From: Jim Carlile (email suppressed)
Date: Tue Oct 30 2007 - 23:28:34 PDT
In a message dated 10/30/2007 9:21:57 A.M. Pacific Daylight Time,
email suppressed writes:
My understanding is that we spend half our time in darkness and half
"in light" while watching a film. As for what we do with the rest of
our time, the ratio varies :-). I thought that it didn't matter what sort of
projector was being used -
a two-blade shutter flashes each image twice with corresponding
durations of darkness, while a three-blade shutter flashes each image 3
times with corresponding durations of darkness. Either way, the
light/dark ratio is still 1:1. What matters is "bumping up" the number
of flashes to at least 48/second (72/second with three-blade
There's actually more dark than light. What complicates this is that during
the pulldown stroke, there has to be a complete black out to avoid ghosting,
so in many projectors the shutter sector during that phase has to be made a
little wider. That also means that a three-bladed shutter is kicking out a 1/48
frequency, not 1/72, because one of the blades is solely for that pulldown
stroke-- leaving only two for the projected frame.
It's interesting-- the fact that we don't see the dark strokes proves that
optical 'persistence of vision' is a fallacy. If it were true, then the dark
time would be readily apparent, as a dimming or darkness caste over the entire
So why don't we see that? Why don't we amalgamate the dark 'images' the way
we pile together the light ones?
A quick and easy way to prove this is with a variable speed projector.
Project a static image at 18 fps, then do the same down to 4 or 5. If persistence
of vision were true, then there would be a radical change in what we see. But
there is not.
The similarity is even more apparent if we shoot at those respective speeds.
So why don't we ever see the dark? It's because 'persistence of vision' is
not true. Something else is going on.
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