ah, gotcha, thans Jon. So then it's strictly thermal noise that the long-exp. NR function addresses? And it can be mitigated by not using Live View I'd guess, also, shooting in lower ambient temps...


(Just how many types of noise are there with digital sensors?)


So with a tripod, is there ever a reason to boost ISO beyond 100, short of long (5-min.) exposures? (or even then?)


Although come to think of it, even at ISO 1600, what was a 5-min. exposure at ISO 100 is still going to be pretty long...something like...let's see, ISO 100 to ISO 1600 is 4 stops, so 5 minutes plus 4-stops = 18.75 secs. (cross fingers once again) Is that correct?


So I guess this is when knowing how ISOs affect HL headroom comes into play. What would be the lesson here?