The SLR mirror has to be in the viewing (down) position for light to reach the AF sensor, unfortunately. Thus, the typical DSLR phase-detection AF system can’t work in Live View mode, where the mirror has to be in the up position so light can reach the sensor. So traditional DSLRs have had to use contrast-detection AF off the image sensor for live-view and video shooting. Contrast-based AF is very accurate because it reads contrast right at the image sensor, and there are no submirrors and assemblies that must be precisely aligned. But, as implemented in DSLRs, contrast-detection AF is also slow because it has to take several readings at different focus settings to establish focus. And by the time it has done that, a fast-moving subject won’t be at that spot anymore.
HYBRID AF SYSTEMS
|A number of mirrorless cameras use hybrid AF systems, including Canon’s EOS M and EOS Rebel T5i and SL1. These systems incorporate a number of phase-detection sensors on the image sensor, which work in conjunction with contrast-detection AF to improve performance.
Nikon 1 mirrorless interchangeable-lens cameras feature the company’s Advanced Hybrid AF, where the camera uses quick on-sensor phase-detection AF (capable of keeping up with a 15 fps advance rate) in good light and automatically switches to contrast-detect AF in dim light. The phase-detection system has 73 AF areas (41 in Auto Area AF); the contrast-detection system has 135 focus areas.
Olympus’ new OM-D E-M1 mirrorless Micro Four Thirds System camera uses a new Dual FAST AF system, employing 37-area, high-speed, on-chip phase-difference AF when Four Thirds System lenses are used and 81-area contrast-detection AF when Micro Four Thirds System lenses are used. In Magnified Frame AF, you can select from more than 800 AF points.
Samsung’s NX300 offers a Hybrid AF system, where phase-detection AF quickly determines distance to subject, then contrast AF fine-tunes the focus to maximum contrast.
Sony’s Fast Hybrid AF (in the NEX-6, NEX-5T and NEX-5R) features phase-detection pixels on the image sensor, with 99 phase-detection AF points.
So, DSLRs have offered quick AF in Viewfinder mode, but slower AF in Live View mode. New technologies, however, are changing the way autofocus works.
Canon has introduced a solution to this problem in its new EOS 70D mid-range DSLR: Dual Pixel CMOS AF. Each pixel on the 70D’s 20.2-megapixel CMOS sensor consists of two independent photodiodes that function as both imaging points and individual phase-detection sensors. Other methods to bring phase-detection AF to live view have either replaced some of the imaging pixels with AF pixels, thus reducing overall sensor sensitivity, or done away with the optical viewfinder (see sidebars).
The Dual Pixel CMOS AF system was actually designed for video, and provides smooth and natural focusing, with no stuttering. And once focus is established on a subject, it stays with the subject as it moves closer or farther away, or around the frame. This makes for much better-looking videos. The EOS 70D also offers touch-screen AF, where you can switch focus from one subject to another just by touching the new subject’s image on the tilting/rotating LCD monitor. (As with other DSLRs, the eye-level viewfinder blacks out when the 70D is in Live View mode, so you have to compose using the LCD monitor.)
Dual Pixel CMOS AF is compatible with 103 Canon EF and EF-S lenses, from the 8-15mm fisheye zoom through the 800mm supertelephoto.
Though not as fast as conventional phase-detection AF, it’s a big step forward. The EOS 70D also has a conventional phase-detection system based on the excellent one in the EOS 7D, which operates when shooting using the eye-level optical SLR viewfinder.
SONY SLT AF
Sony’s SLT cameras offer another solution to the focusing-on-moving-subjects-in-live-view problem in its SLT cameras. The SLTs employ a semitranslucent fixed mirror that transmits most of the light to the image sensor while simultaneously directing a portion to the phase-detection AF sensor. So, you get quick phase-detection AF and good performance with moving subjects, as well as eye-level viewing (via an electronic viewfinder) in live view and for video. Electronic viewfinders haven’t been as good as DSLR pentaprism finders for action shooting, but the OLED finders in Sony’s SLT-A77, A65 and full-frame A99 are very good.