Photo Innovation: Canon's Dual Pixel CMOS AF Offers Quick Live View and Video Autofocus

by JamesNYCJanuary 22. 2014 02:45

Canon’s new Dual Pixel CMOS AF system may be one of the biggest advancements in digital-camera technology we’ve seen in quite a while. Introduced in 2013 with the launch of the Canon EOS 70D digital SLR, this innovative autofocus process—which utilizes a unique approach to implement phase-detection AF—is a major development for solving autofocus issues that arise when a dedicated phase-detection sensor is unavailable and the camera is forced to use the slower contrast-detection AF. This occurs in a DSLR’s live view and movie capture mode, when the mirror must be flipped up.

At the same time, Dual Pixel CMOS AF has strong implications for further improvements in mirrorless cameras, which, like live view and movie capture, depend on sensor-based contrast-detection AF. Canon’s Dual Pixel CMOS AF can also be effective when a hybrid autofocus system is used, where a camera dedicates only a limited number of pixels to phase-detection AF to be used in combination with contrast-detection AF.

While hybrid systems have improved autofocus speed, this approach is still lacking, with lenses hunting or pulling in and out of focus before locking on a subject. This occurs because cameras still depend largely on contrast-detection AF. As the name “contrast-detection AF” implies, a certain amount of contrast between adjacent pixels is needed before focus can be achieved, which is particularly difficult in low light.

With its Dual Pixel CMOS AF, Canon takes a unique approach by creating two separate photo diodes for each pixel, covering about 80 percent of the sensor vertically and horizontally. Hiroshi Miyanari, a Canon staff engineer and one of the technology’s developers, explained in a white paper that each set of photo diodes perform both phase difference AF and imaging functions by capturing two parallax images at the same time: “With Dual Pixel CMOS AF, signals from these two images are used to carry out phase-difference detection AF.” In addition, he noted that, “by combining two photo diodes, the image signal can be output as one pixel,” with no negative effect on image quality. Because the two processes are combined, he pointed out, Canon was able to expand the dual functionality to “all pixels” (albeit with 80 percent coverage).

Thanks to its broad coverage of phase detection across the sensor, Dual Pixel CMOS AF is designed to bring faster and more accurate autofocus to DSLRs in live view and smoother autofocus tracking when shooting video. Additionally, the technology promises better performance in low light, since the autofocus is not dependent on the amount of contrast available in a scene.

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