The ISO setting in your camera allows you to choose the level of sensitivity of the camera sensor to light. The ability to change this sensitivity is one of the biggest advantages to using a digital camera. In the days of film cameras, you had to choose the ISO by film type. This meant that if you wanted to shoot in lower light, you had to replace the film in the camera with one that had a higher ISO. So not only did you have to carry different types of film, but you also had to remove one roll from the camera to replace it with another, even if you hadn’t used up the current roll. Now all you have to do is adjust the ISO dial on the top of the camera to select the appropriate ISO.
Having this flexibility is a powerful option, but just as with the Quality setting, the ISO setting has a direct bearing on the quality of the final image. The higher the ISO, the more digital noise the image will contain. Since the goal is to produce high-quality photographs, it is important to get control over all the camera controls and bend them to your will. If you have the ISO set to Auto, this means the camera determines how much light is available and chooses what it believes is the correct ISO setting. Since you want to use the lowest ISO possible, you should always manually select the appropriate ISO.
Which ISO you choose depends on your level of available or ambient light. For sunny days or very bright scenes, use a low ISO such as 80, 100, or 200. As the level of light is reduced, raise the ISO level. Cloudy days or indoor scenes might require you to use ISO 400. Low-light scenes, such as when you are shooting at night, will mean you need to bump that ISO to 1600 or 3200. The thing to remember is to shoot with the lowest setting possible for maximum quality.