Digital cameras are usually set by default to automatic ISO. The camera automatically sets the ISO according to light conditions: the brighter the light, the lower the ISO; the lower the light, the higher the ISO.
Many users prefer to manually adjust the camera's sensitivity to light. It gives them some control over the amount of noise that may appear in an image. In some cases, you can avoid using the flash by increasing the ISO.
ISO and Noise
On many consumer digital cameras, selecting an ISO number above 100 produces little visible noise. Test each ISO number under a variety of lighting conditions until you become familiar with the noise your camera produces at each setting.
If images are noisy, noise reduction programs do an effective job at eliminating some of it, though fine detail may be sacrificed.
Low ISO settings
High ISO settings
ISO 200 and above
More light needed
Less light needed
More image detail
Less image detail
Larger aperture +/or longer shutter speed
Smaller aperture +/or faster shutter speed
When deciding whether or not to use an high ISO number, keep in mind that the visual presence of noise is often barely noticeable when images are reduced in size for printing and/or viewing on a computer.
Sample Chart of ISO Setting :-
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. Most compact digital cameras produce high to very high image noise.
P/S : Changing ISO also changes the aperture and shutter speed.