Meryem Ersoz
University of Oregon wrote:
>So I'm kicking the question back to the film list. Can anyone answer this
>for me? Why do wheels, when filmed, sometimes look as if they are
>spinning in a direction which appears to be the opposite of the direction
>which logic tells us they actually must be spinning? Does persistence of
>vision have anything to do with it?
The wheels  appearing to go in reverse  is caused by the stroboscopic effect
of the camera shutter.
The camera shutter cuts off the light while the film is advanced by one
frame. If the camera speed and wheel speed are in the correct mathematical
relationship, even though the wheel HAS rotated more than one revolution,
 it may appear to be at a position slightly less than one complete
revolution. In this case, the wheel appears to be rotating backwards ( or in
the opposite direction ).
The persistence of vision is what makes cinema and television work. A series
of slightly different images are integrated by the brain into what appears
to be a moving image.
The phi phenomenon is different. It is what makes a series of still images
appear to move. A typical  example is the light-bulb festooned billboards
outside theatres, cinemas, sideshows etc., where a corrrectly displaying set
of bulbs appears to make a moving arrow or something.
Tony Saffrey
London UK
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