I wonder what Photographers' favorite ISO is. It's probably ISO 100, the clearest ISO of all (I'm right, right?).
In dark areas, I would normally extend the shutter time, except if I'm doing handheld photography.
Always shoot with the lowest real ISO (i.e. not a pulled ISO like ISO 50) that provides you with the shutter speed and f/stop you need to achieve the shot you want.
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Can you please further explain why extended ISO is a bad thing?
"Extended ISO" comes in two flavours: pull or push.
A 'pulled' ISO is like ISO 50 on a Canon DSLR or ISO 100 on an older Nikon DSLR. The camera sets a one stop overexposure at ISO100, and then drops the exposure in software by one stop, making an effective ISO 50. While this allows you to gain a stop of shutter speed (from 1/500 to 1/1000, for instance), you also lose one stop of dynamic range (since you're now one stop closer to clipping the highlights). ISO 50 isn't actually any lower sensitivity than ISO 100.
A 'pushed' ISO is the exact opposite. The camera underexposes by one stop and then pushes the exposure up by one stop in camera. For example, my 5D Mark II has a maximum real ISO of 3200, but can push one or two stops (to 6400 and 12,500) by underexposing an ISO3200 shot by one or two stops, and then jacking up the exposure by that much in camera. This preserves your highlights, but increases the noise floor, thereby lowering dynamic range.
In both cases, you can achieve the same thing by over- or under-exposing your shot at the real ISO, and then altering exposure in Raw processing software like Lightroom. In fact, I would advise this route, because Lightroom (and the like) offer far greater control over the final image than does your camera's built-in software. So if my meter says that the scene is 1/25, f/1.4, ISO3200, and I need 1/100 to eliminate camera shake, I can either bump the ISO to the 'extended' ISO12,500 or take an underexposed ISO3200 shot and deal with it later in Lightroom. I always opt for the latter.