Contrast-detect vs. Phase-detect Autofocus
By way of explanation, the phase-detect/contrast-detect distinction is one of the fundamental differences that separates digicams from digital SLRs, and is the core reason that SLRs focus more quickly. Contrast-detection autofocus involves looking at the image from a camera's main image sensor and evaluating it to see how abruptly brightness values change from one pixel to the next. If an image is soft and fuzzy, brightness changes between adjacent pixels will be relatively slight, but if it's sharply focused, they'll be much greater. The point of ideal focus is found by moving the lens elements back and forth, and determining whether the contrast signal gets stronger or weaker. Achieving focus this way necessarily involves some back-and-forth hunting, which can take a while to accomplish. By contrast (no pun intended), phase-detect AF uses a system of prisms, lenses, and a secondary sensor to determine not only whether the image is in focus or not, but by how much it's out of focus and in which direction. The camera can then adjust the focus setting to exactly the position needed, either in a single step, or perhaps with a quick secondary check once the initial focus adjustment has been made. As a result, phase-detect AF systems are generally much faster than contrast-detect ones.
The catch with phase-detect AF, though, is that it requires some of the light passing through the lens to be diverted to the focus sensor. This is fine in an SLR, when the mirror is down between exposures, as typically part of the mirror can be made to be partially transmissive, with the light passing through it deflected by a secondary mirror down to the AF sensor, usually located in the bottom of the mirror box. When the mirror is raised in Live View mode, though, light from the lens can't get to the separate AF sensor. This accounts for the rather lengthy (and noisy) phase detection AF cycles in most Live View-capable SLRs: To focus the camera the mirror has to be dropped, focus determined, and the mirror raised again, adding several tenths of a second to the normal non-Live View shutter lag.
