Hasselblad Bellows - Exposure Compensation

[pflower]

I have just bought a second hand hasselblad bellows - the one with the dual cable release.  No manual.  Can anyone explain the scale on it for exposure compensation?

the 2 top rails each have 2 scales marked f=8cm; f=15cm; f=6cm and f=25cm.  There are also various numbers along the rails i.e. 2, 21/2, 3, 3 1/2 which I assume are exposure compensations.  They don't make much sense to me. Can anyone explain how to use a 80mm lens on this thing?

Thanks

[dobson]

When you put extension tubes, (or bellows), between your lens and your camera you suffer from light loss. The longer the tube, the more light is lost to the sides. The markings on your bellows just tell you how much to adjust the exposure at different lengths.

[pflower]

When you put extension tubes, (or bellows), between your lens and your camera you suffer from light loss. The longer the tube, the more light is lost to the sides. The markings on your bellows just tell you how much to adjust the exposure at different lengths.
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I figured the theory, but  what confuses me is what these figures mean.  For example at pretty much full extension the rail marked f=6cm shows a figure of 4 which I assume to mean 2 stops.  But the same rail marked f=15cm shows just over 2 which I assume to mean 1 stop.

 Can anyone enlighten me on the meaning of the f=xcm and do the figures of 2, 2 1/2 and 4 related to filter factors -i.e. 2 means 1 stop and 4 means 2 stops?

My instinct would be that the scale for f=6cm would be relevant for a 60mm lens whilst the scale for f=15cm would be relevant for a 150mm lens. Since a 150mm lens is much longer than an 80mm I would have expected to have had to open up the longer lens more  than the shorter one. Any advice?

[howiesmith]

According to Ansel Adams "The Camera", the lens extension factor is:

([Bellows extension]**2)/[(Lens focal length)**2]

This is a factor so that a factor of two is a stop.  

For the special situation for "life sized images (1:1)," the bellows extension is twice the focal length and the lens to subject distance is equal to the lens to sensor distance.  The extension factor is 4, or 2 stops.

Note that a lens with twice the focal length of another but mounted on a bellows equally extended will result in the the shorter lens have four times the exposure factor.  (An 80 mm lens on an 80 mm bellows will have a 1 stop correction, and a 160 mm lens on the same bellows will have a factor of 0.25 stops.)

[pflower]

According to Ansel Adams "The Camera", the lens extension factor is:

([Bellows extension]**2)/[(Lens focal length)**2]

This is a factor so that a factor of two is a stop. 

For the special situation for "life sized images (1:1)," the bellows extension is twice the focal length and the lens to subject distance is equal to the lens to sensor distance.  The extension factor is 4, or 2 stops.

Note that a lens with twice the focal length of another but mounted on a bellows equally extended will result in the the shorter lens have four times the exposure factor.  (An 80 mm lens on an 80 mm bellows will have a 1 stop correction, and a 160 mm lens on the same bellows will have a factor of 0.25 stops.)
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Thanks. I had always avoided anything to do with maths, but having worked through your (or Adams') figures this makes sense.  Well, the fact that a longer lens has less of a factor than a shorter lens still seems counterintuitive to me. But there we go.

Philip