At ISO 100 little noise is present and the image is perfectly sharp
At ISO 200 a small amount of noise is visible but the image remains crisp
At ISO 400 noise is more noticeable but the image is still perfectly usable
At ISO 800 noise is beginning to affect sharpness and detail
At ISO 1600 the image has become soft and noisy
At ISO 3200 the image is now soft when viewed at 100%
At the expanded setting of ISO 6400 it's still just about possible to produce usable images.
The expanded setting of 12,800 is strictly for use only in extreme emergencies