Dynamic range and RAW file "bit depth"

Started Jan 25, 2014 | Questions thread
DSPographer Senior Member • Posts: 2,464
Re: bit depth affects PRECISION, not dynamic range so much

John Sheehy wrote:

xpatUSA wrote:

olliess wrote:

It would make more sense to have wider spacing values as the values increase, e.g., something close to logarithmic spacing. (I believe this is what the old Compressed NEF format did with a lookup table).

Correct, see Bill Claff's article here:

I did the math a while back, and I found that a camera that has a RAW saturation of 65K electrons at base ISO needs no more than 300 levels for the top stop, 213 levels for the next stop down, 150 levels for the next stop down, etc, until you are down to where the read noise is statistically dominant and at least 1.3 ADU of your output levels, where you become linear down to the lowest original RAW values.

You really need to avoid histogram shifts in the mapping, though, as they can result in color shifts after white balance.

The linear section becomes unnecessary if you set the correct black level offset. If the linear data is transformed by a square root function (ie a tone curve of gamma=2.0) with the black level before the transformation set to the square of the read noise standard deviation, then the sum of the read and shot noise will yield a constant noise variance in counts after the square root. So if the PRNU is calibrated-out, then the total pixel noise will be constant in counts after the 2.0 tone curve is applied. I modeled this in Matlab in the following post in a thread where I discussed this mapping in more detail:


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