ACR Overexposure with Nikon D3

Question

Exposure to the right (ETTR) is a standard technique that requires placing of the image highlights just short of clipping in the raw file. Such exposure is often judged by the histogram or blinking highlights in the camera preview of the image, but is best determined by examining the raw file, since the camera preview is derived from a JPEG image to which the camera settings (white balance, tone curve, etc) have been applied. When one brings the raw file into Camera Raw, the white balance setting is applied, but the other camera settings are ignored. If the ACR histogram shows clipping of the highlights and a negative exposure adjustment is needed to recover them, this usually indicates that the image was overexposed. However, with the D3, the default ACR tone curve places the highlights too high and clipping occurs in the rendered file even when the highlights in the raw file are intact.
I exposed a Stouffer step wedge so that step 1 was just short of clipping with the Nikon D3 using 14 bit NEF and the results are shown in an Imatest plot using the built in DCRaw converter. The results are linear as expected.
http://bjanes.smugmug.com/photos/271579079_BGUhM-O.png
Next, I split off the the green1 channel of the raw file with Iris (a freeware astronomical program)and examined the results in ImageJ (a freeware program from the National Institutes of Health). This plot shows that step 1 is just short of clipping:
http://bjanes.smugmug.com/photos/271460382_RqUdu-O.gif
and this is further confirmed by the histogram:
http://bjanes.smugmug.com/photos/271460379_qwjtR-O.gif
However, when the file is brought into ACR, with the default tone curve, the highlights are clipped as shown:
http://bjanes.smugmug.com/photos/271460363_jMYHL-O.png
and -0.6 EV of exposure compensation is required to bring Step 1 in range:
http://bjanes.smugmug.com/photos/271460357_vDqYZ-O.png
Nikon Capture NX renders the highlights of this file correctly.:
http://bjanes.smugmug.com/photos/271569001_ZJwqX-O.png
The conclusion is that the default ACR tone curve for the D3 is too "hot" and indicates overexposure when none exists.
Next, here is the camera histogram of the properly exposed image. The histogram shows some clipping which is not really present in the raw file and which is eliminated by exposing 0.3 EV less. The conclusion is that the camera histogram is slightly conservative.
http://bjanes.smugmug.com/photos/271460374_mRjof-O.png
http://bjanes.smugmug.com/photos/271460388_WDMpW-O.png
The blinking highlight display is even more conservative and indicates overexposure by 0.6 EV:
http://bjanes.smugmug.com/photos/271460368_iFDsP-O.png
For optimum results with ETTR, it is best to perform your own tests to determine the accuracy of the in camera histogram and blinking highlights so that these aids may be used for optimum exposure. Since the camera tone curve affects the results, it is important to use the same tone curve each time. Furthermore, one should be aware that the ACR default tone curve is too hot and that one does not necessarily have to reduce exposure when a negative exposure compensation is needed to bring the highlights into range. This ACR overexposure has been noted by others, but I do not know if it occurs with all D3s. If so, then the default tone curve should be revised by Adobe.
http://www.nikonians.org/forums/dcboard.php?az=show_topic&forum=147&topic_id=53935&mesg_id =53935&page=2

Answer 1

Lawrence,
> How do you open a DNG that would be different than .nef?
The difference between the native raw and the DNG (in this case created by Adobe's DNG converter) is not in the raw data itself but in the metadata; BaselineExposure is a creature of the converter (and if the converter interprets the raw data this way, then ACR too is doing it this way).
I mentioned the DNG format, because *in the DNG file* (as metadata) the adjustment can be seen *explicitely*, in contrast to ACR's processing, which is lying about it: the "Exposure" slider is at 0, despite having adjusted the intensity. (Note, that ACR does not show the adjustment with DNG either.)
> Are you suggesting that converting a suspicious file having highlight clipping might be saved by converting to DNG?
There is no need to convert the file. The data is not lost - it is the same, as if you process a well-exposed image and adjust "Exposure" by (in this case) 1/2 EV: some pixels may become clipped. You can counter this several ways (beside by resetting the exposure adjustment), for example by Recovery (even though there is nothing to recover here, for nothing was lost in the first place), Contrast, etc.
The issue is not, that ACR woud lose image data, but
1. the auto-adjustment is not justified in this case (it is nonsense, that a Nikon D300's exposure should be adjusted to some imaginary "standard", even if that causes loss of part of the image),
2. it can be destructive if the user is not versed enough,
3. it is not indicated explicitely.