Film speed (ISO) - The light sensitivity of film or a digital sensor. ISO 100 produces the nicest image, but requires more light. ISO 1600 works in low light but results in a "noisy" image.
The larger the number the less light is needed to capture the image.
This sensitivity is measured by a formula developed by the International Standard of Organization and is abbreviated as ISO. Occasionally film speed is still designated by ASA, which is an older measurement standard from the American National Standards Institute.
Common Film Speeds
Low ISO numbers are referred to as "slow" and high ISO numbers are fast. This is because low ISO numbers require longer exposure times. Generally, low ISO numbers (100-200) are used in bright light, with stationary objects and/or a tripod.
ISO 400 or ISO 800 would be used in low-light situations, with a fast-moving object, or if the camera is hand-held. ISO 1600 or 3200 would be used if you had a combination of low-light, fast motion, or a hand-held camera. You should usually try to avoid using anything over ISO 400, if possible.
You can set ISO to "Auto" if you prefer to have less to think about. However, some situations may present themselves where you must change the ISO manually in order to get the shot you want.
For example, let's say you're on Tv mode and you've selected a shutter speed of 1/250, but even your largest aperture results in an image that is underexposed. What can you do? Increase the ISO.
Each ISO stop will give you one additional stop of speed to work with. ISO works in stops, just like shutter speed and aperture. If you adjust the ISO by one stop, you must also adjust the shutter speed or aperture by one stop to compensate.
So, if an ISO 100 setting needs an exposure of 1/30 second at f/8, you could switch to 1/60 second at f/8 with ISO 200 or to 1/125 second at f/8 with ISO 400.