B/W offset. Gain, this one shifts the entire signal. Tied to Saturate offset.
Saturate offset is black level control. Does not effect white. Tied to B/W offset.
Digital gain, B/W offset and Saturate offset seem to be digital level controls. Probably only subject to quantisation errors. They also clearly have defined points where they work well together (produce bell curve), and Digital Gain seems to only have 1 sweet spot (subject to contrast).
Saturate offset is not adjusting the black level
per se, it's only adjusting the offset*.
B/W offset isn't any form of gain, it's digitally
shifting the analog signal into the recorded bit depth.
When they produce a negative effect on histogram, simply they have not been optimised for the incoming analog signal, and the round off errors have become large.
Digital gain would seem to be the gain of the digital stage. Judging by the effect on the histogram, this would appear to be
gaining the digital signal. ie: 1 in, 1.5 out. Rather then using gain to boost the digital signal, it's preferable IMO to use B/W offset to map the bit depth of the recorded signal.
I can gain an extra hundredth or two of DR, by not adjusting the black level offset lower (as I was in my earlier tests), and leaving more room for ADTG gain reduction. Such a shame that CMOS gain cannot be reduced further, Canon is probably (over)saturating that gain also.
Does this mean Canon did not fully optimize their sensor for low noise?
I'd say they simply left a safety margin in their code to make sure the ADC is always saturated (that is, to make sure white is always recorded as white).
*The offset is so far down into the noise floor that it does not effect black in the recorded image. The offset can be adjusted higher then the default value, but there doesn't appear to be any gain to DR.
http://www.cool.id.au/astronomy/Neb_tut/Scientifically%20determining%20CCD%20Gain%20and%20Offset.pdf
