This is probably the dumbest question but what is it? On my camera i can change it between 100 - 3200 but i dont really see the difference when i change it?
On a 5D (not sure about other Canon) the multiples of 160 are "native". 160 actually is cleaner than 100. Some claim that 640 is actually the cleanest ISO.
In addition, you don't want to go above 640 unless you have to, you don't want to go above 1250 under ANY circumstances.
