Bellows factor

[Joanna Carter]

Joanna got the first part (the multiplicative factor) correct. However, the conversion to stops is incorrect...

8.8x is 3.1 stops
19.75x is 4.3 stops

Ooops!! I started counted on the wrong finger

[Eric Woodbury]

Here's yet another 2-bit option on how to convert. 405mm is about 16", so call that f/16 and 4 feet is 48 inches, so call that f/45. From f/16 to f/45 is 3 stops. 6 feet is 72 inches which is closest to f/64 (and some change), so a little more than 4 stops difference. You get the idea. It's not very metric, but you can do it with a piece of string.

[aduncanson]

... Much simpler than squaring one thing, then squaring another, then dividing, then translating the factor.

Well I suggested dividing and then squaring, it gets you the same thing and saves a step.

But I just created a table and pasted it into my shooting notebook along with tables for filter factors and reciprocity corrections for my usual film.

Use a tape measure to measure in inches the bellows extension, and find that extension down the first column of the table, then go across to find the f-stop compensation required under the focal length of the lens I'm using in top row of the table.

I have to admit that my table would not go to a 48 or 72 inch extension.

[Steve M Hostetter]

the lens will be a Kodak 405mm 4.5 which is the aperture I'd like to shoot at

film is iso 400

[Joanna Carter]

the lens will be a Kodak 405mm 4.5 which is the aperture I'd like to shoot at

film is iso 400

Neither the aperture nor the film speed have any effect on the bellows factor, it is purely based on the relative length of the bellows to the focal length.

When people use f/stops in the calculation, it is purely a simple mnemonic to help you do the calculation without too much complex maths.

[Kevin Crisp]

Google "salzgeber disc bellows," which takes you to the quick disc .pdf. Print it out and use it, problem solved.

[Steve M Hostetter]

I understand Joanna,,, I thought I might include that info I got a reading at f32.7 so I'll shoot at f11 for iso 400 which is a 4 stop difference..

[Shen45]

ExpoDev.

[Steve M Hostetter]

steve, that a program?

[Steve M Hostetter]

Google "salzgeber disc bellows," which takes you to the quick disc .pdf. Print it out and use it, problem solved.

thx Kevin everyone