The ISO test scene in full and at ISO 100
As this 100% crop shows ISO 100 is clean, sharp and free of noise with plenty of shadow detail.
ISO 200 shows no discernible difference from ISO 100.
ISO 400 there's the merest hint of softening in shadows, but nothing to really worry about.
By ISO 800 this softening in shadow areas continues, but again it's not really doing the image any harm.
At ISO 1600 noise has become visible, although it's only really noticeable at 100% and above.
ISO 3200 is the tipping point - where image noise and the softening effects of noise reduction begin to become noticeable. Overall quality at smaller image sizes is still very good though.
Image noise has become much more prominent at ISO 6400, although images remain usable at smaller sizes.
Moving towards the top of the sensitivity range, ISO 12,800 shows a marked decline in quality.
The top setting of ISO 25,600 is really only for use in emergencies.
Of course, there's only so much 100% crops shot under studio conditions can show, so here's a real-life scene shot at different ISOs.
ISO 100 (for comparison)