Controlling Exposure: ISO settings
Put very simply: The higher the ISO number, the brighter the image will be.
This sounds like good news, cranking up the ISO when shooting in low-light situations can be a quick and easy way of combating under-exposure. And, the difference between ISO100 (generally the lowest) and ISO1600 (generally the highest) is immense. Basically, it doesn’t matter how dark it is - if your ISO is high enough, the camera will get something.
Note the following progression from low to high ISO numbers:
Like all good things, however, turning up the ISO has a catch. The higher the ISO number, the grainier the photo will be - this becomes especially apparent during post-processing (although it rarely shows up on a camera’s LCD screen).
The first of these next two pictures is a magnified portion of the same photograph as before, with an ISO1600 setting.
The second is of the same thing, but with ISO100 (and the brightness of this image coming from a much slower shutter speed).
So, here we see the downside of ISO in action. If you are shooting in low-light situations, then, and simply cannot achieve adequate exposure through other means, then cranking up the ISO is a useful last resort - but the higher it is, the more the quality of the image will suffer.