The way Photoshop displays its histograms makes the results a little confusing, in that the vertical scale shifts to accommodate the highest peaks. Particularly when there's a large spike at one or the other end of the tone scale (as above in the shadows), the vertical scales can change a fair bit between shots. (The graph above right shows higher values than the one on the left, apparently because the High Tone Priority exposure pulled some deep-shadow pixels up out of the murk on the extreme left edge of the graph.

Despite the vertical distortion of the two curves, you can see HTP at work in both the highlight and shadow areas. (The latter was a surprise to us.) At the right-hand edge of the plot (corresponding to highlights), neither image shows any evidence of lost highlight detail, but there's clearly more headroom between the large spike corresponding to the white shirt and the right-hand edge of the plot, corresponding to the saturation point.

Surprisingly, the shadow end shows a bit more detail as well, as there's now some separation between absolute black (the left-hand edge of the plot) and the spike corresponding to the deep shadow areas of the image. (The spike is probably from Marti's dark blue pants, strongly shadowed with this lighting.)

Looking at Marti's shirt in these shots, it's clear that the HTP version holds a lot more subtle detail.

Even though the histograms didn't show any outright clipping in either image, there's definitely a lot more detail in the highlights with HTP enabled.

While HTP clearly seemed to be doing something in the shadows, the effect is barely noticeable without fiddling with the tone curve in Photoshop to see what's down there.

Here, we've jacked the gamma up to 3.0, to open up the shadows. (Marti moved slightly, so there isn't an exactly corresponding point in the two images. Lighting under the leaves of the bouquet can be trusted to be very equivalent though.)

There's definitely more to see in the shadows of the HTP version, but there's also more image noise. How much of that is the work of HTP itself, and how much is simply from the doubling of the ISO between the two shots isn't clear at this point. (I'll reshoot this test once we get a production sample of the Mark III, and at that time will do both sets of shots at ISO 200, to keep it more apples-to-apples.)

When we go to a greater overexposure, we can see more of HTP's effect. Viewed on-screen, both shots look pretty blown out, the one on the right only less so. While it doesn't look it though, there's actually a lot of detail preserved by HTP here.

These adjusted thumbnails clearly show that full detail is preserved in the white shirt. (Checking in Photoshop, it turns out the only things blown out in the image are parts of the pink and yellow flowers, both in the red channel.)

There's still some detail lost in the pink flower in the HTP version, but the difference between it and the normal-mode exposure is pretty dramatic.

Unadjusted shadows are still pretty hard to see, although there's a bit more detail visible in the HTP version.

Pushing the midtone slider down to a gamma value of 3.0 does indeed show more detail in both shots, but more in the HTP version than in the normally-exposed one.