David Pogue's Digital Photography: The Missing Manual

Goose the ISO

Finally, inside every modern camera is an anti-blur technique of last resort: Boost the ISO.

That awkward abbreviation stands for International Standards Organization. In the film-camera days, every roll of film had an ISO (or ASA) number like 100, 200, or 400. This number referred to the film's light sensitivity. If your film was more sensitive to light, your shutter speed could be faster.

If you were going to shoot some beach or skiing shots, where the scene would be flooded with glorious light, you'd buy ISO 100 film. For overcast days, you might go with 200. If you wanted to shoot indoors without the flash, you might load your camera with 400 or even 800 film.

Why didn't everyone want the most light sensitivity all the time? It had three downsides:

• Higher ISO film (known as faster film) cost more.

• There was such a thing as film that was too fast. In bright light, you might not even have a fast enough shutter speed to keep your shots from blowing out with brightness.

• Above all, higher-ISO film also increased graininess in the photos. And that, in general, was bad.

Note

Every year, cameras get better. Used to be, nothing over ISO 400 was usable; today, the best SLRs don't start showing bad noise until ISO 1600 or so. But you get the point: In general, ISO-boosting is not a free lunch.

Today, now that you've gone digital, ISO is still with you. Now it refers to the sensitivity of the sensor, not the film, but the basic idea is the same.

Fortunately, you don't have to worry ...