The whole topic of ISO in digital cameras is quite facinating the ISO rating is controlled through the image processing and it basically amplifies the charge caused by light hitting the photosites. In theory, when you expose a photosite to light, it should charge to the same level every time, as determined by the amount of light it receives. In practice, this does not happen; the pixel will not produce the same brightness every time, this produces one form of noise Read noise: Dark current noise is caused by a latent charge building up while the camera is turned on.. Current noise is increased as the sensor gets hotter.. Best when your camera is kept cool!! Canon also overcame some problems with the development of the CMOS sensor rather than the CCD in most other makes which had a better signal to noise ratio by in camera processing they only require 1% of the power and can manage respectable noise levels at even 3200 ISO As always for most of us it is a compromise always use as low as ISO as you can get away for the given subject matter to keep noise down in your images.. The other type of noise with ISO settings is caused by hot or stuck pixels. These occur when you use long exposures of more than 2 secs evident in images as patterns of red green or white dots

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt