[LargeFormat] Shutter speed lengthened by very small apertures

[Richard Knoppow]

----- Original Message ----- 
From: "philip lambert" <philip.lambert@ntlworld.com>
To: <largeformat@f32.net>
Sent: Wednesday, December 17, 2003 12:44 AM
Subject: Re: [LargeFormat] Shutter speed lengthened by very
small apertures


> In terms of accuracy of measurement:
>
> Does anybody know if the small Calumet speed tester is as
accurate at speed
> testing a shutter when the lens is wide open compared with
when it is
> stopped down?
> It's the accuracy of measurement I am on about, not
whether the exposure
> speed genuinely varies with stop.
>
> Philip
>
>
  The Calumet tester has a sensor about 4mm in diameter. Its
quite accurate. For leaf shutters it will give you the full
aperture speed although there is a complicated way to get
effective speed.
  There is also a simple technique for _effective_ speed of
focal plane shutters.
  The factory repair instructions give the speeds that
should be measured at full aperture.
  If you want to measure the effective speed you must set up
the tester so that the image of a source is focused on it.
This can be a diffused source like an illuminated card.
Focus the lens on the card and place the tester so that it
is sampling the center of the image. Then, set the aperture
for 2 stops down from the maximum. At that setting the
aperture will be half the total diameter of the full
aperture. Then adjust the intensity of the light on the card
so that the counter on the tester just stops. This is the
half light point which is were the effective speed is
measured. To measure the speed open the lens up to full
aperture again and make a test. The indicated speed should
be higher than the speed measured by placing the tester
directly in the light path behind the shutter.
  Focal plane shutters are measured similarly. Calumet gives
a simplified method where the distance of a light source is
adusted so that the counter just stops. The source is then
moved closer to the camera by 0.707 the distance. This will
exactly double the intensity of the light assuming a non
focusing source is used. The latter is important because the
fall off of light with distance does not follow the inverse
square law when a focused source is used.
  While many assume a focal plane shutter is perfectly
efficient in fact they are far from it. The efficiency
depends on the distance of the shutter from the actual focal
plane and the angle of cone of light from the lens. The
longer the lens focal length or the smaller the stop the
more efficient the shutter is. Also the closer the shutter
is to being in the actual focal plane the more efficient it
is. The ratio of the slit height compared to the distance
from shutter to film is an important factor.
  The reason is different from that for the leaf shutter but
can amount to as much as a stop for large cameras, like
Graflex and Speed Graphics, where the shutter curtain is
spaced well away from the film and some of the shutter slits
are on the same order of size as this distance.
  The Calumet tester is well worth having. I am not sure
what the price is now. When I bought mine several years ago
it was about $60, when I looked fairly recently it was up to
$80 and may well be at $100 now. Its still worth it. The
tester will also measure the  length of strobe flash. This
can be useful for calculating whether a correction for
reciprocity failure is necessary and to check for motion
stopping ability.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com