# D600 High ISO in DX

Started Nov 23, 2012 | Questions thread
Re: Roger's optimum pixel pitch of 5um

Leo360 wrote:

Joofa wrote:

Leo360 wrote:

Eric Fossum wrote:

These processes do not "add up" and make the noise bigger. Actually, the noise is smaller, but so is the signal, and SNR is smaller as well. Still equal to the square root of the number of photoelectrons in an rms sort of way.

This is surprising. Is there a simple explanation why these two noise processes in the same conversion chain do not add up ??

Because, both are not "in the chain". One is the 'output' noise (photoelectrons) and the other is the 'input' noise (photon).

Those electrons are triggered by the photons and ideally e- count should be proportional to the photon count. They are "in a chain", so to speak.

Let us consider the following thought experiment. We have a sensor with 1MP and a scene with a single solid color and constant uniform luminosity of 10,000 photons per pixel per exposure.

Let's consider two cases separately and then together:

(A) The ideal sensor with no read noise and 100% QE and the photon statistics is Poisson. In this case the readings from one pixel to another fluctuate with the variance of 100 photons solely due the photon shot-noise.

(B) Ideal deterministic photon arrival process of exactly 10000 into each pixel. But the pixels now suffer from the read-noise of some variance. Output will fluctuate even when the input is perfectly constant.

(C= A&B) Now we have Poisson photon arrivals AND read-noise in the pixels. And the read-noise and Poisson arrivals are independent of each other.

I still do not understand why the resulting variance of (C) is not larger than the variances of (A) or (B).

The issue that Prof. Fossum mentioned is not read noise related. It is all photon or photoelectrons related, i.e., either the photon shot noise or the photoelectron shot noise.

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