I still don't get your "BaselineExposure offset" argument, since you are essentially saying that you are applying negative EC to ACR's default -- this strikes me as being a semantical disagreement. Also (as I already said), there are other RAW converters, and the use of negative EC is close to universal to all of them for RAW conversions of ETTR files (again, assuming a lower contrast scene). Of course for high key photographs you will probably skip negative EC in RAW conversion.
I would venture that most scenes are not low contrast. Also, negative exposure is also not need for images where the dynamic range of the scene is equal to that of the camera. Thus, negative EC is hardly close to universal.
BaselineExposure is defined in the
Adobe DNG Specification on page 32, which is quoted below:
BaselineExposure
Camera models vary in the trade-off they make between highlight headroom and shadow noise. Some leave a significant amount of highlight headroom during a normal exposure. This allows significant negative exposure compensation to be applied during raw conversion, but also means normal exposures will contain more shadow noise. Other models leave less headroom during normal exposures. This allows for less negative exposure compensation, but results in lower shadow noise for normal exposures.
Because of these differences, a raw converter needs to vary the zero point of its exposure compensation control from model to model. BaselineExposure specifies by how much (in EV units) to move the zero point. Positive values result in brighter default results, while negative values result in darker default results.When you take a picture, you probably want mid gray (L* = 50) in the scene to be mid gray in the image. In a 2.2 gamma space, L* of 50 has a pixel value of 118. If you take a picture of a gray card (or any other uniformly reflecting surface) you would want the raw file to have a saturation of 18% (mid gray), which would represent a pixel value of 2949 in the 14 bit raw file or 739 in a 12 bit file. When you take such a picture according to the light meter reading, the resulting pixel value will reflect the calibration of the system, which includes the meter calibration and the tone curve. Nikon (and the ISO standard) specify that light meters are calibrated for 12% reflectance (see
Thom Hogan). This allows 0.5 EV for highlight headroom. However, for ETTR exposure, one does not want much if any headroom.
To test my own D3, I took 3 shots of an 18% gray card and looked at the raw pixel values with Rawnalize.
[attachment=19043:Rawnalize.png]
The 12 bit green pixel value was 497; the saturation is 497/4095 = 12.1%, corresponding to a pixel value of 99.8 in a gamma 2.2 space, as shown by Rawnalize. The system is at spec for exposure. The sRGB pixel value of the file rendered with Capture NX and the Standard Picture Control was 149. Now that is a hot tone curve and mid gray is lighter than it should be. This is why many photographers complain that the camera "overexposes". With ACR and the Camera Standard calibration, the pixel value was 149. Using an ACR exposure of -0.5 EV gave a pixel value of 126, which is close to the desired value of 118.
[attachment=19045:ACR_scrCap.png]
Using the Adobe Standard calibration and a linear tone curve and compensating for the baseline offset with -0.5 EV, the rendered value in Adobe RGB was 99, just where it should be with a gamma 2.2 space. With a full ETTR exposure with the raw value just short of clipping, the rendered value in sRGB would also be just short of clipping with the exposure offset of -0.5 EV, but the highlights would be clipped in the rendered file without the compensation. If you didn't use the offset, the highlights would falsely appear to be blown and you would think that you overexposed and might cut back exposure in the future. If you judge exposure with ACR (or any other raw converter, including the camera JPEG) you have to know the BaselineExposure offset. Many inexperienced photographers naively assume that the raw converter histogram at default setting represents the raw histogram.
According to tests done by
Bill Claff, the D300 behaves in the same way as the D3. If you want 18% saturation for the metered value, you have to use an ISO of 138 rather than the nominal 200. In other words, you have to increase the exposure by 0.5 EV just as Thom described. If you use the real ISO for your Nikon camera (recent digital models), you wouldn't need the -0.5 EV compensation. Of course, I am applying a negative exposure value to the ACR default to bring the rendered values down to where they should be. The raw values were correct, so this is not correction.
You may consider the difference to be one of semantics, but a failure to understand a positive BaselineExposure may result in underexposure. Other cameras may have a negative BaselineExposure. For example, the ISO of the D70 is near the nominal value; however, I don't know what value Adobe uses for that camera. The important thing is to know the behavior of ones camera and raw converter and not be hung up on semantics.