Choosing the Right "Film" Speed

Ah, the imponderables of modern life! Why can you be overwhelmed, occasionally underwhelmed, but never just plain whelmed? Why do people think their dogs won't understand when a stranger speaks in a foreign language? And what do those ISO controls actually do on your digital camera? Should you use them, or just keep ignoring them?

First Some Theory

Your camera's ISO setting is simply a measure of how sensitive the camera is to light--in other words, the film speed, even though there's no real film in the camera. If you have a 35mm camera, you probably know that film comes in a wide variety of speeds--from ISO 25 all the way up to about ISO 1600. The lower the number, the slower the film will respond to light, requiring a longer exposure or larger aperture to properly expose the picture. Higher ISO numbers are more sensitive to light and consequently need less time to expose a picture.

How much do you need to adjust the exposure time for different ISO settings? Doubling the ISO number changes the exposure required by one step. So, if ISO 100 film would expose a picture for 1/125 second, ISO 200 film could take the same picture in 1/250 second. It's almost as if the film is competing in a photographic version of Name That Tune. "I can take that picture in...1/60 second!"

The Digital Angle

That's all well and good, but how does it apply to digital cameras? After all, there's no film in the camera.

Actually, the CCD, which is the light-sensitive chip that plays the role of the eye for most digital cameras, can be configured to be more or less sensitive to light. By default, most digital cameras use a sensitivity that's roughly equivalent to an ISO of about 64 or 100. But many cameras let you throttle that up to 200, 400, or sometimes even higher.

There are a few situations in which you might want to change the ISO setting. With a 35mm camera, you'd have to actually change the film to one with a different ISO rating. But it's a lot easier on a digital camera: You can just twist a dial or push a button. You can easily take pictures with different ISO values on the same memory card. Try that on a film camera!

When to Set ISO

When set to automatic, your camera's ISO is typically configured for its lowest setting, probably in the neighborhood of ISO 64 or 100. That's fine for most photography, but what if the light conditions are low? Frame your shot and see what shutter speed your camera wants to use. If the shutter speed is really slow--like 1/15 second or more--you can crank the ISO up a notch or two to bring the shutter speed into a more hand-holdable, jitter-free range.

You can use the same trick to avoid using a flash. I try not to use the flash because I don't like the hot, artificial look of flash photography. You might also be trying to shoot a subject that's just too far away for the flash. Remember, a digital camera flash generally only lights up about a dozen feet or so. By increasing the ISO setting, I can often take pictures indoors without turning the flash unit on.

Finally, if you're trying to be creative, changing the ISO setting can again come to the rescue. Suppose you want to take a picture with a long depth of field, so that the entire picture, foreground and background, is in focus. For that, you'd need a tiny aperture setting, like f16. But--you guessed it--it's too dark and the camera wants to open the aperture way up. The solution? Increase the ISO so you can close the aperture.

ISO Pitfalls

These are some reasons, in a nutshell, when and why you'd want to increase the ISO level of your digital camera. But beware: When you make the CCD more sensitive to light, you increase the odds of getting digital "noise" in your picture. That's why you should always step the ISO back down to its normal level when shooting in ordinary lighting. An ISO of 400 is too much for most digital cameras, and your photos will suffer.

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