The Effects of Camera ISO on Your Image
To understand digital camera ISO and what it does, it first helps to understand how film works. In the days before digital photography when everything was shot on film, you needed to select the specific film stock you wanted to get your images looking perfect.
Film stocks come in different speeds, which refers to how much light is needed for exposure. They were measured in ASA. The lower the ASA, the less light you need to expose the image. But, like all things in photography, a lower ASA that does not need much light looks grainy while the higher ASA with a lot of light look sharp and clear.
The ISO setting in your camera is the equivalent of what film speed was in older cameras. Your ISO setting controls how sensitive your image sensor is to light.
ISO's numbers and their relationship to exposure is different from film speed ASA. With ISO, the number is set lower, and the more light is needed to expose the image. While a higher ISO needs less light to be exposed. Raising your ISO instead of your light sources produces the same grainy effect you get from film stock with low ASA.
Getting the Best Quality
Ideally, you want to be shooting with a low ISO to get the best image quality that is possible. Even if you're in a situation where you're in low light and your aperture is opened all the way, you would want to use your flash to help you out instead of raising the ISO. But, there are times where you can't use a flash because it doesn't look right or you're in a situation where they're banned, like in a concert.
You'll need to raise the ISO to get the exposure. This is actually one situation where digital has a definite advantage over film because with a film camera you would need to change your roll and if you didn't have something with a low ASA you weren't getting the picture. Now you can get the exposure you need with a quick adjustment in your camera's menu settings.
The grainy noise on a high ISO is particularly a problem on consumer digital cameras because their image sensors are smaller than what you would find on a professional camera. Professional cameras still suffer from grain at a high ISO, but it is much more tolerable. However, as a rule of thumb, you want your camera set to the lowest ISO possible to produce the highest quality results.