D600 High ISO in DX

Started Nov 23, 2012 | Questions thread
Leo360 Senior Member • Posts: 1,141
Re: Roger's optimum pixel pitch of 5um

Eric Fossum wrote:

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.

This behavior is typical for Poisson processes. And many very different physical phenomena lead to Poisson as long as underlying events are independent and memoryless (radioactive decay, back-body radiation, telephone call arrivals, Geiger counter counts, etc.)

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 ??

And one more question as in the subject line. Roger Clark from clarkvision thinks that 5um is the optimal pixel pitch for a CMOS FX sensors. There has been an interesting discussion in this thread about it. In you opinion is it a reasonable question to ask?? Naturally,  photon absorption length and wavelength should put natural limits on the minimal pixel size.  But, of course, there are many more other factors to consider.


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