The AutoISO-feature, available on most modern DSLR cameras (digital system cameras), enables the camera to set its own ISO-values. The user selects a shutter speed as the “minimum speed”, telling the camera that it should attempt to keep the shutter-speed at this level or faster. Based on the amount of light in the picture, the size of the aperture (the f-stop) and the selected minimum shutter-speed, the camera carefully chooses a custom ISO-sensitivity. By “custom ISO-sensitivity” I mean a value that’s not limited to ISO200, ISO400, ISO800 or ISO1600 but a value that can be set anywhere on the scale (ISO240, ISO530, ISO670). The maximum ISO-sensitivity can also be defined to lower noise, meaning that the camera never goes past this ISO-value but instead lowers the shutter speed.
How does AutoISO help you then? Well, imagine you’re shooting indoors. The lighting conditions can change rapidly and you need to take well-exposed shots all the time. Keeping a firm ISO is not flexible enough whilst constantly changing the ISO-sensitivity is not time-efficient. The AutoISO-feature keeps you shooting at a fast shutter speed all the time by making much faster and more accurate changes to the ISO-sensitivity on-the-fly.
All-in-all the AutoISO-feature is a great tool and is well worth the time to try it out!