ISO Basics
ISO is the sensitivity of the film or sensor in the camera. In film, this is decided by choosing a more or less sensitive film. With digital, this is a setting of how much the light signal is amplified from the sensor. Both systems use the same ratings for their sensitivity.
ISO, like shutter speed and Aperture, use a system of stops to measure the change in light sensitivity. The common ISO sensitivity ratings are
100, 200, 400, 800, 1600, 3200, 6400
With 6400 being the most sensitive, and 100 being the least sensitive to light.
As you move up this scale from 100 to 200, you have doubled the sensitivity and therefore double the brightness of the image. If you move from 6400 to 3200, you have cut the sensitivity in half and made the image half as bright.
By controlling the sensitivity of the light, you are able to influence the choices you make with your other camera settings. If the camera is more sensitive to light you can use faster shutter speeds as well as smaller f-stops. And if the camera is less sensitive to light you can use slower shutter speeds and wider f-stops in order to get the photograph you envision.
As a side effect, ISO also influences the amount of noise and grain in an image. The more sensitive the ISO, the more noise or grain you will get. Digital sensors are getting better and better at managing this noise, but it is still something to look out for when shooting at very high sensitivity.
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