2014 16:30
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2. Animation is the result of the phi phenomenon

When you break it down, animation is really just a sequence of images. The optical illusion of perceiving motion in these static images is called the phi phenomenon, which occurs when the timing between the images and the change in the images reaches certain thresholds.

A fundamental contributor to the phi phenomenon is "persistence of vision," otherwise known as "afterimage." Remember how we said that "The vision centre in the brain retains each individual image for one-fifteenth of a second?" That's the persistence we're talking about, and it's what stops us from noticing the blank sequences between sequential images.

Want to see a freaky example of the phi phenomenon? Take a look at the image on the right, and you'll see a spinning, open circle of pink dots. Now focus your eyes on the crosshair at the image's center - suddenly, you see a green dot moving among the pink dots. Whoa.

3. The reason for the 24 FPS standard? Audio, not video

Before 24 FPS became the movie industry standard, silent films were projected via hand-crank at speeds that varied from 16 to 24 FPS - you try to maintain a consistent speed on a hand-crank. Projectionists would actually deliberately slow or speed up their cranking speed during key parts of a film for stylistic effect. In other words, changes in frame rate were not deemed unpleasant to the experience.

However, when audio began to be incorporated into films in 1926, this was no longer feasible - as you can imagine, hearing audio slow down and speed up is just plain annoying. So a consistent speed needed to be selected, and at the time, 22 to 26 FPS was what most theaters were using, so 24 was chosen.

4. 24 FPS is actually 72 FPS

While 24 FPS has been the golden standard ever since, 24 FPS may not actually be what you think it is. Yes, you are seeing 24 different images per second in a cinema, but the number of images being flashed per second is actually 72. What?

Remember the flicker we mentioned earlier? As a simplification, a projector works by flashing light through an image. The light then turns off while the current image is removed and the next image scrolls on up. In order to avoid flickering caused by the strobing light, every image is flashed three times before the next image is swapped in. At 24 different images per second, we are then getting 72 total images per second - or 72 flashes of light per second - and a flicker-free experience.

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