Sorry auto-focus -- as mentioned above, quite correctly, ISO 100 is ISO 100. A full-frame sensor, a cropped sensor, a tiny P&S sensor -- all are calibrated so that ISO 100 means the same thing. That's the point of ISO -- it's defined independently of the film or sensor that you are using.
You may be thinking of the idea that full-frame sensors do better at high ISOs. This is true -- ISO defines sensitivity to light, but on cameras with smaller sensors, high ISOs look grainier. On full-frame sensors, the high ISO photos look better. But my D40 at 1600 and someone else's D3 at 1600 both have the same sensitivity to light, even if mine will look terrible while the D3 will look great.
As some have mentioned, not all cameras are perfectly calibrated, which does mean that my ISO 100 might actually be truly ISO 150. That could account for a little variation, but even a full stop is pretty extreme to be miscalibrated.
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