To understand ISO speed, you first need to understand the basics of how a camera works. At its simplest level, a camera is a box with a hole in it. When light passes through the hole, it shines onto a light-sensitive medium inside the box, either traditional film or a digital sensor, which absorbs the light and captures the image.
ISO speed controls the sensitivity of the film or sensor that absorbs light. The higher the ISO speed, the more sensitive the film or sensor is to light.
The word ISO is not exclusive to photography. It’s the shorthand name (oddly not an acronym) for the International Organization for Standardization [source: ISO]. This standards body was the first to combine competing ratings for film speed into a uniform system.
With the ISO system, the lowest speed, and thus the least sensitive film, is 100. From there, each setting or stop doubles the sensitivity and number: 200, 400, 800, 1600, 3200, etc. . The highest ISO speed on high-end digital cameras is 102,400 [source: Nikon Australia].
When you use a traditional camera, you buy film labeled with a certain ISO speed and set the ISO speed on your camera to match. One disadvantage of film cameras is that you must shoot the entire roll at the same speed. On a digital camera, you can change the ISO speed with each shot based on available light and the subject of the photo. Which brings us to the role of ISO speed when taking pictures.
ISO speed is one of three settings that control the amount of light each exposure captures. The shutter speed controls how long the hole is open. The aperture controls the size of the hole.
And the ISO speed, as we mentioned, controls the light sensitivity of the film or sensor. By adjusting this setting to different light conditions, experienced photographers can get the clearest, sharpest shots.
For outdoor shots in daylight, low sensitivity ISO 100 is all you need because there is so much light. ISO becomes more important in low light conditions. If you don’t want to use a flash, one solution is to slow down the shutter speed to let in more light.
The danger is that your subject (or your hand) will move while the shutter is open, causing a blurry image. But if you increase the ISO speed, the increased sensitivity allows you to maintain a fast shutter speed.
One drawback of higher ISO numbers is an increase in noise or graininess. This is because higher ISO film is made with larger photosensitive grains , the individual particles that absorb light. Grain is not a problem with digital cameras, although image sharpness can be affected at higher ISO speeds.
A nice advantage of high-end digital SLR cameras is that they have built-in software that reduces noise even at ISO 1600 and higher [source: Canon Australia]. Sometimes grain is a desirable artistic effect, such as in low-light black-and-white photography.