ISO 100 is sharp, detailed and free of noise.
ISO 200 shows no discernable change from ISO 100.
At ISO 400 image quality is still very good with no noticeable noise.
ISO 800 shows the merest hint of softening, but only at 100% and you really have to look hard for it.
By ISO 1600 some softening of the image has begun to occur, but overall quality is still very good.
By ISO 3200 this softening has become more pronounced, especially in shadow areas.
ISO 6400 is the cut-off point in terms of overall quality, with images much more noticeably affected by noise.
ISO 12,800 and overall image quality is starting to plummet.
The extended setting of ISO 25,600 is best avoided if possible.