ISO 100 is sharp and detailed.
ISO 200 shows little change from ISO 100.
By ISO 400 softening has begun to occur.
ISO 800 is the cut-off point in terms of overall image quality.
ISO 1600 is fairly soft and noisy.
By ISO 3200 detail in darker areas of the image has all but disappeared.
ISO 6400 is very much for emergencies only.
ISO 12,800 is best avoided altogether.