Quote Originally Posted by Lee L View Post
Mr. Koehrer is correct. Lens extension tubes increase the lens to film distance, and so light intensity per square unit of area decreases. However, diopter lenses shorten the lens to subject distance, and do not change lens extension from the film plane, so light intensity per square unit area on the film isn't changed.

If you want to stick with the enlarger analogy, projecting the same size negative with a 50mm or a 100mm enlarging lens, both to the same size final print, will still give you the same light intensity on the paper with the same f-stop.

How is magnification achieved without spreading out the light? The fact that 50mm and 100mm enlarger lenses at the same print size will give the same times just confirms my point that it is the enlargement size of the print and not the distance that matters.

I had the same idea from John Shaw that diopters don't lose light. But Bjorn Rorslett corrected me on another forum and I have since done many tests to confirm his claim. See this thread for that exchange.