I know I've had problems with non-uniform illumination. Here's an example:
Originally Posted by Photo Engineer
That big inflection is my fault. I'm learning that it's difficult to photograph a back-lit step-wedge through a lens. A uniform light-source usually isn't uniform enough, and lenses introduce variation in brightness from centre (spelled correctly ) to corner, especially wide open. Here are results after supposedly fixing the problem:
My developer (D316) and XTOL are close, and not wildly different, so I think D316 is fine. The curves are much better, but that inflection is still there in both. Rather than blame Kodak, I'm going to assume this is entirely my fault and (1) add more diffusion to the light-source and (2) use a longer lens. I like Michael R's method of simply taking a bunch of shots at various f-stops of a known light-source.
@Michael R: Flare is another problem that your method avoids. I've recently started taking two shots of the step-wedge, the 2nd with the upper row of wedges blocked to eliminate their blast of light that adds flare to the bottom row. You are correct: That flare pushes up the toe-area. The only reason I'm still trying to get step-wedges to work is so that when experimenting with developer formulas, I can develop two-frame-long strips instead of a roll at a time. I'm interested in seeing your results, and TIA for doing those measurements.