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March 1996, Week 4


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Thu, 21 Mar 1996 17:11:53 GMT
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In message  <[log in to unmask]>
 [log in to unmask] writes:
> 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?
Well as with all film, persistence of vision is all of it!
The answer is quite simple, doubt I will make a hash of
explaining it.  The speed of the camera frames, and the speed of the
wheel, have a very narrow 'mismatching' window.  basically, the
camera records the wheel slower (or faster) than the whell is turning,
which means  the wheel has turned more (or less) than a full cirlce
for the frame speed. The eye references on wheel spokes, and as the
camera is replaying the spokes in the 'wrong' order, the wheel
appears to be moving backwards uniformally.  persistance of vision
supplies the eye with the information of where the spoke was
last time, and it's 'moved' back', rather than forward, and thus
the carriage wheels move back, whilst the carriage moves forward.
The speed ratio is interesting, on some extended carriage scenes,
you can watch the wheels go foraward, slow, stop for a moment,
and then roll back, although such shots rarely get through to
final cuts.
> Does anyone have a good sound bite about this which they are willing to
> share? Or at least can someone explain that wheel thing in case I bump
> into any more armed and dangerous, precocious first graders?
> Thanks--
> Meryem Ersoz
> University of Oregon
Give them circles of paper, marked with spokes, and flag one
spoke a different colour.  Then, they can fool around with frame
speed, through  piece of paper with a square hole cut in it!
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