With phase-detection AF, a portion of the light transmitted by the lens is diverted to the AF module, where it’s split into two parts and directed onto a pair of CCD line sensors. (If the AF system provides multiple AF points, it does this for each active AF point.) The spot where the two beams strike their pair of sensors tells the camera’s AF computer whether the image is in focus or not, and if not, in which direction and by how much it’s out of focus. Thus, the AF system can establish focus with a single reading and adjustment, making it quick and effective with moving subjects.

Contrast-based AF is much slower because the system has to make multiple readings and lens adjustments to establish focus. This also means contrast-based AF isn’t as good for action subjects.

While the AF systems in today’s D-SLRs are excellent, passive AF can have trouble focusing in the same circumstances that make it hard to focus manually by eye: dim light, extremely bright subjects (sun glare on water), subjects with no contrast (a plain wall or clear sky) and fast-moving subjects. If your camera won’t autofocus on a subject, try focusing manually.

AF AREAS
Today’s AF SLRs provide multiple AF points, indicated by small brackets or squares in the viewfinder. You can activate just one point or all the points. When all the AF points are activated, the camera will pick the appropriate one to use (generally, that will be the one covering the closest object). In single-area mode, you select the point you want to use (handy for compositions with off-center subjects). I gener-ally use all the AF points for action shots so I don’t have to fret about keeping a single small AF point on a fast-moving target and single-area AF for subjects like birds in trees, where branches might fool the AF system in all-area mode.

SINGLE-SHOT AND CONTINUOUS AF
In single-shot AF, the camera focuses on the subject using the active AF area(s), then locks focus until you take the shot or let go of the shutter button. In continuous AF, the camera focuses continuously on the subject in the active AF area(s). Generally, you’ll get the best results using single-shot AF for stationary subjects and continuous AF for moving subjects.

To use single-shot AF effectively, aim the AF target at the part of the subject or scene you want focused, press the shutter button halfway to activate the AF system, and when the viewfinder’s in-focus signal lights up (and the focus beeper beeps, if this feature is activated), fully depress the shutter button to make the shot. To use continuous AF effectively, acquire the subject, pan the camera to track it, press the shutter button halfway to activate the AF system, give the AF system a beat to acquire the subject, then fully depress the shutter button to make the shot.

Most D-SLRs provide predictive continuous AF. The AF system takes successive focus readings of the subject and, from these, determines its direction and speed, then uses this data to calculate its likely position at the instant of exposure and adjusts focus accordingly. Predictive AF works best on subjects that are moving at a constant speed in a constant direction. If the subject moves erratically, predictive AF may have trouble with it.

With most D-SLRs, single-shot AF is focus-priority, meaning the camera will lock the shutter until it has confirmed focus—the idea being this will prevent you from accidentally taking an out-of-focus photo. What focus-priority actually does is prevent you from capturing decisive moments; believe me, you can still get out-of-focus photos. Confirm focus visually, using the viewfinder.

With many D-SLRs, continuous AF is release-priority, meaning you can trip the shutter at any time, whether or not the AF system thinks the image is in focus. Some cameras let you select focus-priority or release-priority via a custom function. I usually set release-priority because I’d rather have a possibly slightly soft image of a great moment than no image at all.