"persistence of vision"

[Andrew Foley]

-----Original Message-----
From: s~Z <keith at pfmentum.com>
To: pynchon-l at waste.org <pynchon-l at waste.org>
Date: 20 October 2000 23:30
Subject: vv2: "persistence of vision"


<snipped a bit>

>http://www.exploratorium.edu/snacks/persistence_of_vision.html
>
>Your eye and brain retain a visual impression for about 1/30th of a second.
>(The exact time depends on the brightness of the image.) This ability to
>retain an image is known as persistence of vision. As you swing the tube
>from side to side, the eye is presented with a succession of narrow,
>slit-shaped images. When you move the tube fast enough, your brain retains
>the images long enough to build up a complete image of your surroundings.
>
>Persistence of vision accounts for our failure to notice that a motion
>picture screen is dark about half the time, and that a television image is
>just one bright, fast, little dot sweeping the screen. Motion pictures show
>one new frame every 1/24th of a second. Each frame is shown three times
>during this period. The eye retains the image of each frame long enough to
>give us the illusion of smooth motion.


A film frame is only projected three times on a projector with a three-plane
shutter.  Many projectors use a two-plane shutter, which only shows each
frame twice.  A projectionist I consulted about this says that the light
levels are very slightly lower with a three-plane shutter.  The only
disadvantage with a two-plane shutter is that it causes a visible strobing
flicker when the projector is set at 16 or 18 frames per second for silent
films; this is right on the borderline of persistence of vision, and the
switch between frames becomes very marginally visible.