This is part of a four-part series on exposure.
Once your aperture has allowed your chosen amount of light into the lens, and the shutter speed has controlled the length of time the light has passed through the lens, the ISO controls how much of that light is absorbed.
The ISO is the camera’s measurement of light sensitivity. In analog photography, a higher ISO film absorbs the light faster, and film with a lower ISO number absorbs light more slowly. In digital photography it works in the same way, just with an electronic sensor in place of the film. A low number equals slower absorption.
Some newer dSLRs have a higher range than this, but the principle is still the same. These stops correlate with the shutter speed and aperture stops. One ISO stop up or down equals one f/stop up or down, or one shutter speed stop.
Okay, that’s interesting, right? But how is it useful?