Below are two screenshots, the first is the full image with ACR 4.6 defaults with histogram to show I've correctly exposed this image. The second is a 200% zoomed in portion of the rock with the lightest highlights that I've attempted to add definition within the rock detail. You'll note the posterization isn't just in the highlights where it would be understandable for this to occur but also down into the upper mids.
Tim,
Another thing to consider, in addition to what Gabor and Guillermo have pointed out, is that ACR uses an exposure offset of -0.5 EV for your camera. See page 32 in the
DNG Specification on the Adobe web site. This means that you have to use an exposure correction of -0.5 EV in ACR to get an accurate histogram of your image. You can determine this offset by converting the image to the DNG format with the DNG converter and then looking at the exif file. Here is what I got
[attachment=12112:PentaxBaselineExp.gif]
When you use the ACR defaults, you will see quite a bit of highlight clipping in the rocks in the upper right area of your image If you look carefully at the histogram you will see the clipping on the right in the histogram. I held down the alt button when using the exposure slider to show clipping. In the white areas of the preview, all channels are clipped. In the yellow areas, green and red are clipped.
[attachment=12113:PentaxSCRCapB.png]
If you use an exposure of -0.5 EV, the situation is improved, but there still is clipping as shown in Gabor's illustration and in the ACR preview shown below. Here, yellow indicates clipping in the green and red channels with an intact blue channel, and white indicates clipping in all channels.
[attachment=12114:PentaxSCRCapC.png]
In looking at your file, an exposure correction of about -0.65 was needed to eliminate all clipping. Try this and repeat your experiment. ACR highlight recovery works reasonably will when at least one raw channel is not clipped. Usually the green clips first (as in your file), since most sensors are most sensitive to green. Since all clipping could be removed with an exposure compensation of -0.65, this means that the blue channel was not clipped in the raw file--ACR can't bring a clipped channel below 255 (in 8 bit notation) regardless of how much highlight recovery is used. This is consistent with Gabor's raw histogram, which showed no clipping in the blue channel. The BaselineOffset of -0.5 was only approximate.
[attachment=12115:PentaxSCRCapD.png]
Bill