From: andpph@ritvax.isc.rit.edu
Subject: Re: Testing shutter speeds.
Reply-To: andpph@ritvax.isc.rit.edu
Organization: Rochester Institute of Technology
Date: Sun, 24 Jan 1993 03:02:35 GMT

In article <1993Jan19.234939.1@cc.curtin.edu.au>, zrepachol@cc.curtin.edu.au writes:
>In article <1993Jan18.220114.6206@bnr.ca>, poole@bnr.ca (Thomas Poole) writes:
>> What is the cheapest, most accurate way of testing shutter speeds? I 
>> recently acquired some old cameras, most of them with sticky leaf shutters.
>> I would like to test the upper speeds to see if they are affected as well. 
>> I have heard of photographing the tv and counting lines. Assuming the tv 
>> scan rate it 1/30 sec, 1/60 shutter speed would give 1/2 a picture, 1/125 
>> would give 1/4 picture, etc.  Has anyone ever used this method? Does it work
>> for focal plane shutters and leaf shutters? Is there a better way?
> ....
>
>But, yes the TV method does work, and quite well. You have made one error 
>already though, the TV in the US is 30 FRAMES / sec, 60 FIELDS / sec.

It is probably best to think of the TV as a device that scans out, in NTSC,
15,750 lines per second. So one should be able to count lines and divide by
15,750 to find the sutter speed.Shutter speeds that are longer than 1/60 second
are not easy to deal with and anything longer than 1/30 second is just about
impossible.

>For leaf shutters, a big problem is that they spend a large part of the time
>opening and closing.

This is true but is really only of major concern when one is using the shortest
exposure times in combination with the smallest apertures. This is the case
because leaf shutters are generally calibrated with the aperture of the lens
set to the largest aperture. The consequence of this is that under the above
conditions leaf shutters will tend to overexpose becasue the lens will be at
the preselected aperture for a longer time than if a large aperture were used.
For these situations it is advisable to stop down an additional 1/3 to 2/3
stop.

>Ilumination of film plane vs Time:
>
>         /-........-\                 |-........-|
>        /            \                |          |
>       /              \               |          |
>      /                \              |          |
>     /                  \             |          |
>----/                    \----     ----          ----
>
>
>   Leaf shutter                      Focal plane
>
>Note that the ... section in the middle is the bit that varies, sooo if we
>reduced the ... to a hypothetical zero, we would have 0 exposure for the FP,
>but quite a bit in the skirts of the leaf. This is why a CRO is more use for
>a leaf shutter.
   
                     /-..-\              f:2.8
                    /      \
                   /        \
                  /          \
shutter          /            \
calibrated ------------------------@ 50% full open
               / ^  cal time  ^ \
              /                  \
             /                    \
            /......................\      f:22
       ----/ ^ small aperture time ^\----


clearly shutter is fully open at f:22 for longer than it is at f:2.8 or even
than at the smaller 50% open position. The discrepancy, however, is mostly
significan at the smallest apertures and only when using the highest speeds. At
low speeds the error introduced by this characteristic is for all intents and
purposes insignificant. The sloping end pieces having a much smaller wjhen the
area of the figure is large.

Focal plane shutters are not totally immune from this effect. In their case it
is usually referred to as an error in efficiency but the effect is the same. In
FP shutters the error creeps in because the moving slit travels across the cone
of illumination coming to any given point on the film from the lens aperture.
The diameter of this cone of illumination is larger as the aperture is made
larger and the slit edges take a longer time to reise the level of illumination
to 100% associated with any given aperture when the light cone diameter is
large than when it is small.

The consequence of all this is that with any shutter the efficiency is highest
when small apertures are used in combination with long exposure times. Anyway,
that is my 2c worth and most of it credited to the Materials and Processes of
Photography book by Stroebel, Compton, Current and Zakia which is used here in a
intro to the science and technology of photography course. Though this may not
be for much longer as there seems to be less and less interest in technical
matters and more and more on "show me where the button is"... i can figure out
the rest... sort of... besides, when i need to know something i can always ask
somebody like you!

humpf.....
andy

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.....|   |..                                                              \
            | Andrew Davidhazy, RIT, High Speed Photography Lab.           \
            :____________________________________________________ |'''''''\ \
            : IN%"andpph@ritvax.isc.rit.edu"    fax 716-475-5804  |......./ /
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