>-- and I think this is what Brian is driving at -- is whether >the SNR will be reduced more by taking three high-ISO shots >and merging them compared to taking one low-ISO shot, albeit >at a longer exposure.
I think it depends on what the Gain in “AutoGain” means.
You are taking it to mean ISO or A/D amplification where it could mean a setting in post capture processing.
For example I shot these test images at base ISO of 100 (the D7000 has no Lo ISO settings).
This is single capture.
This is a three shot multiple exposure with AutoGain enabled.
They are NEF's using LR3 for conversion, using the Auto button (there was an apparent difference in exposure between the two) and a 71 sharpening setting only. Then I handed off to PS for cropping to 100% and saving.
To me there is significant difference in the darker background.
This technique has been used in Astrophotography for some time to improve overall image quality. The only limitation I see with the in camera process is that you are limited to only 2 or 3 shots. In Astrophotography they might often use 10 to 30 exposures (or more).