Canon Dual Pixel CMOS AF: Autofocus secrets of the Canon 70D explained

Autofocus precision with Canon's groundbreaking Dual-Pixel CMOS AF technology (as found in the Canon EOS 70D) is a somewhat different beast than with the conventional phase-detect systems used in DSLRs.

In my recent interview with Mr. Go Tokura and Mr. Naoya Kaneda of Canon Inc., the company's Japanese headquarters organization, I asked some technical questions about Dual Pixel CMOS autofocus that turned out to be too difficult to get across the language barrier, despite the excellent translation to which we had access. So I did what I should have done in the first place: checked with Chuck Westfall, Advisor, Technical Information, Canon U.S.A., Inc.

As usual, Chuck delivered the goods, in the form of an exceptionally clear description of how autofocus precision compared between Canon's Dual Pixel CMOS AF technology and the conventional separate-sensor phase-detect AF system, which Canon calls Thru-the-Lens Secondary Image Registration (or TTL-SIR) AF. There's some very interesting information here; see the remarks below Chuck's writing for a few of my thoughts on the matter.

AF Precision: Dual Pixel CMOS AF vs. TTL-SIR AF

By Chuck Westfall / Canon USA Inc.
February, 2014

When comparing the precision of conventional TTL-SIR (Through-the-Lens Secondary Image Registration) AF with EOS cameras to the EOS 70D's Dual Pixel CMOS AF, it is important to understand the following principles.

1. The EOS 70D's TTL-SIR phase detection autofocus system with its 19-point AF sensor has two fixed baselines for AF precision:

• All 19 AF points have a relatively short baseline for use with EF and EF-S lenses that have maximum apertures larger than or equal to f/5.6.
• The center AF point has a dual-cross type AF sensor with a much longer baseline that is used with EF and EF-S lenses with maximum apertures of f/2.8 or larger.

2. The EOS 70D's Dual Pixel CMOS AF system uses every pixel to measure phase-detection AF with supported EF and EF-S lenses. With a supported lens, the baseline for AF measurement is proportional to the maximum aperture of the lens in use. Dual Pixel CMOS AF does not aggregate AF measurement from multiple pixels to create longer baselines for large aperture lenses. Each individual pixel is fully capable of generating AF measurement data.

Keeping these differences in mind, one of the consequences of the TTL-SIR AF system is that except for the center point, AF precision is not proportional to the maximum aperture of the lens in use. With off-center points, AF precision is not as high as the center point when the maximum aperture of the lens is larger than or equal to f/2.8, in order to allow AF with maximum apertures as small as f/5.6. It's a workable system, but it's a compromise between precision and AF compatibility. Even with the dual-cross-type center point, precision at a maximum aperture of f/2.8 is the same as it is at f/1.2. At the other end of the scale, autofocusing is not supported at all with maximum apertures smaller than f/5.6 with the 70D's TTL-SIR AF system. Finally, because AF data is measured from a secondary sensor (not the imaging sensor), there can be a need for AF Microadjustment to compensate for focusing discrepancies with individual lenses.

On the other hand, because AF precision is proportional to maximum aperture with Dual Pixel CMOS AF, it scales itself automatically and dynamically according to the supported EF or EF-S lens in use. For example, Dual Pixel CMOS AF achieves a longer baseline for AF precision with an f/4 lens compared to an f/5.6 lens, and so on all the way down the line to f/1.0. At the other end of the scale, Dual Pixel CMOS AF supports autofocusing with maximum apertures as small as f/11, a full two stops smaller than is possible with the 70D's TTL-SIR-AF system. And because the AF data is measured directly from the imaging sensor, there is no need for AF Microadjustment.

It should be noted that AF precision at f/1.2 with Dual Pixel CMOS AF is basically the same as AF precision with the Dual Cross center point of the 70D's TTL-SIR phase-detection AF system. And accordingly, AF precision at f/2.8 with Dual Pixel CMOS AF is not quite as high as AF precision at f/2.8 with the Dual Cross center point of the 70D's TTL-SIR phase-detection AF system. However, the trade-off is that Dual Pixel CMOS AF maintains its precision throughout the entire AF measurement area (approx. 80% of the width by 80% of the length of the imaging area), rather than limiting high precision to the center of the picture area.

This is very interesting information, indeed. Confirming earlier but more tentative information from our initial briefings on the camera, it is, in fact, the case that the Canon 70D's Dual Pixel CMOS AF system will set phase-detect baselines proportional to the maximum aperture of the lens in use, so its accuracy will improve at maximum apertures beyond f/2.8. What I found particularly interesting, though, is that the Dual Pixel phase detect operation is actually somewhat less accurate than the 70D's separate TTL-SIR AF system.

Note, though, that I said phase detect operation, as that's only part of the process the EOS 70D uses in live view AF, at least for still-image focusing. Part of the reason the 70D's Dual Pixel AF isn't as fast as the conventional separate-sensor approach is that the 70D only uses Dual Pixel technology to achieve rough, initial focus; it then switches to contrast-detect focusing to check focus accuracy and, if necessary, fine-tune focus. (The other part is that data most likely can be read out from the dedicated AF sensor more quickly.) This means that the slightly lesser accuracy of Dual Pixel CMOS AF doesn't affect ultimate focus accuracy, just the speed with which you achieve it.

When it comes to video focusing, though, even Full HD (1,920 x 1,080 pixel) resolution is so much lower than the 70D's 20-megapixel still image resolution that the slightly lower PDAF accuracy of the Dual Pixel CMOS AF system matters not a bit, while its ability to focus smoothly and continuously at apertures down to f/11 is a huge benefit.

Special thanks to Chuck Westfall for taking the time to write such a detailed and informative answer to my queries!