ISO setting 
A measure of how sensitive a camera's film--or CCD, for digital cameras--is to light. Film for 35mm cameras comes in a wide range of speeds, from ISO 25 up to ISO 1600. The speed or light-sensitivity of a digital camera's CCD is rated in ISO numbers as well. The lower the number, the slower the response to light. Higher ISO numbers indicate a higher sensitivity to light, so less time is needed to expose a picture. Most digital cameras let you control the ISO setting to allow the use of higher shutter speeds and/or smaller apertures, according to circumstances.