The ISO test shot in full (shot outside on an overcast day).
The baseline sensitivity of ISO 80 is sharp and free of noise.
At ISO 100 the image continues to be noise free, retaining good shadow detail.
By ISO 200 the first signs of softening are visible - but only at 100% or larger.
By ISO 400 the softening effect of noise-reduction has become visble.
By ISO 800 noise has become visible even when the image is viewed at less than 100%.
ISO 1600 is both noisy and lacking in detail.
ISO 3200 is best avoided at all costs.