Technology explained

The EOS 70D's headline feature is undoubtedly its unique 'Dual Pixel CMOS AF' sensor. This offers 20.2MP resolution, but uses two photodiodes for every single pixel ('facing' left and right), so that they're all capable of on-chip phase detection. This in turn promises hugely improved autofocus in both live view and movie recording. For example Canon says the EOS 70D will be able to hold a face in focus when the subject is moving back and forwards relative to the camera, even when filming with a fast prime lens at large apertures.

To understand what this technology is, and why it's a big deal, it's perhaps best to start with looking at how phase detection autofocus works, and some of the limitations of existing systems.

How phase detection autofocus works

Put simply, phase detection autofocus works by looking at the images projected by the left and right sides of the lens separately. When the subject is out of focus, these are 'out of phase' and don't coincide on the sensor, giving a blurred image. When the lens is moved into correct focus the images coincide, giving a sharp photograph. The key point is that it's possible to determine from a single measurement precisely how to move the lens to achieve correct focus.

Back Focus
In Focus
Front Focus
In this simplified schematic, you can see what happens to the image cast by the light passing through the left (blue dotted line) and right (red dotted lines) sides of the lens.

When in focus, the light from both sides of the lens converges to create a focused image. However, when not in focus, the images projected by two sides of the lens do not overlap (they are out of phase with one another).

Of course this is a massively simplified diagram with a single, vertical straight line as the subject (and no inversion of the image as it passes through the lens). The point is that we can derive information about focus if we can separately view light coming from opposite sides of the lens.
 
How does a phase detection sensor 'see'?

And we don't need the whole image to do this. Think about a strip of pixels taken from the sensor in the previous diagram. If you could make one such strip that receives light only from the left hand side of the lens and another that 'looks' only to the right-hand side of the lens, then you have enough information to find focus.

By comparing images from just these two strips it's possible to work out not only how far but also in which direction the lens needs to be moved to bring them into phase.
Back Focus
In Focus
Front Focus

This basic approach has been used by SLRs since autofocus first appeared in the mid-1980s. Some light is allowed to pass through the main mirror and redirected down to an autofocus sensor in the base of camera. This works when shooting with the optical viewfinder, but the moment the camera is switched to live view and the mirror flips up, the autofocus sensor no longer receives any light and so can't be used. One solution to this is Sony's 'SLT' design, which uses a fixed mirror that always feeds some light to the AF sensor. But these cameras can no longer use optical viewfinders, and have to use electronic ones instead - the isn't a bad thing per se, but not all photographers like them.

An alternative solution that's used by many current cameras (including Canon's recent entry-level SLRs, and many mirrorless cameras) is on-chip phase detection. Some of the pixels on the main image sensor are masked black on one half, so that they only register light coming from one side of the lens. By using left- and right-facing pixels, it's possible to implement phase detection focusing using the image sensor itself.

This diagram shows how on-chip phase detection was implemented on the first cameras to use it, the Fujifilm F300 EXR and Z800 EXR. Click here to read more about their PDAF system.

The problem with on-chip phase detection is that these pixels only register half the usual amount of light. So when it comes to taking the photograph they output a lower-quality noisier signal, and can't always be used for imaging, especially in low light; instead their values have to be interpolated from the surrounding pixels. This in turn means there can't be too many of them on the sensor, or image quality will drop noticeably. And as light levels drop, the signal quality from the masked pixels can deteriorate to the point that the focusing system doesn't work either. Canon's fix for all of these problems is, rather than masking the pixels, to split them into two.

Canon's new 'Dual Pixel CMOS AF' sensor

Canon's schematic of its Dual Pixel CMOS AF sensor structure. The top layer illustrates the light-gathering microlenses and conventional Bayer-type colour filter array. The lower layer shows how each pixel is split into two photodiodes, left and right, which are coloured blue and red respectively. (Note that this does not indicate different colour sensitivity.)

Canon's 'Dual Pixel CMOS AF' sensor looks at the concept of on-chip phase detection, and takes it to a logical conclusion. Instead of masking some pixels so they only 'face' left or right, it uses two photodiodes for each pixel, so every single one has a left- and right-facing component that can be used for phase detection. When a photograph is taken, the output from the two photodiodes is combined. The diagram below illustrates what this means for phase detection AF.

Lens back-focused
Overall view
Lens back-focused
Left-facing photosites
Lens back-focused
Right-facing photosites
   
Lens in focus
Overall view
Lens in focus
Left-facing photosites
Lens in focus
Right-facing photosites

This immediately brings many theoretical advantages. In principle there's no significant light loss overall, so the quality of the output pixels shouldn't be lower compared to a conventional sensor. But because there are many more pixels capable of phase detection on the sensor, they can be used together for more accurate focusing, and in principle the system should work much better in low light.

Because the phase detection sites are more closely-spaced, the system can also work at small apertures, which allows continuous focusing in movie mode with the lens stopped down, or functional autofocus when using long lenses with teleconverters. Last but not least, because the main image sensor is used for focusing, it should be effectively immune from any systematic front- or back-focus problems that can come from an AF sensor with a separate light path.

According the Canon the EOS 70D's Dual Pixel CMOS AF system has the following key characteristics:

  • Usable phase detection AF area covers 80% of the frame horizontally and vertically
  • AF works at apertures down to F11
  • AF works in light levels as low as 0 EV
  • Can work with face detection to keep moving subjects in focus
Canon says the EOS 70D's 'Dual Pixel Hybrid AF' is active across 80% or the frame by width and height, as outlined here. (Pixels towards the edge aren't used for AF for technical reasons, despite having the same dual-photodiode structure.)

This means that almost wherever your subject is in the frame, the camera can attempt to track focus on it during live view and movie shooting using its on-sensor phase detection.

Lens compatibility

Canon has made no fewer than 156 EF lens models in the 26 years since the EOS system first appeared, and it says that 103 of them fully support Dual Pixel CMOS AF, including all current lenses. This means that these lenses only ever use phase detection for live view and movie focusing - there's no need for a slower contrast detection step. The other 53 lenses count as partially supported, which means that for One-Shot AF (either stills or movie) they use a hybrid system with phase detection to determine the initial focus movement, and contrast detection to fine-tune correct focus. So these lenses won't focus as fast and decisively in Live View.

It's really important to understand that this compatibility list relates only to live view and movie mode autofocus - it doesn't mean that other lenses won't work at all. Third party lenses from the likes of Sigma, Tamron and Tokina are fully expected to work absolutely fine on the EOS 70D - Canon hasn't listed them simply because it has no interest in determining how well other manufacturers' lenses work with its new AF system. So if you own some nice third party lenses, there's no need to worry that they might not work on the 70D.

Click here for Canon's list of fully compatible lenses (from Canon USA).