Bellows extension math assistance, please

[Ulophot]

I ended up creating a simple Excel calculator yesterday, for bellows extension exposure compensation with my forthcoming additional focal length. Before you reply, "But there are 39 gazillion phone apps already," please understand that I do not use a smartphone and like it that way. Thank you. If similar Excel sheets are already available, at least I have had the satisfaction of solving a problem myself.

The calculator is simple, because my Excel and math skills are elementary. One simply enters the focal length; calculations for extension and exposure factor are automatically calculated for a supplied range of distances (which, of course, can be changed for different lenses or preferred distances). It's fine for the work I do.

However, although I created a column for exposure factor, I don't know how to create a formula for a final column rendering that increase in f/ stops based on that factor, which would be formatted as a fraction (that, I can do.)

For example, with a 5.31-inch (135mm) lens entered in one cell, the following calculations are done, but I had to enter values in the last column manually.

Distance, feet Bellows extension Exposure factor Exposure increase in stops
0.84* 11.24 " 4.47 2 1/2
1 9.54 " 3.22 1 1/2
1.5 7.54 " 2.01 1

* Excel sheet has an inches column as well

Can someone provide me an Excel formula for the "Exposure increase in stops" column?

I don't know if anyone else would find this handy. I have written our site owner to ask if he would be interested in posting a link on the home page, once I finish it. Otherwise, since I don't have a website, I may post the file to a Dropbox link for anyone who happens to see it or would like to host it on his or hers.

[Bob Salomon]

Why make something that is very simple complicated?

1:1 open 2 stops.
2:1 open 4 stops
1:2 open 1 stop
1:4 open .5 stop

It’s that easy!

[Alan9940]

Bob's method is really simple! I like it!

I carry a small retractable tape measure with me and measure from the ground glass to the center point of the lens, then convert inches to the closest f-stop. For example, on 8x10 with a 12" lens extended to 18" I would convert the lens to f/11 and the 18" extension to f/16; doesn't have to be exact. This extension would result in 1 stop additional exposure needed (as Bob shows above.) If you feel the need to be more accurate, extrapolate the in-between numbers.

[Dan Fromm]

In pidgin Excel, = log10(exposure factor)/log10(2)

I never used a cheat sheet, just set up, used a tape measure to estimate extension, then calculated magification given extension and focal length, then calculated exposure increase in stops from magnification. Mental arithmetic, easy.

[Bob Salomon]

Bob's method is really simple! I like it!

I carry a small retractable tape measure with me and measure from the ground glass to the center point of the lens, then convert inches to the closest f-stop. For example, on 8x10 with a 12" lens extended to 18" I would convert the lens to f/11 and the 18" extension to f/16; doesn't have to be exact. This extension would result in 1 stop additional exposure needed (as Bob shows above.) If you feel the need to be more accurate, extrapolate the in-between numbers.

I use two rulers, one at the subject plane and one to measure one inch on the ruler on the gg. If they are both 1” open two stops. If the one on the gg is 2” open 4 stops. If the one on the gg is half an inch open 1 stop.
Why do anything more complicated?

[Ulophot]

Thanks to all for your replies. It's fine if no one else is interested. Since I find that I have enough to concentrate on with my portrait subjects without asking them to hold a ruler or other object for me, or to pull out a tape measure, I have a made scale on my camera bed, as some others do, that tells me how much to open if the standard is between one point and another. I am using the calculator to determine that for my second lens as well. (I don't foresee having any additional lenses for my work, and the first will get more use.) It's as quick reference, right at the camera where I am making the adjustments.

[Peter De Smidt]

Philip, I do that on my portrait 8x10. I used one of Calumet's targets and rules to set extension compensation in 1/3 stop increments.

[Bob Salomon]

Thanks to all for your replies. It's fine if no one else is interested. Since I find that I have enough to concentrate on with my portrait subjects without asking them to hold a ruler or other object for me, or to pull out a tape measure, I have a made scale on my camera bed, as some others do, that tells me how much to open if the standard is between one point and another. I am using the calculator to determine that for my second lens as well. (I don't foresee having any additional lenses for my work, and the first will get more use.) It's as quick reference, right at the camera where I am making the adjustments.

Back in the day we used a string tied to the camera that we stretched to the sitter’s nose.

[Tin Can]

That Day is still here.

But I use sewing measure tapes as they have both metric and inches and cheaper by the dozen.

Back in the day we used a string tied to the camera that we stretched to the sitter’s nose.

[Mark Sawyer]

Why make something that is very simple complicated?

Why not make it even simpler? An f/stop is a simple ratio, focal length divided by aperture. You're measuring anyways, just measure the focal length and aperture, divide and get your actual f/stop.