The most basic answer is that gamma only lets you control one thing, the midtone value, and changing this drags a lot of other values up or down. You can’t boost the shadow without boosting other tones you might not want to boost. Shadow/Highlight doesn’t have this limitation.
Now I’m going to write out my impressions of Shadow/Highlight here partly so I can see if my understanding of this feature needs to be corrected!
Shadow/Highlight lets you boost shadows or drop highlights independently, leaving the other ranges untouched. It’s as if you applied multiple Curves or Levels adjustment layers, one to boost shadows, another to cut highlights, and another to control midtone contrast, and carefully masked each layer to protect the other tonal ranges. That’s a vast oversimplification of what I think is going on. They key idea is that the complex processing applied through S/H does cannot be duplicated with a single Curves/Levels tweak, so you can’t really compare it to that.
The Midtone contrast, as I understand it, doesn’t move the midtone up or down (as gamma would); it’s like adding a couple of control points close to either side of the midtone of a curve to steepen the curve there.
Tonal Width and Radius are tricky to understand but I think they’re actually similar in principle. Radius is spatial, like the width of a blur on a mask on either side of a mask edge. Tonal Width is like a tonal radius, the range of the tones affected on either side of the control.
You can probably figure out by now why overusing S/H makes it look weird. If you boost shadows and cut highlights too much, and you have these mask edges going on, tones start to cross over and you get that solarization effect. So far I’ve had success by making only teeny tiny adjustments in S/H.
The quailty of the image seems to be critical. S/H probably works best with very clean (noise-free) full range images, like those from today’s scanners or good digital cameras. Otherwise, applying S/H just accentuates the defects in the image, like making Curves or Levels moves that are too large.
I like to use it because, while I could do most of what it it does, it would take me a lot longer with a bigger stack of layers and masks. Shadow/Highlight is pretty smart.
No controls have any effect when you yank them from end to end? Even Midtone Contrast?
The only thing I can think of–and I’m almost afraid to mention it because it sounds like you know what you are doing–is that the image layer isn’t selected, but another layer or a mask is selected?
