In photography, ISO is a measure of sensitivity to light. ISO--along with aperture and shutter speed--make up the "photographic triangle," the balance required to capture the correct exposure in each photograph. Choosing the right ISO is the first step in taking quality pictures.
Film Vs. Digital ISO
In traditional film photography, the film's ISO (previously known as ASA) indicated how sensitive the film was to light. Film ISO commonly ranges from 100 to 1600 and is printed on the film canister. The camera is set to match the ISO of the film loaded and remains the same for the entire roll of film. Changing ISO requires changing film and resetting the ISO on the camera.
In digital photography, the ISO is a measure of how sensitive the image sensor in the camera is to light. Digital ISO covers the full specturm of film (100-1600) and more, extending the range ever upward. Without the limitations of film, ISO can be set on a shot-by-shot basis, if desired.
Effect of ISO on Exposure
The lower the ISO, the less sensitive the camera or film is to light. Conversely, the higher the ISO, the more sensitive the camera or film is to light. Higher ISOs can be used to freeze action or to handle low-light settings, however, as ISO increases, the exposure is subjected to more "noise."
ISO and Noise
Noise, or graininess in film photography, is a loss of clarity appearing as speckles or color artifacts. As ISO increases, so does noise. Noise is more prevalent in compact digital cameras due to the small size of the image sensor, but even the very best high-end cameras are not immune from noise. Typically, the better the camera, the higher the ISO at which noise becomes an issue. As a rule of thumb, above 400 ISO noise is a factor and should be considered when selecting the best ISO for the shot.
An ISO of 100 is adequate for a broad range of photography, and, in fact, many photographers choose to leave their ISO constant at 100, adjusting aperture and shutter speed instead. Others will adjust their ISO based on the type of photography.
ISOs under 100 are considered slow speed, and when used with a tripod and a stationary subject, produce the finest-grained photography. ISOs between 100 and 200 are referred to as medium speed and can be used for all-purpose photography--scenics, pets, family or sporting events and just about any photo opportunity. ISOs 400 and up are fast speeds, best for freezing action or shooting in low-light situations.
Using Higher ISO
With limited light and fast-moving action combined, increasing the ISO is frequently a good choice. Other situations warranting higher ISO include concerts, plays and other "no-flash" events. With digital photography, consider "bracketing" critical shots--shooting the same picture at a range of ISO settings to ensure the optimum shot is captured. More importantly, take time to experiment with higher ISO before you need it to gain confidence in chosing the right one instantly.