Digital Photography Basics: All About Exposure

Use your camera's LCD, histogram, and exposure compensation control to set the optimal exposure for your shots.

By Dave Johnson, PCWorld

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Learn Practical Rules, not 'Magic'

Thanks to modern digital cameras, even casual snapshots are sharper, better exposed, and more vividly colorful than photos of old. But cameras can't work magic. They aren't good at figuring out challenging lighting situations, so it's up to you to choose the right settings.

Don't expect Photoshop to transform your overexposed snapshots, either--photo-editing programs can tweak and nudge pixels in the right direction, but they can't save a train wreck. The bottom line: It pays to learn the basics of exposure, and to apply your knowledge instead of relying on your camera's Auto mode.

Choose the Right Program Mode

If you leave your camera in Program mode, it will handle the details--but you'll often get better results by exploring other options. Aperture priority mode lets you set the aperture (handy for controlling depth of field), while the camera sets the matching shutter speed. Shutter priority mode does the opposite: Pick a shutter speed, and the camera selects an aperture.

ISO controls make the camera more sensitive to light, so you can use a shorter shutter speed to freeze action in low light. And your camera's exposure-compensation control--identified by the letters EV (Exposure Value) or a +/- symbol--lets you over- or underexpose the photo in small increments.

Use Your LCD as a Laboratory

The display on the back of your camera is like a photography lab, just waiting for you to experiment (without any smelly chemicals). The LCD, after all, is not just for showing the photos you've just taken. It can preview the photo you're about to capture, and that means it can simulate the various exposure settings quite accurately. But there's one big caveat: If you have a digital SLR, its LCD might not give you a "live view" of the photo you're about to take. Most digital SLRs use the LCD as a playback screen only, though some models have a live-view mode. Check your camera's user guide if you're not sure.

Experiment With Shutter Speed

The easiest way to experiment with your camera's exposure controls is to set it down on a table, put the camera in Manual exposure mode, and watch the LCD to see how adjusting the shutter speed and aperture changes the brightness of the image. Start, for example, with a shutter speed of 1/10 second and an aperture of f/4. You might see something like the dim image on the left.

Now spin the shutter-speed dial. As you slow down the shutter speed (lengthening the time the shutter stays open), the image becomes brighter and overexposed, as you can see here on the right. Speed up the shutter speed again, and it gets darker and underexposed.

Tweak the Aperture

Now switch gears and change the aperture. As you make the f-number bigger, the image grows darker. That's because a big f-number corresponds to a smaller aperture opening. Make the f-number smaller, and the image brightens.

Why does the f-number matter? It affects the depth of field of your photo. A large f-number puts more of the photo in sharp focus, while a small f-number gives you a relatively narrow depth of sharp focus in your scene--as in this photo of a flower. That gives you a lot of creative control, but a narrow depth of field generally means that you have to use a slower shutter speed. Beware of the potential for blurry photos.

Add Light With the ISO Control

It's easy to find yourself in situations where you don't have enough light to take a sharp photo. To prevent blurry shots, you can increase the camera's ISO until the shutter speed is fast enough to freeze the action. In this nighttime shot of a football field, I used an ISO of 1600 to coax the shutter speed into being fast enough to get a sharp shot.

Just remember to reduce the ISO to its lowest value again later, because high-ISO photos tend to have a lot of digital noise. Read "Use ISO to Take Low-Light Photos" for more tips.

Use Your Camera's Histogram Display

You can't necessarily look at the LCD to know whether the scene is properly exposed. Although the image might look good there, the LCD itself might be set too bright or dark, which will affect how the scene appears on the screen. Many digital cameras allow you to turn on a histogram display while previewing your photos, and that's a better indication of whether the photo will be properly exposed. If you're unfamiliar with histograms, read "Use the Histogram to Avoid Exposure Issues" to review the basics.

Learn How to Interpret the Histogram

If you overexpose a scene with too much light from the aperture and/or shutter, you'll see something like the image on the left, in which the curve bunches up on the right side of the graph. If the scene is too dark, the curve moves to the left side instead. If you use the camera controls (shutter, aperture, and ISO) to position the curve so that it doesn't crawl off either extreme, you'll see something like the image on the right.

Use the Exposure Compensation Control

If you vary the exposure compensation (EV) level, you can force the camera to over- or underexpose the picture by a certain amount based on your own judgment. It's a great compromise, since the camera is still doing most of the work by measuring the overall exposure. You're just tweaking it.

Consider these pictures. I focused on the penguin in both shots. In the first image, the camera overexposed the tiger; I noticed this bad exposure as soon as I reviewed the photo in the LCD. Without changing anything else, I dialed down the exposure by setting the EV control to -1 and then shot again, which resulted in a much better photo.