ISO is the number indicating a digital camera sensors sensitivity to light. The higher the sensitivity, the less light is needed to make an exposure. Digital cameras automatically select the ISO but most have a setting to change it manually. Auto ISO generally works best for bright scenes.
Shooting at a lower ISO number requires more light than shooting at a higher number. Lower numbers result in images with the least visible noise, which is desirable. The higher the number, the more noise. The amount and degree of noise varies from camera to camera.
Digital single reflex cameras (dSLR), because they have larger sensors, are best for producing noise-free images. However, some consumer digital cameras now have improved sensors that produce acceptable images at higher ISO numbers.
ISO settings
AUTO ISO - digital camera automatically sets the ISO speed according the the brightness of the scene, increasing or decreasing the sensitivity. User has no control over which ISO number is used.
ISO 50 - 80 - for taking photos in bright light; excellent for close-ups, landscape, and portraits. Produces fine detail and image quality.
ISO 100 - for extra sensitivity with little, if any, reduced image quality.
ISO 200 - cloudy and overcast days. Acceptable image quality, with some visible noise.
ISO 400 and above - suitable for indoor photography whether or not a flash is used. Useful for "stop-action" and sports photographs. High to very high noise.
HTH
