Light metering with bellows - How to?

IANUS · Joined: 14 Oct 2008 Posts: 276 Location: Germany

I've got a bellows unit for a Praktina camera and want to test it. Unfortunately I don't have a description for it and so I am questioning how I have to do the light metering. I never used a bellows for makro shots, only extension rings where I had to use factors for longer exposure. Obviously there is no ttl metering, which would be much easier.

Can anyone give me a description how to use the light meter with this set. I do hav a Gossen Profisix meter where I can dial in the factors for longer exposures but I have to somehow calculate them for myself.

Another question would be using a (manual) flash. How do I set up the flash.

Excalibur · Expire: 2014-04-21

As a film user this would be the time I wished for a DSLR, and I would think with that camera you would have to bracket your shots for exposure.

IANUS · Joined: 14 Oct 2008 Posts: 276 Location: Germany

alex · Joined: 18 Apr 2009 Posts: 561 Location: UK

If you can use a scientific calculator, a very rough measure of the exposure compensation can be calculated this way :

Measure the extension of the bellows from the camera lens mount to where the bellows meet the lens.

Calculate log10 ( (extension + lens focal length) / lens focal length )

Divide result by 0.3. [This is to convert a log10 calculation to a log2 calculation in case your calculator doesn't have a log2 key.]

That's the exposure compensation in stops.

Example : bellows extension 30mm, lens focal length 50mm

log10 ( 80 / 50 ) = 0.204

0.204 / 0.3 = 0.68 or 2/3rds of a stop.

There are other factors that come in to play, such as the focus distance, because this calculation assumed the lens is focussed at infinity, but it's a starting point. You could probably draw up a little table for different bellows extension lengths and attached lens focal lengths.

IANUS · Joined: 14 Oct 2008 Posts: 276 Location: Germany

thanks alex, I will try this.

Farside · Expire: 2013-12-27

Rough rule of thumb based on the inverse square law: if the lens is twice the distance from the film than it is normally, you get a quarter of the light intensity on the film, so you'd have to open up two stops or two shutter speeds.
Extrapolate from there, using your bellows distance as it actually is, when you're set up on the subject, compared to normal lens to film plane distance.
This gives you a starting point, but the relationship might not be utterly reliable as you go out further - trial and error to a certain extent thereafter.

Here's a useful device http://www.salzgeber.at/disc/

RioRico · Posted: Sat Dec 11, 2010 5:10 pm Post subject:

Another way to calculate this, starting with magnification:

MAG = (TE-FL) / FL where TE is total extension and FL is focal length

So with a 50mm camera lens on 100mm of extension, TE= 150mm. Magnification is (150-50)/50 or 2X. Now we can calculate f-stops:

EA = NA * (MAG+1) where EA is effective aperture and NA is nominal aperture

So with the previous setup and the lens set to f/11 (the nominal aperture), the effective aperture is 11 * (2+1) or f/33. You need to adjust exposure by 3 f-stops. More simply, exposure adjustment is (MAG+1) f-stops.