Markus,
'Nsig' is the wanted signal. In the absence of any other noise contribution the equation reduces to Nsig/sqrt(Nsig) =sqrt(Nsig).
For photographic purposes, quantization noise, either in the original sub-frames or properly stacked multiple frames, is negligible compared to the various shot noise components - provided that the camera ADC bit depths are not absurdly low (say <8-10 bits)
Mark
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