Tuesday, November 13, 2012

Hybrid AF

By Mike Stensvold Published in SLRs
From top to bottom: Nikon J2; Sony SLT-A99; Canon EOS M
From top to bottom: Nikon J2; Sony SLT-A99; Canon EOS M
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.

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