Originally Posted by Sheiky
I'm not sure this is correct, Jan. Photon noise is a function of sensitivity and the amount of light hitting the camera. When you set your camera to ISO 2000, it uses a slower shutter speed and thus lets more light in than it would have if you had used ISO 3200.
The problem with fake ISO's isn't that you get too much noise, but rather, you lose range (see below)
Originally Posted by Sheiky
The camera also "overexposes" the picture by 2/3 of a stop as compared to an ISO 3200 shot. Thus you lose highlight headroom.