How do you calculate the flash power fraction, assuming you want to keep constant the other variables of ISO, aperture, and distance from subject to flash?
The Guide Number formula for a flash (strobe) is GN = distance * f-stop
Let's say the Guide Number is 174 at ISO 200. You want to shoot at f/8. This gives a distance of d = 174/8 = 21.75 feet
Now, let's say you're shooting in a room that doesn't have that much room to play with.
But you do know you can move the flash 10 feet away.
What fraction of power do you use?
Is it linear, e.g. 21.75/10=2.175, so use 1/2? Or something else?
The numbers in this example are for the Nikon SB-800 and a Nikon D90 camera, but the principle is likely the same.