[LargeFormat] Shutter speed lengthened by very small apertures

[philip lambert]

What a wealth of information you are.. Thanks 
Philip
----- Original Message ----- 
From: "Richard Knoppow" <dickburk@ix.netcom.com>
To: <largeformat@f32.net>
Sent: Wednesday, December 17, 2003 9:17 AM
Subject: Re: [LargeFormat] Shutter speed lengthened by very small apertures


> 
> ----- 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
> 
> 
> 
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