Dual Pixel AF vs. Conventional AF accuracy

We set up a few tests to compare conventional, dedicated phase-detect autofocus to sensor-based phase-detect, to better understand the new system's behavior.

In general, we would expect any on-sensor focus method to be more accurate than an SLR's conventional AF system, because it is measuring focus from the plane where the image will be captured. The conventional, dedicated AF sensor sits elsewhere in the camera, behind its own optics and a dual mirror assembly (each aspect of which will always be slightly mis-aligned), which means that it isn't directly measuring focus - it's taking a measurement as a proxy for focus.

However, with most lenses, we expect the difference in focus between the two systems to be quite small. When working with the relatively small apertures offered by kit zooms, the greater depth of field is likely to hide any slight imprecision. But with large aperture lenses, differences are likely to become more noticeable.

AF accuracy and consistency

Canon EF 85mm f/1.8 USM

The Canon EF 85mm f/1.8 USM is a relatively challenging lens to focus, especially with the aperture wide-open, due to its narrow depth of field and lens aberrations at maximum aperture. However its axial chromatic aberration (green and purple fringing in front of and behind the focus plane) has the advantage of making it easy to assess where the camera has focused.

The above target provides a good focus subject, and its angled sides reveal front and back focusing. Roll over the apertures listed below to see the 70D's performance at different apertures, and click the crops to see a full-size image.
Conventional, OVF autofocus Dual Pixel, live view autofocus Manual focus

As we'd expect, the Dual Pixel AF system outperforms the conventional autofocus system - it's more consistent, shot-to-shot and it's more accurate, in terms of where it focuses. Interestingly, it's not quite as accurate as manually focusing the camera by using both live view and the depth-of-field preview button to ensure you're focusing at the aperture you're using to take the photo.

Interestingly, when we subjected the EOS 60D to the same test, we got similar, but slightly different results. The conventional phase-detection AF produced almost identically soft images, but switching to live view autofocus yielded the same results as manual focus - putting its contrast-detect AF method ahead of the 70D's Dual Pixel AF in terms of accuracy. It's perhaps not a surprise that contrast-detect autofocus is more accurate than phase-detect, since it keeps moving the lens until optimal focus is achieved, but it's important to remember that it's quite a bit slower than the 70D's Dual Pixel AF.

Real-world Portrait

EF 85mm f/1.8 USM, shot at F1.8
Conventional, OVF autofocus Dual Pixel, live view autofocus Magnified live view

Here you can see what those differences mean in a real-world shooting situation - informal portraiture with the 85mm f/1.8. Using the conventional, optical viewfinder focus we got pretty good, if not bitingly sharp results. The shot used here represents a fairly typical result but, for one of our five shots, the camera was able to get even better results. However, the Dual Pixel AF system consistently produced better results than the conventional system - the five shots we took are almost indistinguishable.

Autofocus consistency comparison - Canon 85mm f/1.8
Conventional, OVF AF - 100% crops
Dual Pixel, live view AF - 100% crop
Magnified live view AF - 100% crops

However, manually focusing or using live view autofocus with greatly magnified (10x) live view, we were able to get the very best results. And this, to a degree, highlights one of the limitations of the Dual Pixel AF system: its AF points are considerably larger than the conventional PDAF sensor's, meaning that it's not always possible to precisely position the focus point where you want it.

While we were able to use the 10x magnification in this shot to achieve more precise positioning, it's hardly the ideal method when shooting portraits. While zoomed in to focus, we didn't notice our subject had drifted around in the frame.

The Dual Pixel AF system offers a relatively large focus point. It can be moved with some subtlety around the central 80-or-so percent of the frame. Pressing the magnify button on the right-hand shoulder of the camera zooms in. Pressing three times takes you to the maximum 10x zoom, at which point half-pressing the shutter button uses this small square to fine focus.

Autofocus microadjust (for viewfinder phase-detect AF)

It's possible to get much better results from the conventional phase detect AF system by using Autofocus Microadjust. This option resides in the camera's custom function menu (C.Fn II 13), and allows you to bias the lens's focus position foward or backward compared to where the AF sensor thinks it should be. So if the lens is consistently focusing behind or in front of your subject, you can fix this.

OVF focus, no AF Microadjustment Dual Pixel AF, no AF Microadjustment
OVF focus, -10 AF Adjustment Dual Pixel AF, -10 AF Adjustment

Here you can see that with a microadjustment of -10, the 85mm f/1.8 can be made to focus much more accurately using conventional phase-detect AF. It's also important to understand that this setting doesn't affect the Dual Pixel AF focusing, so using microadjustment won't throw it out of alignment.

With 3rd-party lenses: Sigma 18-35mm F1.8 DC HSM

The Sigma 18-35mm F1.8 DC HSM is optically superb, but when we reviewed it, we found that most Canon bodies struggled to focus it correctly. Like the 85mm, it's a lens whose wide aperture can really show off any inaccuracy in focus.

