Phase-detection AF, used in DSLRs for non-live-view shooting, is quick and very good at tracking moving subjects. Contrast-detection AF, used in compact and mirrorless interchangeable-lens cameras—and DSLRs (other than Sony’s) when shooting in Live View mode—is very accurate, but historically slow, and not good at tracking moving subjects.
Conventional DSLRs can’t use phase-detection AF for live view and video operation because the SLR mirror must be in the down (viewing) position for light to reach the AF sensor, while it must be in the up (exposing) position for live-view to function. Sony’s SLT cameras use Translucent Mirror Technology to solve this problem: the nonmoving semitranslucent mirror lets most of the light pass through to the image sensor, while simultaneously directing a small amount of light up to the AF sensor. So you get quick continuous phase-detection AF at all times, even for video.
Now a growing number of new cameras are using another method to get phase-detection AF for live-view and video shooting—"hybrid" AF systems, which employ phase-detection AF sensors on the image sensor itself, the camera automatically using those or contrast-based AF to suit the situation.
The Nikon 1 mirrorless cameras—the J1, V1 and new J2—use 73-point phase-detection image-sensor AF when the light level is bright enough (this depends on both the ambient light level and the speed of the lens being used), automatically switching to slower contrast-based AF when the light level drops. The phase-detection system is quick and can handle action subjects very effectively in good light. These cameras can shoot at 10 fps with autofocusing for each frame.
Canon’s EOS Rebel T4i DSLR features a new Hybrid CMOS AF system that uses phase-detection AF sensors in the center of the CMOS image sensor along with contrast-based AF for live-view and video operation, using the phase-detection to quickly "ballpark" focus, then letting contrast AF take over to fine-tune focus. With normal viewfinder operation (non-live-view), AF is conventional phase-detection, via a 9-point (all cross-type) AF module in the camera body. This system can provide AF for each shot at 5 fps.
Canon’s new EOS M mirrorless camera features the same 18-megapixel image sensor as the T4i with an on-sensor phase-detection AF system, but with a couple of differences. There’s no conventional phase-detection AF system (since the M isn’t a DSLR, it operates only in Live View mode), and you can set focus just by touching the camera’s 3.0-inch touch-screen LCD monitor.
Sony’s new NEX-6 and NEX-5R mirrorless interchangeable-lens cameras feature Sony’s Fast Hybrid AF, with 99 phase-detection sensors at the focal plane working with 25-area contrast AF to optimize AF performance in a wide variety of shooting situations. The phase-detection sensors quickly determine the subject’s distance and direction, then the contrast-detection system fine-tunes
focus. In Speed Priority Continuous AF mode, the camera uses phase-detection AF to track fast-moving subjects at up to 10 fps. Note that Fast Hybrid AF initially is possible only with the E 18-55mm ƒ/3.5-5.6 OSS, E 55-210mm ƒ/4-6.3 OSS, E 18-200mm ƒ/3.5-5.6 OSS and E 24mm ƒ/1.8 ZA Sonnar T* lenses. Firmware upgrades should add compatibility to more lenses in the future.
Sony’s new 24-megapixel, full-frame, translucent-mirror SLT-A99 DSLR provides Sony’s TMT fixed-translucent-mirror technology, which offers full-time 19-point continuous phase-detection AF for both stills and video, like all SLT cameras. But it also features 102 phase-detection AF points on the image sensor itself to complement the standard phase-detection AF system. The new sensor AF points provide more information about the subject for better tracking of fast-moving subjects against complex backgrounds. Also new is in-body AF Range Control, which further helps AF speed and accuracy by allowing you to limit the range of distances through which the AF system will seek focus.
Phase-Detection Vs. Contrast-Detection AF
|With phase-detection AF, the system divides the light coming through the lens into two images, then measures the phase difference—the difference between where each image strikes its respective AF sensor. This tells the system in a single reading whether or not the image is in focus, and if not, in which direction and by how much it’s out of focus. It’s a very quick process, and by using data from successive readings, the system can calculate the speed and direction of a moving subject, and adjust focus for its predicted position at the instant of exposure. The main drawbacks are that its accuracy depends on all components of the AF system being perfectly aligned, and in conventional DSLRs, it functions only when the SLR mirror is in the down "viewing" position—so it can’t be used for live-view and video shooting.
Contrast-detection AF works by measuring contrast right off the image sensor, so it’s inherently more accurate than phase-detection AF—there aren’t a number of lenses, sensors and sub-mirrors that can get misaligned. And contrast-detection AF works in live-view and video modes. The drawbacks are that it’s typically slower than phase-detection AF, and it’s not as good for action.
Contrast AF takes a contrast measure-ment, changes the focus setting and takes another reading. If the new reading shows more contrast than the first one, the system adjusts the focus more in that direction and takes another contrast reading, repeating until contrast starts to drop. It then moves focus back one step and makes the exposure. If the second reading shows less contrast than the first, then the system adjusts focus in the opposite direction, takes another contrast measurement and repeats until contrast is maximized as above. This takes time, during which moving subjects are moving to a new distance.
Some of the newer mirrorless cameras have increased the reading rate to where the contrast-detection system can establish focus very quickly, but it’s still not as effective as phase-detection for fast-moving subjects.