Persistence of vision

From Academic Kids

This article is about the theory on human vision. For other uses of the term Persistence of vision, see Persistence of vision (disambiguation).

According to the theory of persistence of vision, the perceptual processes of the brain or the retina of the human eye retains an image for a split second. This theory supposedly accounts for the fact that when a motion picture flashes a series of progressive images, instead of the mind seeing the flashing of a series of images, it sees the illusion of motion.

Psychologists and physiologists have abandoned this theory's applicability to film viewership, though film textbooks, film professors, and film theorists have largely not.

Persistence of vision should be compared with the related phenomena of beta movement and phi movement. A critical part of understanding these visual perception phenomena is that the eye is not a video camera: there is no "frame rate" or "scan rate" in the eye: instead, the eye/brain system has a combination of motion detectors, detail detectors and pattern detectors, the outputs of all of which are combined to create the visual experience.

The frequency at which flicker becomes invisible is called the flicker fusion threshold, and is dependent on the level of illumination.


Film systems

Through experience in the early days of film innovation, it was determined that a frame rate of less than 16 frames per second caused the mind to see flashing images. Audiences still interpret motion at rates as low as ten frames per second or slower (as in a flipbook), but the flicker caused by the shutter of a motion picture projector is distracting below the 16-frame threshold.

Modern theatrical film runs at 24 frames a second. This is the case for both physical film and digital film systems.

It is important to distinguish between the frame rate, and the flicker rate, which are not necessarily the same. In physical film systems, it is necessary to pull down the film frame, and this pull down needs to be obscured by a shutter to avoid the appearance of blurring, there needs to be at least one flicker per frame in film. To reduce the appearance of flicker, virtually all modern projector shutters are designed to add additional flicker periods, typically doubling the flicker rate to 48 Hz, which is less visible. (Some newer projector shutters even triple it to 72 Hz.)

In digital film systems, the raster scan rate may be decoupled from the image update rate. In some systems, such as the DLP system, there is no flying spot or raster scan at all, so there is no flicker other than that generated by the temporal aliasing of the film image capture.

The new film system MaxiVision 48 films at 48 frames per second, which, according to film critic Roger Ebert, offers even a flickerless tracking shot past picket fences. The flickerless shot is due to the higher sampling rate of the camera.

Video systems

Video records at an equivalent of 25 or 29.97 "frames" per second depending on the national system used; television thus displays a complete new image at 25 or just under 30 times a second.

Again, with video, the flicker rate is not the same as the frame rate. To provide a visibly superior picture "interlacing" is employed. Each complete frame is divided into two video fields of alternate lines, and the two fields are shown consecutively to make up a frame. Thus, the field rate of video is twice the frame rate.

Some modern video systems also decouple display from image update, for example systems using LCD monitors add intermediate frame buffers to increase the display rate.

Cartoon animation

In drawn animation, moving characters are often drawn "on twos", that is to say, one drawing for every two frames. Even though the image update rate is low, the fluidity is satisfactory for most subjects. However, when a character is required to perform a quick movement, it is usually necessary to revert to animating "on ones", as "twos" are too slow to convey the motion adequately. A blend of the two techniques keeps the eye fooled without unnecessary production cost.

Optical toys

Persistence of vision was used to create a number of optical toys that were popular before movies. Most used simple mechanical methods, such as spinning disks or turning spindles, to quickly transpose a series of pictures and so create the illusion of motion. Examples includes the zoetrope and the thaumatrope.

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