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Does it, or does it just mean there is noise upon noise which makes it look like less noise but at the expense of detail?
Certainly in Snapsy's example it seems there is an apparent loss of detail in the lettering
I can't help thinking skillful use of PS layers and good noise reduction software would be both quicker and better
Regarding the tests I'd done, I saw less noise and more detail. More precisely, when I compared a 5-shot-averaged multi exposure at 20K ISO with a single one at the same ISO, I was able to see subtle details in the former that were more or less hidden by noise in the latter. With the benefit of such a clean photo.
I forgot to take shots at lower ISO to compare, but I wouldn't be surprised of a gain of 2 to 3 stops - the photo looked very clean. I remember doing similar with the 7D (taking a burst of the same shot and blending them at PP), and having a gain of about 1.5 stops (blending 5 shots at 6400 ISO was less noisy than a single shot at 3200 ISO, but more noisy than a single shot at 1600 ISO).
My 2% cpu !