Sigma 18-35mm F1.8 DC HSM, shot at 35mm with a focus distance of around 4.5m
Conventional, OVF autofocus Dual Pixel, live view autofocus

Here we see the same pattern as we saw with the Canon 85mm, with the on-sensor, Dual Pixel AF system doing a much better job of focusing the Sigma 18-35mm F1.8 lens. As you can see, switching to the the Dual Pixel AF system produces greater accuracy than the conventional AF system; we found the consistency to be improved, too.

This particular copy of the Sigma 18-35 F1.8 focused very well up close on our EOS 70D, but from about 3m to infinity, the camera just couldn't do better than you see above. However, Dual Pixel AF made this very sharp lens perform as well as we'd expect at all distances.

Kit Lens

EF-S 18-135mm f/3.5-5.6 IS STM zoom, shot at 135mm and around 10m
Conventional, OVF autofocus Dual Pixel, live view autofocus

The other factor to bear in mind is that real-world shooting doesn't always involve shooting high-contrast targets with fast telephoto lenses. With the EF-S 18-135mm f/3.5-5.6 IS STM kit zoom, any difference in accuracy between the conventional focus system and the Dual Pixel AF system disappears. Part of this could be down to the new STM design, but its smaller maximum aperture also means you're unlikely to see a big difference in a lot of shooting situations. These tests were performed at several focal lengths and distances, and the above results are representative of what we saw from each combination.

Limitations of Dual Pixel AF - tracking and continuous shooting

These tests are all very impressive, but Dual Pixel AF does have its problems, most notably to do with tracking a moving subject during continuous shooting. The 70D offers a 'Face Detect + Tracking' mode, in which it can follow your subject's face as they move around the frame, and keep them in focus. But if you combine this with continuous shooting - both high and low speed - the camera locks focus at the first frame, and doesn't attempt to refocus for successive shots. Worse still, the screen blacks out completely during continuous shooting, making the whole thing something of a guessing game, particularly when panning to follow a moving subject. This means that Dual Pixel AF is effectively limited to being a 'one shot' mode when shooting stills.

This behaviour is disappointingly reminiscent of first generation mirrorless cameras, which worked in much the same way. It looks very dated compared to the most recent models, which offer continuous shooting modes that show live view between frames, and can refocus between shots. This requires them to shoot at a reduced rate compared to their fastest possible, but the Olympus OM-D E-M1's 6.5fps with focus tracking isn't exactly sluggish. Hopefully Canon will develop and improve future generations of Dual Pixel AF in much the same way.

Overall assessment of Dual Pixel AF

Overall, our impressions of the Dual Pixel AF system are pretty positive - it's one of the fastest live view focus systems in a current DSLR, so there's not such a dramatic shift in behavior when you switch to live view shooting as with most DSLRs. We've also found that it offers clearly greater accuracy and consistency than the conventional AF system. It's worth noting that while you can use AF adjust to fine-tune the behavior of the conventional AF (and may well need this to get usable results), the setting has no effect on the live view autofocus.

It's also worth noting that the Dual Pixel AF system doesn't necessarily use the same aperture as will be used to take a photo - instead it will use the diaphragm to control the amount of light reaching the sensor. In our tests, we found that this could result in a very small drop in consistency, if you're shooting images with a wide aperture in bright light, because that's where you'll have the largest discrepancy between the camera's working aperture value and your chosen shooting aperture.

Overall, any inaccuracy we have found with Dual Pixel AF has been very small. Anyone sticking with the kit lens will likely never encounter the difference we found between conventional phase-detect and Dual Pixel autofocus, but those with larger lens collections or who plan to purchase faster zooms and primes will benefit from shooting in live view mode with the 70D. Far from being 'just for video,' Canon's Dual Pixel autofocus does seem to offer a good balance between speed and accuracy, making it a good go-to mode when sharpness is critical, espeically when shooting at large apertures.

Overall, our testing makes it appear the 70D's Dual Pixel AF gives most of the speed advantage of conventional phase detection AF and most of the accuracy benefits of contrast detection, but doesn't quite manage to offer all of both.

Tracking AF

Sadly Dual Pixel AF is only really used for single image AF or movie shooting (see the movie page to see how it does). If you need continuous autofocus with tracking, you have to use the 70D's conventional AF system. We looked at how well it could track a subject walking towards the camera and left-to-right across the frame.

In wide-area AF, the Canon EOS 70D did reasonably well with both an older 85mm USM lens and the new 18-135mm STM lens (set to 85mm to match). There were more dropped frames with the USM lens (just one), but the LCD overlay points lit up and followed our subject as she walked toward the camera and tracked as well as we expected. Below we show only the 18-135mm results for the sake of brevity.