D3 sensor efficiency

Started Nov 18, 2007 | Discussions thread
Bill Janes Senior Member • Posts: 1,848
Re: Maximum black with reflective target

ejmartin wrote:

For the purpose of measuring the read noise, you don't need to go all
the way to black; in fact this is somewhat preferable for Nikons due
to the problem that Nikon clips the blacks in the raw data, thereby
skewing the std dev of the deepest shadows, whose ave raw level is
within a std dev or two of zero. Instead one can measure the noise
in deep shadows somewhat above black and extrapolate the result to
zero.

For my rough measurements with the D3, read noise was indeed measured
by extrapolation from nonzero values, taking care of the narrowing of
the st dev near black due to the fact that Nikon clips the blacks
rather than adding a bias. For details see the following:

Emil,

Thanks for the explanation, which I find very interesting. The illumination in the post you quoted was about 60 in 14 bit ADUs, in line with what I thought it might be. Since Nikon clips the blacks, is this background illumination akin to the bias that Canon adds to prevent this problem?

I see that you did obtain blackframes, and I presume they were similar to what you obtained by extrapolation. Which method is preferable? Certainly one does not need a regression such as Emil Martinec performed.

BTW, I see that he had some trouble with Iris reading the TIFFs from DCRaw. ImageJ, the freeware program from the National Institutes of Health can read and analyze 16 bit TIFFs (but not the compressed ones output by IRIS) and also FITs that Iris outputs. Roger Clark likes ImagesPlus. Which do you prefer for this purpose?

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Bill Janes

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