Quote Originally Posted by Steve Smith View Post
Not relevant as we were referring to moving lights, not the focus (well, I was anyway!) and inverse square rule in general. But yes, on an enlarger the re-focussing will affect it a bit. but only because it will also change the coverage so the inverse square rule will still apply.

You can think of re-focusing as thing that changes the effective aperture. As the lens gets racked out the effective aperture changes. Otherwise you could easily calculate the exposure changes based on conservation of energy and the spread of the light. The change in aperture screws up the simple equation.