At ISO 100 images are clean and sharp and free of noise.
ISO 200 is virtually identical to ISO 100.
At ISO 400 and the A77 is still producing clean, noise-free images.
ISO 800 shows the merest hint of softening, but you really have to go looking for it.
By ISO 1600 there is some noise, and some softening, although it's still very usable.
At ISO 3200 noise is more pronounced.
ISO 6400 is the cut-off point in terms of overall image quality.
At ISO 12,800 noise is clearly visible even at reduced image sizes.
ISO 16,000 is really for emergency use only.