I’ve done a similar exercise in terms of assessing noise in the DPP course see:-
But in this case it’s a demonstration of how boosting up the ISO is a valuable option. With modern cameras the range of ISO is staggering (particularly when, in days gone by, I thought pushing Ilford 400 was fast.) My camera’s ISO ranges from 50 to 25600 and is useable in my experience up to 6400. Compare this to my first digital 1.3 megapixel camera which had three ISO settings – 160, 320, 640. This increase in useable ISO is particularly relevant when shutter speed and/or aperture cannot be altered for creative or circumstantial reasons. You may for example wish to record a shot with a large depth of field and with a fast shutter speed. At conventional ISO speeds, say 100-400, this may not be feasible but by boosting ISO the desired shot may be available.
As an example these shots show again the bird feeder outside my window and the first three shoots are taken at ISO 100; F8 and 1/15. Because of the rapid movement of the birds at that aperture the movement will be blurred. Increase the ISO to 6400 however and there is a better chance of getting a useable shot as the shutter speed is now much higher at 1/800.
The second series of three is at ISO 6400
Of course the big compromise with increasing the ISO is the amount of noise usually in the shadow areas.
For comparison I show below the same area the edge of the UVPC widow frame and the shadowed hedge behind it, at 1:1:-
As can be seen at this level of viewing there is a significant increase in the noise. Of course whether this is an issue depends on the use the shot is put to and how big it will be viewed at. As an A4 print the ISO 6400 shots would probably still be acceptable (aesthetic considerations notwithstanding) but any larger and it may be an issue. There is the further point that noise can sometimes, with its graininess and edginess, be an artistic or compositional choice – for instance in concert shots. It should also be noted that I have not applied any noise reduction control in either of the above shots, but this would, under Lightroom as far as my experience goes, reduce the issue somewhat, but not entirely.