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March 1996, Week 4


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Film and TV Studies Discussion List <[log in to unmask]>
Bet MacArthur <[log in to unmask]>
Thu, 21 Mar 1996 23:31:08 -0500
Film and TV Studies Discussion List <[log in to unmask]>
text/plain (37 lines)
On Tue, 19 Mar 1996,  Meryem Constance Ersoz <[log in to unmask]>  asked:
> 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 asked this very question of a grip at a film shoot for a Western once, and
it was pointed out to me that this illusion appears in 'real life' too, not
only on film.
     It occurs when the angle of the spinning object (wagon wheel, fan,
flywheel, etc) changes in relation to a light source, causing light to
refract off the spokes (or hole, or other 3-dimensional feature of the
spinning surface) a tiny fraction of a second _sooner_ than before. You will
notice that not only is the the image one of the wheel turning 'backward,'
but that of a wheel or set of spokes doing so _very slowly_. --ie, at the
rate at which the angle in question is changing.
      This illusion or shift can occur in the vehicle's 'forward' direction
too, creating the image of a very-slowly-rotating spoke going in the proper
direction.  As far as I can tell, this illusion has nothing to do with
persistence of vision per se.
     Joining your 6-year-old questioner in discovering the joys of physics,
         Bet                      "All the arts begin in
Bet MacArthur                   the world of the senses --
Arts Analysis Institute        This doesn't mean that
Cambridge, Massachusetts      they don't use ideas,
Mass: (617) 455 6189                but the ideas
Los Angeles: (310) 313 5059        are never first."
Online:   [log in to unmask]          --Robert Motherwell
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