OK, I've "thunk" a little harder on this. I was in error when I said that the sum of the two curves would lead to a curve with regions that would block up mid tones. After drawing a few curves and adding them together on paper, the curve would actually have regions, in what would have been the straight line portion, that have a steeper slope instead of a flatter slope. The curve would still be more complex than a simple sigmoid curve and would have up to seven regions of different slope characteristics. I'm still wondering whether anyone has measured the d/log(e) with enough resolution in the exposure to detect these variations from the ideal. If I had a densitometer I would do it myself, but...(posted 8874 days ago)