Even snel gezocht, hier bijvoorbeeld een uitleg; http://dslr-astrophotography.com/iso-dslr-astrophotography/ van Christian.
Hier een interessante discussie met snel dezelfde conclusie op DPReview: https://www.dpreview.com/forums/thread/3848840 of hier voor diegenen die DSS kennen, een stukje uit de manual: http://deepskystacker.free.fr/english/theory.htm waarin geschreven staat:
Which ISO speed?
The question is common enough and the answer is simple: it doesn’t matter…sort of.
In fact the ISO speed is just a setting of any DSLR, but since the CMOS or CCD chip is the same (you don’t change it when changing the ISO speed, do you?) then the results are really the same.
This is not because you are using a higher ISO speed that you get more photons, it’s just that the signal is more amplified (noise and all).The good news is that you don’t need to change the ISO speed to try to capture a fainter target. You just need to take longer exposures.
Of course it’s a little more complicated than that since there is a good ISO speed for each DSLR.
However it depends on the characteristics of each sensor chip (readout noise and electronic noise) and is not simple to compute.
Christian Buil has computed the values for a few Canon DSLRs”En dit laatste heeft Christian ook op zijn website staan, aan de hand van eigen onderzoek.