Originally Posted by Terry Bowyer
I'm somewhat new at this too, but the easiest method I've heard described is that every 25% increase in your bellows extension from infinity focus = 50% increase in required exposure.
For example, you have an 8 inch lens (210mm), then your bellows extension for infinity focus is 8 inches. If you're focusing closer, you extend your bellows. So if you extend your bellows to 10 inches, you've increased your bellows extension by 25%. Increase your exposure by 1/2 stop.
This is born out by the more complicated formula (for me to do in my head) of bellows extension squared divided by focal length squared = exposure comensation.
Other methods are given in earlier posts to this thread or at: