Definitions for "ISO Speed"
The rating of a film's sensitivity to light. Though digital cameras don't use film, they have adopted the same rating system for describing the sensitivity of the camera's imaging sensor. Many cameras allow manual and or automatic of ISO selection. As a rule of thumb, the higher the ISO selected, the more degraded or grainy the picture.
ISO (International Standards Organisation) and is the international criteria for representing film sensitivity or speed. The higher the ISO number (eg. ISO 400) the greater the sensitivity.
A measure of a film's sensitivity to light. A 400 speed film is more sensitive to light than a 100 speed film. Consequently, the faster film is better suited for low-light photography. Unfortunately, faster films also create more noise and reproduce colors less accurately than slower films. Digital cameras either have a preset ISO sensitivity or they allow you to choose from a variety of film speed settings.