What is ISO?
In traditional (film) photography, ISO, or ASA, indicated how sensitive a film was to light. It was measured in numbers, and the lower the number, the lower the sensitivity of the film, and the finer the grain in your shots.
In digital photography, ISO instead measures the sensitivity of the image sensor. The same principle applies as in film photography: the lower the number, the less sensitive your camera is to light, and the finer the grain.
Higher ISO settings are used in darker situations to allow for faster shutter speeds, an indoor sports event, say, to freeze the action in lower light. That said, a higher ISO also means noisier shots. 100 ISO is generally accepted as “normal”, giving you nice crisp shots with little noise or grain.
Most people leave their digital camera in auto mode, letting it select the appropriate ISO for the conditions, but most cameras also let you choose your own ISO manually.
When you override your camera and pick a specific ISO, you’ll notice the knock-on effect on the aperture and shutter speed needed for a well-exposed shot. For example, bump your ISO up from 100 to 400 and you’ll find you can shoot at faster shutter speeds with a smaller aperture.