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[OM] Shutter testing

Subject: [OM] Shutter testing
From: Patrick Greenlee <patrickg@xxxxxxxxxxxxxxx>
Date: Mon, 11 Jan 1999 08:49:08 -0700
 USED BODY TESTING
>
> I heard that a useful and inexpensive (cost of 1 roll of film) way of
> testing the shutter speed of my new camera would be to shoot at a computer
> screen (or TV) at different shutter speeds. Would anyone have more detailed
> information on that?

        Never heard or thought of that  B U T  given the persistence of the
TV screen's phosphors it might lower the contrast enough to interfere with
interpretation.  What popped into my head was to take a picture in a
darkened room of a moving LED powered by full wave rectified but not
filtered AC.  this would give 120 pulses of light per second (100PPS with
50 Hz mains).  By moving the light or panning the camera you should get a
picture with a series of bright spots on it.  Each spot would be 1/120 of a
second.  This would allow checking the shutter speed to a resolution of
1/120 sec.  Useful perhaps for longer shutter speeds but not for the
speeds above say a 30th of a second.
        But you don't have to power the LED from a wall socket, i.e. 60
Hz(sometimes 50 Hz outside of the US).  If you have access to a calbrated
audio oscillator or signal generator you could flash the LED at say 4000
pulses per second or whatever is required for the shutter speed in
question.  A frame exposed at 1/500 while panning across the LED should
yield a negative with 8 exposed dots on it and so forth.
        If you have access to a storage oscilloscope or a 'scope with
fairly long persistence you could use a photodetector placed where the film
would be in the light path and trip the shutter at various speeds and read
the shutter speed on the 'scope.  For example if the horizontal sweep speed
of the scope were set for 0.0005 seconds per horizontal unit (graticule,
centimeter, whatever) then with the camera aimed at a light source you trip
the shutter, set at 1/1000 and should get a pulse on the 'scope about 2
units long, i.e. 1/1000 of a second.  If the pulse is longer than that the
shutter is slow, if shorter the shutter is too fast.
        If you were to take a series of pictures at different shutter
speeds of a uniformly moving object, say a ceiling fan running at or near
its top speed, the length of the blurring of the blade tip should follow
the inverse of the shutter speed, i.e. as shutter speed increases the blur
length should decrease in precise proportion to the increase in shutter
speed.  This would immediately identify one or a few bad speeds Given that
you are confident that most settings are OK.  In the unlikely event that
most or all of the shutter speeds were wrong this test would only confirm
that most or all were wrong.  If the moving object's speed (fan blade in
our example) were known to a decent precision then absolute measurements
could be made instead of relative ones.
        Dontchajustlovit when you ask for the time of day and get a lecture
on the theory and practice of precision time keeping and its study through
horology.

Patrick      (OLY content = these ways would work with an Oly)

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