Intermediate Modern camera technology has done a lot for photographers like you and me. Intelligent metering systems have made it possible for us to take pictures on the fly in changing lighting situations, without having to stop to take new readings and make adjustments. Digital technology has vastly reduced the daily expenses of photography, allowing us to take a lot more photos than we used to, thereby increasing the number of truly amazing shots we capture in any one outing. And let's not forget the often under-appreciated autofocus. Imagine having to manually twist that focusing ring every single time you wanted to take a picture, like they did in the olden-days. But do you truly understand your camera's various autofocus modes and how they work?
How does autofocus work?
Different DSLRs have different autofocus systems; the two dominant systems are active and passive. An active system has an infrared beam that can determine the distance between the camera and the subject, and adjust the focus accordingly. A passive system measures changes in contrast - to these systems, maximum contrast equals maximum sharpness. If you've ever felt like flinging your camera against the wall that it couldn't figure out how to focus on, it's probably because your camera has a passive autofocus system. Subjects that are flat and have only a single color will confuse a passive system, because a passive autofocus system needs an edge to give it some contrast to focus on. If it can't find that edge/contrast, it will "hunt" for focus, which will result in an annoyed photographer.
All cameras have focus points--some only have three, and others have as many as 45 or more. In general, your entry-level DSLR will have fewer focus points than a professional-grade DSLR. But what exactly is a focus point?
Simply put, a focus point is a specific, selectable point in your viewfinder where your camera will focus. You can manually switch between them if, say, you want your focal point to be on the left of the screen instead of in the center. Or you can "focus and recompose," which just means that you use the center focus point to lock focus on your subject, then you recompose the shot so your subject is in the part of the viewfinder where you'd like him/her to be in your final image.
I doubt the value of having a lot of focus points, because as a general rule that one point in the center provides for the sharpest focus. So you may have better results with the "focus and recompose" technique than you would dialing in a different focal point every time you want to take a picture. Manually switching between focal points has additional drawbacks, too, since it takes time to dial in that focus point, and you may lose the shot.
Now that we have some of those technical details out of the way, let's talk about the different autofocus modes that can be found on most DSLRs: single, continuous, automatic and manual.
If you're shooting subjects that aren't moving, you will probably want to set your autofocus to "single" or "one shot," (depending on your camera's manufacturer). In this mode, you press the shutter release button down halfway and the system will focus one time. For basic portraits, macros, architecture etc., "single" mode will be as much as you need - unless you think that potted flower or church steeple might grow legs and walk away.
If your potted flower or church steeple does grow legs and walk away, you'll want to switch to continuous auto-focus. In this mode, you depress that shutter button halfway and the autofocus system will track your moving subject, making constant readjustments as the subject moves. This is very useful in theory but is not foolproof - no autofocus system can always accurately predict where a subject will end up (children at play, for example, don't always move in a single direction) so you aren't guaranteed to get a sharp photo in this mode. You will probably get better results than you will in single mode, though, although another drawback is that you'll use up those batteries a lot faster.
I don't have to tell you too much about manual focus: it's for chumps, right? If you're like most hobby photographers you probably avoid it as cumbersome and undesirable. There are times, though, when you'll need it - when shooting in low light, when shooting low-contrast subjects, or when trying to shoot one object through another (an animal behind a wire fence, or a subject as viewed through the branches of a tree). In these situations your autofocus may get confused about what it needs to focus on, so you're better off switching to manual so you can make the focusing decisions yourself.
Note: if you really, really, really don't want to switch to manual, or if you yourself are having a tough time focusing on your subject because of poor light, many cameras come with "autofocus assist," which is essentially just an infrared or visible beam of light that helps your autofocus system "see" your subject. This does slow things down a bit, but since the AF-assist works best on subjects that aren't in motion anyway, this won't matter so much to you unless you are particularly impatient.
Newer DSLRs are starting to feature this setting, which basically just allows the camera to choose whether it is going to use a continuous or single autofocus mode. This can be useful because in a situation where you are rapidly switching from shooting stationary objects to moving ones, you may not particularly want to bother with that awkward little switch that lets you change modes.
Getting the best results
Barrel Racing by Flickr user billdubreuil
For all that technology does for us, it's worth keeping in mind that it's not always a perfect substitute for human decision. The best way to make sure your fast-moving subject is sharply focused may not be to rely entirely on that continuous AF mode, but to focus ahead of time on a stationary object that is at the same distance as your subject will be when you take the picture. So though your well-equipped, top-of-the line DSLR may tempt you into brainless shooting, it is always best to maintain some awareness of what type of focusing situations you're working with, so you can take control back when you need to.
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