Originally Posted by Bruce Osgood
You're dealing with the Inverse Square Law. Moving a light source farther from the subject it diminishes proportionately to the distance.

I've worked out a simple Excel program that I simply plug in three variables and come up with New Elevated Exposure

New elevated exposure equals:

((New elevation / Old Elevation) ^2) X Original Exposure

Supposing your 8x10 elevation from lens to paper was 24 inches @ 16 seconds and your New elevation is 30
inches, then: ((24/30)^2) = 2.44
2.44 X Original Exposure (16 seconds) = 39.4 New Elevated Exposure.
I read this thread last night and decided to put this theory in practice.

While I don't agree with your example(!?), I found that the formula worked fine. Well, fine enough to be a pretty good starting point (I always test strip regardless).

I also found this one elsewhere on the net. Same formula in essence:

(ot/oh^2)*nh^2=nt, (old time divided by square of old height) times square of new height equals new time.

So, thanks to this thread, I am now saving a lot more time in the darkroom!!

Cheers