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.
Answer
Light falls off according to the inverse-square law. Basically it boils down to this equation:
I ~ 1/r2
Where I is the intensity and r is the radius (which is subject distance for us) and the ~ means 'approximately equal'. Anyways, a couple of good articles on the subject can be found at Cambridge in Colour and at Portrait Lighting.
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