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March 1996, Week 4


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David Tetzlaff <[log in to unmask]>
Reply To:
Film and TV Studies Discussion List <[log in to unmask]>
Thu, 21 Mar 1996 08:48:03 -0800
text/plain (32 lines)
>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?
It is a strobe effect. It occurs when the period of rotation of the wheel
places a repetitive elements of the wheel - a spoke say - in almost the
same position at the point of one frame exposure as the next, but not
quite. Let's say the first frame exposed captures a spoke at 12 o'clock (or
0=B0). If the forward rotation of the wheel is at such a speed that another
spoke is at 12 o'clock 1/24 of a second later, the wheel would appear not
to be moving. If the rotation speed is such that the first spoke is caught
at 12, the second at 11:59, the third at 11:58, and so on - the wheel
appears to be rotatinf backwards at a speed much slower than it is actually
rotating forward.
It is exactly the same effect as using a neon strobe to set the speed of a
turntable. Marks are placed on the rim of the turntable so that at the
correct speed they will pass the light at exactly the frequency the light
flashes, appearing to stand still.
So I guess the answer is conditionally yes, the reverse wheel effect is as
much an example of persistence of vision as a moving image of a forward
rotating wheel. It is generated by consecutive still images that show the
subject in a pattern of progressively different positions, and our mind
stitches them together into motion. I say conditionally, because I believe
there is some controversy as to whether persistence of vision per se - the
tendency of the human optic system to hold information, has anything to do
with any of this.
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