Increasing ISO increases noise, four times the ISO - two times the noise. Detail is not really affected by underexposure, unless noise reduction is applied. On the other hand noise can cower over fine contrast.
The low end of the grayscale is normally dominated by readout noise. Cameras like Canon amplify the signal when increasing ISO (within limits) and can thus reduce noise. Cameras using sensors of more modern designs (most Nikons, all Sonys) gain little from increasing ISO.
If you work with JPEG, the camera's ASICs will apply noise reduction at higher ISO, so there will be a loss of detail.
If a sensor's dynamic range exceeds a subject's DR by 5 EV, then does a 4 EV increase in ISO cost much detail, add much noise? For example: a painting whose dynamic range is 7 stops (as lit in studio), imaged on a sensor whose dynamic range is 12+ stops and whose baseline ISO is 100. If one exposes to the right without clipping highlights, then will the fact that all the shadow detail is at least 5 stops to the right of the left edge of the histogram mean that one can shoot at ISOs higher than baseline ISO without compromising detail and increasing noise? If so, how much higher?
I have read (see quote below) that this is the case for a scanning back; if it is, then how much less true is it for a DSLR?
A few years ago, a superbly helpful representative at BetterLight wrote this:
"[This scanning back] has an 11+ stop dynamic range, and seldom does fine art repro exceed 6 or 7 stops, which means we theoretically have 4 or 5 stops of overhead to use for increasing the ISO without intruding on the detail, so let's see... ISO 200 to 400 to 800 to 1600 is 3 stops while still leaving a stop or two of "underhead" at the highlight end of the scale. We are not film and we are not instant capture, so 2 or 3 stops of gain dialed in by increasing the ISO should have no effect on shadow detail because there is no "black" or "white" in any painting that is the equivalent of Zone I, II, or III or Zone IX or X. Black paint is not the same as black velvet or the deep shadows found in grass under the shade of a bush under the shade of a tree under the shade of a cliff wall. Practical experience in the studio shows that our Super 6K HS with a Base Line ISO of 200 can be easily turned up to ISO 800, 1200, even 1600 without concern of noise in the deeper tones. ISO 2400 and 3200 (and a little beyond) are available for problem shots under special circumstances but noise becomes visible at these settings."
But in cases where one must use a single-capture DSLR, how much of this would fly out the window?