Quote Originally Posted by RodKlukas View Post
To all,
Here is a simple way to calculate compensation requiring only an inch tape measure.
take the focal length of your lens in inches and the extension of the bellows in inches and using these numbers change the unit to Fstops.
What is the difference? that is the number of stops you must compensate.
example a 210mm is approximately an 8" lens and with a 16" extension you derive f8 and f16. What is the difference between F8 and F16? 2 stops!
You're done.
Remember though that the extension in this methode is the full lens to film distance.

I mention that, because when SLRs are involved it is normal practice to use only the extra extension (the length of an extension tube, or bellows put between lens and camera) in calculations.
So when using the above method to calculate the compensation you need for an X" focal length lens with an Y" length tube, you have to add those X" to the length of the tube Y before 'converting' them into f-numbers.