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SO is the measure of film speed for a digital camera sensor, used to express how sensitive the imaging surface is to light. This measurement is called “ISO” because its method is set forth by the International Standards Organization.

Effect of ISO on exposure

ISO has a direct effect on exposure. Think of the camera sensor as “absorbing” more or less light at one time as the ISO is set higher or lower. A low ISO setting means less light is picked up, so you will need to use a longer exposure time and/or a larger aperture to get more light in order to capture the image. Conversely, if you choose a higher ISO setting, you need less exposure time and/or a narrower aperture.

Units of Measure

Without getting too technical, a digital camera’s ISO setting is expressed as a simple number. The higher the number, the more sensitive the setting, the faster the film speed. By doubling the ISO rating you double the sensitivity, and by halving the ISO rating you cut the sensitivity down by half.

For example, ISO 200 is twice as sensitive to light as ISO 100. By changing from ISO 100 to 200, you become able to use a shutter speed that is twice as fast as what was needed at ISO 100 to get the same exposure.

Let’s say you’re outside and the scene requires the camera to expose at ISO 100, f/16 and 1/100 sec. You see a moving subject and want a little faster shutter speed. If you just increase your shutter speed and nothing else, the image would be too dark. But if you double the ISO to 200 you can now shoot at ISO 200, f/16 and 1/200 sec. Because your ISO setting of 200 is now twice as sensitive, your exposure time can be twice as fast.

Digital camera noise

There is a trade-off of image quality when shooting at high ISO settings. On film, photographers saw grain appear. In digital photography, we see noise. The general consensus is that noise reduces image quality and is not desirable, therefore photographers in general tend to choose the lowest ISO possible while still getting a good exposure.

The ISO race

The current trend in digital camera design is to produce sensors capable of imaging at high ISO sensitivity. With a camera that is able to sense a great deal of light at once, photographers can use faster shutter speeds which makes it possible to shoot sharp hand-held photos without a tripod, and even photos of moving subjects, in low-light environments.

As of Spring 2010 we are seeing cameras like theNikon D3s and Canon EOS 1D Mark IV, both of which are capable of shooting at speeds up to ISO 102,400. These kinds of cameras get increasingly acceptable image quality at incredibly high ISO settings, opening the doors to shooting environments in which it was previously incredibly difficult to get good images.