Fundamentals: Autofocus In Digital Cameras
All you wanted to know about using AF that fits on one page
There are two basic types of AF systems in wide use in digital cameras today. All of the D-SLRs use passive phase-detection AF, while most consumer models use passive contrast-based AF. "Passive" means the camera doesn't send a ranging beam out to the subject as do the "active" infrared (or near-infrared) AF systems used in many compact film cameras.
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.