Perhaps I'm being unreasonable in wanting to see only one version of the histogram for any particular phase of image development.
I have to conclude that the phenomenon whereby Photoshop can display two conflicting histograms for the same image, in the absence of any modifications, is damned misleading! Photoshop loads up an image, shows me a nice histogram, but as soon as I click the old "!" I see a horribly clipped histogram! But then I go to make an adjustment, and without doing anything except clicking on the adjustment layer I'm back to the un-clipped version or the histogram. Gimme a break!
I assume it's a performance related compromise. Very likely a fossil of the days when we had 16mHz processors. But to my simple mind it seems that an overly large and no longer needed compromise is being accepted here. Hey, I want to use that histogram to control clipping!
PS. Just noticed that "PhotoLine" does not share this two-faced phenomenon. Its histogram always fully tracks the current state of the image in exquisite detail and never speaks with a forked tongue. That's how it should be, right?
(And BTW, the gamma slider on PhotoLine's Curves adjustment panel is awesome!)
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Oh Duh! I just realized that I've been using a Quantum Image Editor. I can never know both the histogram and the appearance of my image at the same time! That's as it should be. Sorry for the fuss.