D3 sensor efficiency

Started Nov 18, 2007 | Discussions thread
OP ejmartin Veteran Member • Posts: 6,274
Re: Some clarification, please

cpw wrote:

Hello Emil,

Very intereting work. As I'm sure you know, pixel-to-pixel
nonuniformities can also contribute to your standard deviations.
Since I don't know much about the degree of pixel-to-pixel
nonuniformity of these sensors, do you have a feel for the size of
these nonuniformities, and whether it is so small as to not be a
worry?

The only way to know for sure is to have two identical images so as to be able to subtract them, response nonuniformity then cancels in the difference image leaving only the read and shot noise. I have measured this effect on the Canon 1Dmk3 and 20D. There the response nonuniformity leads to fluctuations in the raw level that are roughly equal to photon shot noise, in the highlights at ISO 100.

Now, the Nikon D3 measurements I did were at ISO 1600, precisely in order to mitigate this effect (I then converted the answer to ISO 200 by multiplying by the ratio of the gain factors).

Let us assume that the response nonuniformity of the Nikon D3 sensor is at about the same level as on the Canon sensors. Then in going from ISO 100 to 1600, the response nonuniformity goes down by a factor of 16 since it is proportional to the gain, while the photon noise goes down by a factor of 4 since it is proportional to the sqrt of signal. So at ISO 1600, the response nonuniformity should be about 4 times smaller in the highlights (where I did the measurements) than the photon shot noise. When adding the noises in quadrature, the response nonuniformity will produce a roughly 3% correction, which is well within the uncertainty of the measurements I was doing. But you are right, my value might be a teensy bit low due to this effect.

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