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Your digital camera's automatic or program mode is a wondrous thing. It's really amazing how it can take good pictures in a diverse array of lighting conditions.

There are times, though, when you and your camera have a difference of opinion regarding what constitutes a good picture. At times like those, it's a good idea to experiment with some of your camera's other exposure modes. In particular, most digital cameras have a pair of modes that can help you get a little more creative: aperture priority and shutter priority. This week, we'll talk about the aperture priority mode, and next week we'll take a look at ways to use shutter priority.

Why Use Aperture Priority?

Okay, let's get this out of the way along with similar questions, like "Why do I need to eat my vegetables?" (because they're good for you), "Why do I have to clean my room?" (because I said so), and "Why mess with the camera's ISO?" (so you can take pictures in dark conditions without a flash).

Aperture priority lets you select the size of the camera's aperture, the opening that controls how much light reaches the sensor at the moment of exposure. A large opening (counterintuitively represented by a small f/number), makes a picture with a narrow depth of field; a small opening (a large f/number) increases the depth of field, so more of the picture is in sharp focus.

I've written in length about this in "Master Your Camera's Depth of Field." I've also written about using your camera's ISO setting; read "Use ISO to Take Low-Light Photos" for tips.

Dial A for Aperture

Switch to your camera's aperture priority mode by selecting the "A" or "Av" symbol--typically on a dial on top of the camera, though you might need to enter your camera's on-screen menu system. Once selected, this mode lets you choose the aperture at which to take pictures. The camera automatically selects the proper shutter speed in response--so it's a pretty safe way to take pictures, because the camera is still doing half the work.

Try It Out

The best way to get used to using aperture priority is to experiment with it on a lazy afternoon when you don't need to worry about taking great pictures. If you can convince some friends or family members to pose for you, position them a few feet in front of each other and take a series of photos, each one at a different f/number.

You might get something like this pair of photos: In the first image, both subjects are clearly in focus; in the second, only one is in focus.

Note that where you focus has a bearing on your final image. For example, focus on the foreground subject when using a small f/number, and the background subject will probably be out of focus.

Here's another tip: the farther away that you focus, the deeper the depth of field will be. So focusing on the subject in back will yield more depth of field than if you focus on the closest subject, even at the same aperture setting.

Distance Matters

Knowing the relationship between aperture size and distance is handy for photography trivia games, and it can sometimes help you take a better photo of people sitting on a hill, but it's critical for success when you take close-ups, or macro photography. (DOFMaster has a useful online calculator.)

The closer your subject is to the camera, the more narrow your depth of field becomes. If you're shooting something that's only a few inches away--and virtually all digital cameras let you get so close to a dog that you can count the individual hairs on its nose--then the depth of field can be less than an inch. This is the perfect time to switch to aperture priority and dial in the deepest depth of field you can muster.

In this image, for example, the globe is less than an inch behind the astronaut--yet at f/2, the two can't both be in focus at the same time. But by shooting with aperture priority, I was able to set the depth of field deep enough to get an effective shot anyway.