With phase-detection AF, a portion of the light entering the lens is diverted to the AF module, where it’s split into two parts, which are directed onto a pair of CCD line sensors. The points where the two beams strike the sensor tell the camera’s AF computer whether the image is in focus or not, and if not, in which direction it’s out of focus and by how much. A phase-detection system can thus establish focus with a single reading and adjustment, making it quicker than contrast-based systems and much better for moving subjects (and in dim light).
All of today’s D-SLRs use phase-detection AF systems for normal shooting. Contrast-detection AF measures contrast at the image sensor, the idea being contrast is at its maximum when the image is sharply focused. A contrast-based AF system must take multiple readings to determine and set focus: After the first reading, focus is adjusted and another reading is taken. If the contrast is greater, another adjustment is made in the same direction and another reading is taken, and so on, until contrast starts to decrease. If the second reading shows less contrast than the first, an adjustment in focus is made in the opposite direction, then another reading is taken, etc. The result is that contrast-detection AF requires multiple readings and adjustments, taking longer than phase-detection AF to establish focus. But it can be more precise, and there’s no disruption of the live image during focusing as there is with phase-detection, so it’s excellent for tripod-mounted live-view work, where speed is not of the essence. Some D-SLRs offer both types of AF in live-view operation.
AF In SLRs: A Timeline
1981 Pentax introduced the ME-F, the first interchangeable-lens SLR with TTL autofocusing capability. It took all Pentax SLR lenses, but autofocusing was possible only with one special AF lens: a 35-70mm zoom, which contained both the AF motor and the four AA batteries it needed to operate.
1985 Minolta introduced the Maxxum 7000, which really started the AF SLR revolution. The body contained not only the AF sensor, but a focusing motor as well, so AF worked with all lenses. Caveat: Only the new Maxxum lenses could be used with the camera; the Maxxum cameras couldn’t use previous Minolta system lenses.
1987 Canon introduced the EOS system, with the EOS 620 and EOS 650 models. EOS AF SLRs don’t contain focusing motors; rather, each lens contains its own motor optimized for its requirements. The drawback was that longtime Canon users couldn’t use their earlier lenses with EOS cameras; only EF (and today, EF-S) lenses can be used on EOS bodies. Other manufacturers put the AF motor in the AF SLR body to retain compatibility with previous as well as new AF lenses (only the AF lenses would autofocus, but users could use their existing lenses with manual focusing). Today, most D-SLR manufacturers offer higher-end lenses that contain their own focusing motors. (Nikon has even introduced entry-level D40 and D60 bodies that don’t have focusing motors and thus must be used with the AF-S lenses that do contain AF motors if one wants autofocusing capability.)
2006 Olympus introduced the first D-SLR with a Live-View monitor. The EVOLT E-330 had two Live-View modes, one that used the same phase-detection AF as was employed for normal shooting and a full-time Live-View mode with manual focusing.
2007 Nikon introduced the D3 and D300 with Live-View modes, featuring both types of autofocusing. In Handheld mode, phase-detection AF is used; in Tripod mode, contrast-detect AF is used.