
Exposure Compensation Based on Focal Length and Subject Distance
You can easily generate the table you want with a calculator with the natural logarithmic function and the following information.
Let the number of additional fstops of exposure required for bellows extension be x, f = focal length of the lens, and p = diaphragmtosubject distance.
Then
x = 2*ln[p/(p – f)]/ln2
Example 1: f = 180mm, p = 3 meters = 3000mm.
Then
x = 2*ln[3000/(3000180)]/ln2 = 0.18 stops.
Example 2: f = 180mm, p = 360mm
Then
x = 2*ln[360/(360 – 180)]/ln2 = 2 stops.
You can easily generate a table for each focal length you use. I find it more convenient to simply program the calculator with the generic formula and enter f, p, and execute. The result is the exact additional exposure needed in f stops. The calculator is easily carried in a camera bag or coat pocket.