18 May
2005
18 May
'05
5:02 p.m.
Massively OT, but...
>> I have a small collection of old cameras, all
of which can (or at least
>> could, if I made film to fit them) be used.
I often use a TV screen as a quick check of the
shutter speed.
Even though the screen has persitence, where the electron
beam hits is brighter. If you know the sweep speed, you can
get good enough on the shutter speed.
Another trick is to photograph a revolving disk with a dot/line on it,
the disk being driven by a synchoronus motor. You measure the length of
blur on the negative and thus work out the exposure time.
Measuring time fairly accurately is not a problem. I have an old-but-good
HP 5245 counter here. OK, the timebase hasn't been calibrated for years,
but it's still cloese enough to check a mechanical shutter.
The problem comes with defining the open time of a shutter, particularly
a leaf shutter. Rmemeber that it doesn't open or close instantaneously.
So you get a roughly trapezoidal pulse of light coming through. So do you
measure the time from starting to open to fully closed, the time from
fully open to starting to close, the equivalent width of a recangular
pulse with the same area (which would give the same exposure), or what?
Worse than that. the opeeniong and closing times depend on the diameter
of the ligth beam coming through the shutter, and thus on the lens
aperture setting.
Hmmm...
-tony