I don't think there is any BaselineExposure in ACR. ACR simply can blow information when the exposure slider is set to 0 because of the white balance, which it seems to apply in a destructive way, i.e. with >=1 multipliers. To compensate for this overexposure due to WB, you need to add some negative exposure correction, not being this a problem at all. And it will not always be -0.5EV; some images may need -1EV, some others may be well served with -0.3EV depending on the WB applied and the particular exposure achieved in all three channels in the RAW data.
Regards
Guillermo,
ACR definitely uses a baseline exposure. One can determine the BaselineExposure by converting the raw file to DNG and reading the exif data. The pertinent extract from the EXIF is shown below. The BaselineExposure is discussed in the DNG specification (
Link to PDF).
I have found that if I use the BaselineExposure offset and set the ACR tone curve to linear, I get a fairly good representation of the raw file. For example here are the 10 steps in the image I showed previously. The table shows the ACR linear value for sRGB, the 14 bit raw value, the normalized raw value, and the normalized raw value converted to gamma 2.2 (=normalized raw ^[1/2.2] * 255). There is good agreement.
[attachment=15313:Table.gif]
Processing file D:\DataFiles\Nikon\D3\LowISO\Gabor\Jun092008_LowIso_0005.dng
Byte order: Least Significant Byte First
33432 Copyright ASCII Public Domain
50708 UniqueCameraModel ASCII Nikon D3
50721 ColorMatrix1 SRATIONAL 9336/10000 -3405/10000 14/10000 -7320/10000
14779/10000
2763/10000 -914/10000 1171/10000 8248/10000
50722 ColorMatrix2 SRATIONAL 8139/10000 -2171/10000 -663/10000 -8747/10000
16541/10000
2295/10000 -1925/10000 2008/10000 8093/10000
50727 AnalogBalance RATIONAL 1000000/1000000 1000000/1000000 1000000/1000000
50728 AsShotNeutral RATIONAL 503937/1000000 1000000/1000000 677249/1000000
50730 BaselineExposure SRATIONAL -50/100
50731 BaselineNoise RATIONAL 60/100
50732 BaselineSharpness RATIONAL 100/100
50734 LinearResponseLimit RATIONAL 100/100
50736 LensInfo RATIONAL 280/10 1050/10 35/10 45/10
50739 ShadowScale RATIONAL 1/1
50781 RawDataUniqueID BYTE