Quote Originally Posted by DWThomas View Post
Off the top of my head, I think what may be missing
here is a bellows correction factor. DaveT
The inverse square law applies to systems without optics.
Once a lens is introduced the speed of the lens must be taken
into account. Just as the extension of a camera lens will effect
the amount of exposure so will it with an enlarger. As with a
camera the more extended the lens the slower is the lens.
An enlarging lens extended for 1:1 ratio enlargements
has 1/4 the speed; is two full stops slower. Dan