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March 1996, Week 4


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"Duncan, Iain" <[log in to unmask]>
Reply To:
Film and TV Studies Discussion List <[log in to unmask]>
Thu, 21 Mar 1996 13:31:08 EST
text/plain (54 lines)
In response to this question posted by Meryem Constance Ersoz:
>So I'm kicking the question back to the film list. Can anyone answer
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?<
I suppose that the effect of wheels spinning backwards is created more by
mechanics of motion picture photography than by persistence of vision.
Maybe the best way to answer the question is to use a simplified example.
Imagine you had a disc with a single line drawn on it from the centre to
a point
on the circumference.  You then set the disc spinning at a constant rate
of one
revolution per minute.
Now suppose you were filming this disc spinning using a camera that made
one exposure each second.  As the line would be in the same position each
time an exposure was made, it would appear to be stationary rather than
on the resulting film.
To achieve the 'backward' effect, you would simply need to speed the
camera up
slightly to, for example, 0.95 seconds between exposures or,
alternatively, slow the
disc down.  Now the disc would be completing slightly less than one
between exposures and (hope you can imagine this!) would therefore appear
film to be moving in the opposite direction to which it is actually
To apply this in practice, you simply have to speed the whole thing up -
24 frames
a second and a wheel that rotates perhaps only 23 times a second.
Hope that gives a solution to at least part of the problem.  It would be
easier to
draw diagrams but, unfortunately, that's not too easy to do on email!
Iain Duncan
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