DxOMark Tonal Range Specification

Started Dec 19, 2012 | Discussions thread
DSPographer
Senior MemberPosts: 2,236
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Re: DxOMark Tonal Range Specification
In reply to cpw, Dec 21, 2012

cpw wrote:

Yes hi Dspographer,

Since Dxomark's TR is = counting noise steps, we have to include the effects of PRNU noise (it's not strictly noise, but it acts that way) as well as shot and read noise.

Yes, for a reasonable definition of tonal response we should include the effect of PRNU: as DXO does.

But, in my formula I was most concerned with an encoding to capture all the valid raw data- similar to the intent of a compressed NEF. Since there is the possibility that a user might calibrate their cameras' PRNU (as astronomers sometimes do) I didn't want my encoding to coarsen the step size using the pixel to pixel sensitivity difference.

An interesting observation of my formula is that if you set the mean black level count value to the square of the read noise standard eviation in e-, and use the signal in e- count to add to this black level, then the encoding formula is simply to use a step size proportional to the square root of this e- count total. A square root encoding- for a step size of one after encoding, is just the formula for a gamma=2.0 tone curve.

So if we call the square of the e- read noise stdev the "black signal", we just need a gamma=2 tone curve for the total signal. We could create a compressed raw standard for gamma=2 and then the raw converter just needs to know where the black point is (the black signal) when performing the conversion. This raw standard would provide some counts for levels below black (instead of truncating them like Nikon tends to) which is appropriate to prevent loss of data for the case where there is a very dark region of pixels that have an exposure that is nearly zero.

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