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I have seen the term "Dual Pixel CMOS autofocus" used by Canon (specifically in relation to the 70D). What is it? Is this Canon specific technology? Are there equivalents in other manufacturer's cameras? And why is it better than the conventional autofocus systems?

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2 Answers 2

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With the actual announcement of the camera's release, there's no reason to speculate any longer. At least not any more than one has to when translating brochure-speak into tech-speak.

Essentially, on the new Canon sensor, each pixel consists of two photodiodes side-by-side under a single microlens. During the picture-taking operation, the two photodiodes act in concert (their outputs are summed/binned) to produce a standard single-pixel output. During the focus operation, on the other hand, they are read independently, so each photodiode is receiving light from a different angle through the microlens. Because the angle becomes significant, the imaging sensor can act as a phase-detection autofocus sensor.

That's the mechanics (or optics) of it, but that still leaves some questions that can only be addressed in testing and reviews. For instance, since imaging pixels are being used and imaging pixels are very small and only half of each pixel is devoted to one of two angles of incidence, how will that affect low-light performance? Are adjacent pixels ganged in a different way to provide greater sensitivity? Do the splits occur in only one direction, or in multiple different directions across the sensor. (This is essentially asking "are all of the focus points vertical, horizontal or cross-type?") Is PDAF available across the entire sensor or only in defined focus point areas?

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To answer your last question (from the press release): "Dual Pixel CMOS AF is possible over 80 per cent of the width and height of the Live View frame" –  Matt Grum Jul 2 '13 at 11:42
    
Here is a link with an explanation: dpreview.com/previews/canon-eos-70d/3 –  jrista Jul 2 '13 at 14:29
    
The area doesn't tell us too terribly much; it can still be in discrete "chunks" and have a lot of coverage. (The angles at the frame edges probably wouldn't allow for PDAF using a scheme like this.) It may need to be arranged thus is order to have sufficient sensitivity and prediction resolution. We'll have to wait for behavioural testing. –  user2719 Jul 2 '13 at 19:24

Given that this feature is part of Canon's new 20.2mp APS-C sensor, and is part of the LiveView focal plane CD+PD AF feature, it stands to reason that it is similar to a dual "zig-zag" line sensors in a classic dedicated PDAF sensor. In the 7D 19pt AF system, and further in the 1D X & 5D III 61pt AF system, dual line sensors improve the precision of a standard AF line sensor by using a pair of sensors where the pixels are offset by 50%.

I cannot be sure that Canon's new sensor pixels are "zig-zag", but a pair of pixels in each phase-detection line for each AF point are likely to be more precise, and at the very least more sensitive to light (by approximately a factor of two), than an AF point composed of single lines of pixels.

Regarding whether it is Canon specific technology, cannot say for certain, however Canon is the only one who has mentioned it. One could probably dig through patents related to focal-plane phase-detect AF systems to be sure. To date, this is the first I've heard of any specific design feature for FPPD AF systems.

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