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.