Originally posted by Kryscendo I don't know where I picked it up, but from what I was told by someone very knowledgeable, that digital camera sensors are made to operate at a certain level of amplification without much noise. It usually is the default lowest and that raising or lowering the ISO from there, either forced or through turning on extended ISOs, creates more noise. So if a camera has an ISO that starts at 100, but can be brought to 80 or 50 with extensions, you would suffer a noise penalty. Any salt to that?
Well, the "extended" ISOs are accomplished through over- or under-exposing, which means you will possibly clip data at one end of the spectrum. If your lowest sensor ISO is 200 and the camera as an extended ISO 100 setting, it gets there by overexposing ISO 200 by a stop and then taking it down a stop when processing the signal. It should not yield more noise, but less dynamic range.
As for the rest, assuming the same exposure (i.e. amount of light hitting the sensor) you are actually better off cranking the ISO (up to around 1600 or so depending on the body) as long as you can avoid blowing out the highlights.