ISO

ISO describes how sensitive the film or sensor is to light. The letters ISO come from the Greek word Isos which means equal or equivalent. It has been adopted as a loose acronym for the International Organization for Standardization.

On your digital camera, ISO settings refer to the 'film speed.' Even though your camera is most likely not film at all, but rather digital, the ISO setting still does has the same function as older film cameras.

The higher the ISO number, the more sensitive the sensor is to light...

ISO speeds increase or decrease in FULL STOP increments every time the ISO number doubles or halves.

Here's what that means:

ISO 100 is 1 stop slower than ISO 200, which is one stop slower than ISO 400.

ISO 400 is 1 stop faster than ISO 200, which is one stop faster than ISO 100.

Why change your ISO setting? You use a high ISO when shooting in low light conditions or conditions that require fast shutter speeds and / or small apertures. Why not use high ISO settings all the time? There is a trade off between ISO speed and image quality. Digital cameras tend to produce images that have more 'noise' (unwanted pixels) at higher ISO. click on the image to view a larger example