ISO is similar to film speed on film cameras. Unlike aperture and shutter speed, ISO doesn't control how much light enters the camera, but instead controls how sensitive the camera is to that light. The lower the ISO, the less sensitive the camera is. In other words, a lower ISO will require more light to properly expose a picture than a higher ISO.
Most entry level digital SLR's will have an ISO range from 100 to 1600. More expensive digital SLR's may have ISO 50, 3200, and a select few have 6400. On most cameras, ISO is adjusted in full stops. Only the more expensive cameras will have 1/3 stop adjustments of ISO. In most cases you will be able to adjust your ISO to 100, 200, 400, 800, and 1600. For example, ISO 100 will need twice as much light to expose the same picture as ISO 200.
So then, why not just max out the ISO and never worry about light ever again? Unfortunately, ISO comes at a price. Depending on your camera's ISO performance, it can come at a pretty hefty price too. As you increase ISO you are effectively decreasing the quality of your picture. While it's very hard to see a difference between ISO 100 and 200, ISO 800 or higher is extremely noticeable. On most cameras, taking a picture at ISO 1600 is completely worthless.
This is caused by the increased sensitivity of higher ISO settings. While increased sensitivity means you can expose a picture with less light, it also means your picture will have more noise (distortion). While a little noise can be acceptable -- especially if you'll be reducing the image size significantly for the web -- too much noise can completely trash a picture.
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