Zoom 1:1 12-bit histogram of RAW file showing clipped data due to 256 black substraction.
Definitively I am going to write to David Coffin to find out why he (and it seems ACR too) always substracts an integer power of two (I have developed some RAW files in which the black point substracted was 128, but 256 is more usual).
My guess is that it's because using a power of two is quicker.
As long as you deal with powers of two, a shift left or shift right can divide by or multiply by powers of two easily, and it's often quicker than performing calculations.
If you shift right by N bits, then you've effectively lost the information in the N least significant bits. Then shift left by the same N bits, and you've zeroed out all of these.
It is possible that some architectures will optimize the zeroing of an entire 8-bit byte to an even quicker operation.
