D600 High ISO in DX

Started Nov 23, 2012 | Questions thread
Joofa Senior Member • Posts: 2,655
On random processes

bobn2 wrote:
The random process is occuring in the light not the pixel.

Actually, both. Even if we knew the exact number of photons impinging on a pixel, and consider only the QE among any other effects, the number of photoelectrons generated is random (see bullet #3 below). The mean value of photoelectrons can be estimated as the mean number of photons multiplied by the QE.

The key thing to remember is:

  • the number of photons being impinged is Poisson.
  • the number of photoelectrons generated is Poisson.
  • the number of photoelectrons generated for a given number of photons impinged upon a pixel is not Poisson, rather binomial.
  • the number of photons impinged upon a pixel given the number of photoelectrons generated is not exactly Poisson, rather a variant of it.

No, it gets catered for simply by multiplying the photon count by the QE.

The interesting thing to note regarding the above is that the estimate of the number of photons impinged obtained by dividing the photoelectron count by QE (something that is done many times due to lack of other information) is a relatively poor estimater, with its mean square error being 1 / QE of the mean square error of the best (L2) estimater.

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