You often want to capture the best images you can with the light you have available. Most of the time a tripod isn’t practical, and even when it is, a long exposure can be undesirable. When there isn’t much light, setting a high ISO allows for fast shutter speeds in low light, but with increased image noise and less detail in the highlights and shadows. Remember the golden rule for eliminating camera shake:

Shutter speed must equal double lens focal length (mm) as a fraction of a second
This means that with a 50mm lens, you’ll need a shutter speed of 1/100sec or faster. Image stabilising lenses help things a lot, but just remember this rule and you’ll be fine.

Why not simply use a high ISO all the time?

If a fast shutter speed eliminates camera shake, and a high ISO allows for a fast shutter speed, why not simply always shoot at a high ISO? Well, there are two main reasons, the most visible of which is loss of detail. When working out how much perceived detail is lost at high ISOs, I find it helpful to think of my usable image size halving each stop from 3200 upwards, at least when shooting jpegs. A bit more detail can be recovered from RAW files. read more