# D600 High ISO in DX

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

Eric Fossum wrote:

bobn2 wrote:

The random process is occuring in the ﻿light ﻿not the pixel.

Well, not exactly. The random process occurs in the emission of photons from a blackbody or other source (plasma). There is a mean value to the number of photons being emitted, and a variance. The variance happens to be equal to the number of photons, and the std. dev is the square root.

Then the light is reflected off objects and there should be more randomization because of that, since some photons are reflected and some absorbed. And then there is the whole lens transmission (and absorption/reflection) and color filter absorption (also somewhat random on a photon by photon basis) and finally, absorption in the silicon. Where the photon is absorbed is also "random" in position or depth. Lastly not all photoelectrons are collected -- some escape.

The amazing thing (to me at least) is that this does not change the statistics of the noise. The variance in photons emitted from the blackbody goes like the number of photons, and the variance in the number of collected photoelectrons goes like the number of photoelectrons. 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.

It is also important to understand what we mean by noise. Noise means that when we try to measure the average photon flux (or collected photoelectrons) we get a different value each time, and the std. deviation of that mean signal+noise is equal the square root of the number of photoelectrons. With one pixel, one measurement of course there is no noise.

If you think many pixels should all see the same average flux, then calculating a sort of spatial noise (FPN excepted) willl also yield the same result on a single exposure. So, in other words, shot noise can be manifested across many spatial pixels, or both space and time.

Estimating the noise is also tricky. There is "noise" in any noise measurement.

Hello Eric. While I would agree with everything you say here, I don't think that it disagrees with what I was saying. If one had a 100% efficient sensor, with zero read noise, you would still observe shot noise. I think the point is that noise is apparent only against an expectation of what you think (your words) should be happening. If you knew what should be happening, then noise would be no problem. So to talk about 'correct' values of a pixel is a nonsense. All you have is observed values, and you can make inferences about the 'correct' values based on your expectations, and also on you knowledge of the way the system operates.

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Bob

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