A not so well documented feature of Adobe Camera Raw is its use of a BaselineExposure offset. For details see page 32 of the DNG specification
. When one converts a raw file to a DNG, the BaselineExposure is written in a tag of the file and can be read with an EXIF reader. ACR uses the same BaselineExposure offset, and if you are not aware of this offset, it can confound your efforts to expose to the right, with exposure being judged in the ACR histogram.
For the Nikon D3, the BaselineExposure offset is +0.5 EV, which means a positive exposure compensation will be applied to NEFs from this camera. Because of this offset, the highlights may be blown in the ACR histogram when then are intact in the raw file. To investigate this offset, I took exposures of a Stouffer wedge such that step one was just short of clipping. This is what I consider the optimum ETTR. The raw histogram as shown by Rawnalize is shown below:
One can also look at the histogram of the Green1 channel by using the split_cfa command of Iris to split off the green1 channel for analysis by an editor. I used ImageJ (a freeware program available from the US National Institutes of Health). The histogram of the green channel is shown below. It is just below clipping
If one looks at the histogram of the raw file in ACR with default settings and the Adobe Standard camera profile, the highlights appear blown:
By using a negative exposure compensation of - 0.6 EV, one can eliminate the histogram.
I then rendered the NEF to an aRGB 16 bit TIFF using various ACR settings and split out the green channel in ImageJ and used the Plot Profile command to examine the rightmost 5 patches of the wedge. The image in the top shows that the two brightest patches are completely blown out; instead of five levels, one sees only three. The middle image shows the image rendered with an exposure of -0.5 EV to undo the +0.5 EV offset. One can now see all five levels, but the brightest patch is slightly blown. With an exposure value -0.6 EV, one has a bit of headroom.
The take home point is that if you want your ACR readouts and histograms to reflect the values in the raw file, you need to compensate for the baseline offset. Otherwise, your images will appear overexposed and you might try compensate for this by using less exposure in the camera and would not get the full benefit of ETTR.