David Pogue's Digital Photography: The Missing Manual

Which Focus Mode?

Just as most cameras offer different ways to analyze the light in the scene, most also offer different ways to analyze the focus of the scene. In essence, you have two questions to answer: where to focus and when to focus.

Where to Focus

All cameras can focus automatically. But SLRs and some advanced small cameras let you override that autofocus—and, in fact, let you indicate which part of the scene you want to be in focus. Here are your options:

• Center focus. Unless you change the settings, most cameras, out of the box, use this method: They focus on whatever's in the center of the frame. And most of the time, that works fine.

  Tip

  Here's an oldie but a goodie. Suppose you don't want your subject to be in the center of the picture, but you do want the subject to be in sharp focus. In that case, aim the camera so the subject is in the center. Half press the shutter button (sounding familiar?). When you hear the beep or see the indicators change, you've locked in the focus. Keep the shutter button half pressed and then re-aim the camera, recomposing the shot so the subject is no longer in the center. Doesn't matter—that person or thing is still in sharp focus. Push down the rest of the way to get the shot.

• Spot focus. On an SLR, you can switch to a spot-focus mode. That's where you see little boxes or + signs arrayed on the scene; using the arrow keys on the back, you highlight the one you want to be in focus.

  So why use this method instead of the focus-and-recompose trick described in the previous tip?

  Because if you're using a tripod, or if you've carefully leaned your camera against some immovable object in low light, it's much easier to adjust your focus spot on the screen than to move the camera and recompose the shot.

  Furthermore, half-pressing locks in focus and exposure. In some cases, that could mess up the exposure for the final shot. There may be a patch of light, for example, that's present when you half-press, but not when you re-aim—so the camera locks in the wrong exposure. Spot focusing avoids the whole problem, because you're not changing where the camera is pointing.

• Face recognition. Talk about a mega-hit feature! Face recognition circuits first appeared in cameras in 2006; these days, almost every camera model on earth offers them.

  When face recognition is turned on (and most pocket cameras come with it turned on, at least in auto and Portrait modes), the camera identifies faces in the scene—as many as 20 of them at once. The latest models even recognize faces that aren't looking straight forward at the camera.

  Little rectangles appear around the faces in the scene, even as your subjects move around (shown on Autofocus Assist Lamp). The idea, of course, is that the camera intends to calculate its focus and exposure based on the faces. After all, people are the most common subject of amateur photos.

  Face recognition mode is better than the usual center focus mode, because it keeps the face in focus even if it's not centered in the frame. And it's better than spot focus mode, because you don't have to fuss with moving a little cursor around the screen. It's all automatic and, in general, very successful.

• Manual focus. SLRs and advanced pocket cams also let you use manual focus, much beloved by many experienced photographers. In this mode, of course, you —not the camera—do the focusing. On an SLR, you focus by turning the black ring on the lens; on a pocket camera, you generally use the arrow buttons to adjust focus so that the in-focus spot is closer or farther away.

  Tip

  On some cameras, you can zoom in to the image while you're using manual focus—that is, you can enlarge the preview on the screen or in the viewfinder. That's incredibly helpful in assessing your success as you focus; without zooming in like this, it's tough to tell when you've achieved the perfect focus position. The method for triggering this feature is different on every camera, so cuddle up with your manual for this one.

  Truth is, all cameras generally do an excellent job of autofocus. So who would fiddle with the slower, fussier manual focus in this day and age?

  Consider shooting through fence posts, zoo cage bars, or, worse, a chain-link fence. You might find that the autofocus insists on focusing on the closest element in the scene—the fence—rather than what's beyond it. Switching to manual focus neatly overcomes that problem.

  Dark situations sometimes stymie the autofocus, too. Manual focus, in those cases, may be your only chance of getting the photo at all.

  Then there are close-up portraits. Many cameras come set to focus on whatever is closest—in this case, the tip of your model's nose. Not the eyes, which is what should get the focus in a portrait.

  Finally, autofocus also takes more time—as you now know, it's a key contributor to shutter lag—so manual-focus mode is also handy when split-second timing is of the essence.

When to Focus

Your camera may also offer a choice of when to focus.

• When you half press. This option is available on every camera. It simply means: "I'll focus when you half press the shutter button." It also means: "At that point, I'll lock the focus at that distance—and keep it there as long as you're half pressing, even if you point the camera at something else." In other words, this is single-focus mode. One measurement of focus, one time.

• Continuous autofocus. This option is found in SLRs and advanced smaller cameras; it's sometimes called AI Servo. In this mode, the camera continuously recalculates focus, even if your camera moves or the subject moves, as long as you're half pressing the shutter button. As you can imagine, it's handy for action shots, racing cars, and anyone under five years old.

  Tip

  Then there's predictive continuous autofocus, available in nice SLRs. That's where the camera keeps tracking the subject even if it momentarily leaves the frame, by calculating its trajectory. Neat.

  It's especially great for burst mode (usually on an SLR). Suppose, for example, that an ice cream truck has burst into flames and is now barreling directly toward you, out of control. You put the camera in burst mode, thinking that this will make a heck of a shot.

  But if you just did the half-press trick, then the first shot would be in focus and subsequent shots would be increasingly out of focus as the truck closes in on you.

  But if you put the camera in continuous mode, then every shot in your burst would be in focus as the truck approaches, right up until the moment when you get run over.

  It's not foolproof—distance, lighting, subject speed, and other factors can affect its effectiveness—but it's great when it works.