BaselineExposure for Canon 7D

Question

The Canon 7D does not support fractional ISO steps by hardware gain; they are achieved by numerical adjustment of the pixel values by the firmware.
ACR and the DNG converter "honour" the adjustment by BaselineExposure; however, the actual value, -0.41 EV is incorrect, it should be -0.32 EV.
Not a big deal, but why make it wrong, when it costs nothing to make it right?
Gabor

Answer 1

Erik,
instead of measuring it, it can be calculated easily. For example ISO 320 is from ISO 400. The clipping levels with ISO 400 are around 15300, and with ISO 320 this is around 12645 (these are the same for the other combinations, like 160/200, 640/800). The black level is always close to 2048.
(15300 - 2048) / (12645 - 2048) = 1.2505...
This is the proportion of the "shrinking" the camera performs on the raw data. 1/1.25 = 0.8, and in fact, the ISO 400 pixel values are multiplied by 0.8. 1.25 is a bit less than the cuberoot of 2. 1.25^3.1 is ~ 2 and 1/3.1 = 0.3225.
See the "combing histogram" in http://www.cryptobola.com/PhotoBola/Canon7D_ISO.htm
Added: the clipping levels may differ from copy to copy, but not relevantly. I found these values in shots of about five copies, among others with firmware 1.0.9.
Gabor