ISO is the number indicating an digital camera sensors or your films sensitivity to light. The higher the sensitivity, the less light is needed to make an exposure.

All digital cameras automatically select the ISO, but most cameras let you change it manually. Auto ISO generally works best for bright scenes, though you take control when you manually set sensitivity.

ISO settings are expressed in numbers. Shooting at a lower number requires more light than shooting at a higher number but it also produces the most noise-free images. Using high a ISO number can produce visible noise in digital photos. The amount and degree of noise varies from camera to camera.

When a scene is bright, use a low ISO setting. When taking pictures in low light, select a higher ISO number when you can't use a tripod or a flash.

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 - 64 - for taking photos in bright light; excellent for close-ups, landscape, and portraits. Produces fine detail and image quality.

ISO 100 - for extra sensitivity without degrading image quality.

ISO 200 - cloudy and overcast days. Acceptable image quality, with some visible noise.

ISO 400 and above - set to a high ISO when unable to use the camera flash. Useful for indoor photography whether you use a flash or not. Also good for fast "stop-action" and sports photographs. Likelihood of high very noise with most consumer digital cameras.

Changing ISO also changes the aperture and shutter speed settings, so take note of them if you are trying to prevent camera shake or create special effects.