Re: Bellows factor


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Posted by Les on January 05, 2001 at 07:35:07:

In Reply to: Bellows factor posted by Morris on January 05, 2001 at 05:04:38:

I have a 4 year degree in photography with a heavy emphasis on the technical side, I've been a professional photographer for over 15 years, most of that time was with at least a 4x5 camera.

I still can't quote the formula for bellows factor.

The way I get around all the math is to buy the Calumet Exposure calculator. It uses a 2" square target (which is the same as a 35mm slide mount, should the target dissapear) and a plastic ruler with the exposure factor on one edge and a porportion scale on the other.

Plop the target in the scene, measure the target at the ground glass with the exposure scale and adjust the f-number. Ta Da! done!.

For the most part, unless I'm shooting something close to half of life size (1:2) either shooting a Polaroid and bracketing like I normally do, solves the problem of bellows factor, though technically bellows loss comes into play around 1:8.

For other people just getting started, here's a little theory behind bellows light loss:
The F-number is a calculation. The f-number is the focal length divided by the diameter of the hole. However this is only accurate when the camera is focused at infinity.

Once the ground glass gets far enough away from the lens, a marked f number will act like something smaller since less light is getting to the film.

The formula (I looked it up) for the Effective F-number is Effective fnumber= f-number X [bellows length (image distance) / Focal length.]

Example: Assume you have a 127mm lens and the length of the bellows is 254mm. 254/127=2 so the bellows factor is 2 and f 16 is now f32.


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