Dynamic range and RAW file "bit depth"

Started 6 months ago | Questions thread
DSPographer
Senior MemberPosts: 2,236
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Re: having pondered this, why not have 17 bit raw file option?
In reply to RussellInCincinnati, 6 months ago

RussellInCincinnati wrote:

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.

Come to think of it, it wouldn't be skin off of anyone's back to just have a menu option for 17 bit-depth raw files. Totally linear recording of produced-electron counts. Yes you'd be encoding a ton of noise in the least significant bits, slowing down file write times since the numbers written would be less-losslessly-compressible, etc. But certain folks might want it, just like certain folks might want to encode their JPEGs in some other colorspace besides sRGB (an option my cameras have happily let me ignore for many a year now). It's not like any of us care much about raw file size any more. And the raw files would be even raw-er.

This isn't just wasteful of file bits, it also could significantly slow down the raw converter. In column parallel ramp converters like Sony's Exmor sensors use, having a nonlinear ramp dramatically speeds up the conversion process. So, on many cameras that have "14-bit" raw capability the conversion step size is not really uniform, instead it increases approximately as the square root of the level, so that the time to perform the ramp conversion is not excessive.

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