[rodi]

so basically, digital sensors have a base ISO, where image quality is optimum. moving away from this base ISO brings about noise and reduced dynamic range the further away you go from the base number. So, with today's sensors, being one or two stops away from the base ISO is not going to hurt you much.

I shot a D3 for several years, 200 was its base ISO and if I needed to shoot at 100, I would without hesitation.

as far as how it's done, it's pretty much a negative gain on the sensor. where increasing the ISO is "overcranking" the gain / sensitivity of the sensor.

make sense? it's not really a push/pull of the metadata via firmware/software, as it happens before the image and stays with the image whether you shot RAW or not (as opposed to a metadata function like white balance).