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This is my first question on this site so forgive me for the awkward wording of the question. Basically, my question is why does light from, say, a sparkler, seem to remain where it just came from to you EYE? I attached a sample photograph and i understand that cameras work differently in that they're designed to "retain" light so to speak.

But why does the human eye have a similar, albeit less dramatic effect?

alt text

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thanks for the great link! –  Ramy Dec 28 '10 at 19:45
@Mark Persistence of vision is a myth. :) –  Mateen Ulhaq Dec 28 '10 at 22:50

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The human eye is actually quite similar to a camera. It also requires a finite exposure time to build up sufficient photon signal. If the source is moving over that time (typically about 0.1 sec for human eye), it will be blurred out over the exposure.

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+1, yet human sight (brain is also an important factor here) is much more complex than a camera; a bit more details can be found here: white.stanford.edu/~brian/papers/pdc/IAHV-preprint.pdf –  mbq Dec 28 '10 at 19:44
The eye captures at about 12 fps. (That's why movies are made at 24 fps - twice the speed that the eye can take in.) –  Mateen Ulhaq Dec 28 '10 at 22:51

The human retina and visual perception system of the brain display in combination what is known as "persistence of vision." The eye and brain retain a visual image for somewhere between 1/30th and 1/25th of a second, or even longer for especially bright images. See: the Wikipedia entry on Persistence of Vision and the page on The Nature of Visual Perception by the Center for Visual Computer at Stonybrook.

Persistence of vision is not a myth. If you click on the second link above, you will see an image with 9 yellow circles and one small black dot which demonstrates that persistence of vision is real by showing an afterimage you will be able to detect for yourself. Persistence of vision is directly related to the perception of an afterimage.

Some attribute persistence of vision only to the retina, and some attribute it to the visual processing system in the brain, specifically to what is known as iconic memory. But no one knows for certain what causes this phenomena at this time.

There is indeed a myth called the "myth of persistence of vision" which relates to the reasons for the perception of movement specifically and shows that persistence of vision does not account for the perception of movement in film but that movement is accounted for in film by some combination of the phi phenomenon and beta movement. So the myth is that the very real phenomenon of persistence of vision accounts for movement perception in film which generally only displays 24 frames per second. The myth is not that the idea of perception of vision is itself a myth. See:

The Myth of Persistence in Vision Revisited By Joseph and Barbara Anderson

NOTE: Even the wikipedia entry says that: "Some scientists nowadays consider the entire theory a myth" which is confusing because if one follows the reference for this statement, one reaches the link above from Joseph and Barbara Anderson which clearly is talking about movement perception only and not the general idea of perception of vision.

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