ISO 1600 vs correcting exposure in photoshop

[Jon Ruyle]

So then it's strictly thermal noise that the long-exp. NR function addresses?

Yes. Well, almost. I believe it just takes a dark exposure of the same length and subtracts. This is primarily to reduce thermal noise, but it also must mitigate read noise as well (though if one wants to reduce read noise in a low signal image, more standard practice is to take several very fast exposures, combine eg by averaging, then subtract)

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

If the subject isn't moving and the tripod is sufficiently steady, then I don't think so. Thermal noise is less than linear, so longer exposure means better snr.

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?

That's what I get []

I can't think of why it would
trade off any detail. (Maybe he's thinking of Nikon's long exp NR,
which does eradicate small details.)

I assume the canon long exposure nr just takes a dark exposure of the same length and subtracts. Does Nikon do something different?