Feature: Digital Photo Basics--Exposure Control

In many ways, digital cameras aren't all that different from traditional film cameras. To capture an image they still need to expose a light-sensitive material to light; it just so happens that the material is a hunk of silicon, not a reel of thin plastic coated with a chemical soup. After reading a recent flurry of e-mail messages from readers who want to understand the essentials of topics like exposure and image resolution, I thought we should start the new year by covering the digital photography basics. This week: everything you ever wanted to know about exposure.

Balancing Speed and Size

It's probably no surprise that all digital cameras expose photos in more or less the same way: by opening the lens for a brief period of time to admit light that registers as an image on the camera's CCD or CMOS sensor. But there are two factors that you or your camera can control to perfect the exposure: the duration of the exposure (the shutter speed) and the size of the lens opening (the aperture).

Here's the complicated bit. The size of the lens opening at any given moment is called the f/stop. F/stops are represented with numbers following the symbol "f/," like f/2, f/4, and f/11. For mathematical reasons, the bigger the number, the smaller the opening, and each "whole" f/stop increment admits either twice or half the light of the nearest f/stop. So an aperture set to f/8, for instance, admits half the light of f/5.6. But f/8 admits twice as much light as f/11.

It stands to reason, then, that a long exposure with a large aperture (say, f/5.6) will admit more light than the same shutter speed with a small aperture (like f/11). When your camera is set to its automatic exposure mode, it figures out how much light the scene should get for optimum exposure, and then sets the shutter speed and aperture accordingly. Most digital cameras will try to pick the best combo--a shutter speed that's fast enough to minimize camera shake, for instance, with an aperture to match.

There are many aperture/shutter speed pairs that will give you the same overall exposure. If an aperture of f/5.6 and a shutter speed of 1/125 second is just right for a given scene, then making the aperture smaller (f/8) and shutter speed slower (1/60 second) will give you the same results; or conversely, a larger aperture of f/4 and faster shutter speed of 1/250 second would take the same picture.

Get Creative

Because f/stop and shutter speed settings can be changed independently, you can take control of your camera to get creative.

Suppose you're at a racetrack taking pictures of cars whizzing by, and you'd like to introduce some motion blur. If your camera wants to shoot a scene with f/4 and 1/250, you can change the settings to f/8 and 1/60. You'll be admitting the same amount of light, but you're exchanging aperture size for exposure duration. When you take the shot, the longer exposure will blur the cars.

Or what if you're taking a portrait? You'll probably want to blur the background by using a relatively open aperture. A very small aperture opening--like f/11 or higher--creates a sharp image with a deep depth of focus. That means much of the foreground and the background will be in focus. A wide-open aperture, like f/2 or f/4, gives you a shallow depth of focus. The subject will be sharp, but most of the background will be fuzzy.

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