Don't rely on Auto
Thanks to modern digital cameras, even casual snapshots are sharper, better exposed, and more vividly colorful than photos of old. But digital 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 just relying on your camera's Auto mode.
Choose the right program mode
If you leave your camera in Program mode, it will pick the settings—but you'll often get better results by exploring other options. Aperture priority mode (marked with an A) lets you set the aperture (handy for controlling depth of field), while the camera sets the matching shutter speed. Shutter priority mode (marked with an S) does the opposite: Pick a shutter speed, and the camera selects an aperture. And your camera's exposure-compensation control—identified by the letters EV (Exposure Value) or a +/- symbol—lets you over- or under-expose the photo in small increments.
Use your LCD as a laboratory
The LCD display on the back of your camera isn't just for looking at the photos you've already taken. It can show a live preview of the photo you're about to capture, and that means it can simulate the various exposure settings quite accurately. But there's one caveat: Some digital SLRs' LCD screens might not give you a "live view" of the photo you're about to take. Most older digital SLRs use the LCD as a playback screen only, though many newer 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 the camera down on a table, put it in Manual exposure mode, and watch the live view on the LCD screen to see how adjusting the shutter speed and aperture changes the brightness of the image.
For example, start 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 on the right. Speed up the shutter again, and the image looks darker and underexposed.
Tweak the aperture
Now switch gears and do the same thing with the aperture setting. 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.
How else 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. For a more detailed explanation of how aperture settings impact your photos, check out this article on depth of field.
Add light with the ISO control
You may find yourself in a situation where you don't have enough light to capture a sharp photo. Try changing your ISO settings—high ISO controls make the camera more sensitive to light at the expense of more noise. To prevent blurry shots, increase the camera's ISO until the shutter speed can be set to a fast enough speed 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.
Use your camera's histogram display
You can't just look at the LCD to know whether the scene is properly exposed. Although an image might look good on the screen, 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.
Learn how to read the histogram
If you overexpose a scene with too much light from the open aperture and/or slow shutter speed, 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 adjust your camera's exposure controls (shutter, aperture, and ISO) until the curve is positioned 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.