Understanding EOS Autofocus: The EOS 70D

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EOS 70D autofocus

The new EOS 70D is an exciting camera in many ways. But for me, one of the more interesting features is the autofocus. That’s because every APS-C EOS camera so far (except the 7D and some of the earlier models) uses a nine point autofocus array. Up until the EOS 70D, if you wanted more autofocus points you had to buy either an EOS 7D, a 5D Mark III or a 1 Series camera (none of which is particularly inexpensive).

EOS 70D autofocus

Now, the EOS 70D (pictured above) gives you an extra option. Its autofocus system features the same nineteen point AF array found in the EOS 7D. The AF of the two cameras is virtually identical (you can learn more my reading the article I wrote about the 7D’s autofocus here). The only exceptions are that the EOS 7D features spot autofocus and autofocus point expansion, but the EOS 70D doesn’t. If these features matter to you then this means that the 7D Mark still has the most advanced autofocus of any APS-C EOS camera (as long as you are not in Live View or Movie mode).

In short, the improved autofocus of the EOS 70D alone makes it a decent decent improvement over the 60D, which is always nice to see. The specification gap between Canon’s enthusiast cameras (EOS 600D, 700D etc.) and mid-range models (EOS 60D, 70D etc.) has also just got wider, marking a clearer separation between the two.

Here’s a diagram showing the AF point array found in the EOS 60D (and most EOS APS-C cameras):

EOS 70D autofocus

Here’s a diagram showing the AF point array in the EOS 70D (the same arrangement as the EOS 7D). The icons at the top tell you which AF mode you are in:

EOS 70D autofocus

Dual Pixel CMOS autofocus

The other significant development is Dual Pixel CMOS autofocus. It’s a feature aimed at users of Movie mode and Live View. If you only use your camera to shoot stills while looking through the viewfinder, then it is largely of academic interest only. But if you shoot movies, or use Live View a lot, then it is a very interesting feature indeed.

If you’re not into video, then you may be wondering why there has been so much fuss about the incorporation of Movie mode into EOS digital SLRs. The excitement exists for two reasons:

  • The quality of video footage from EOS SLR’s is extremely high. EOS cameras have been used to shoot footage for movies, television series and documentary films.
  • Movie makers are attracted to EOS SLRs because the relatively large size of the camera’s sensor means they can use prime lenses at wide apertures to shoot footage with extremely shallow depth-of-field – an effect they love. This works with APS-C cameras and even better with full-frame cameras (the larger the sensor the less depth-of-field you get at any given aperture and focal length combination).

Tied into both these reasons is price. It’s much less expensive to buy a digital SLR than it is to buy a purpose made video camera offering the same image quality. Producing high quality video footage is expensive. The software costs a lot, and so do essential accessories like microphones and video tripod heads. EOS SLR’s lower the cost of entry, enabling more independent and hobbyist movie makers to produce video.

The autofocus problem

EOS 70D autofocus

Shallow depth-of-field may look good, but it brings its own problems, especially when shooting footage of people, as in the photo above. Accurate autofocus is essential, and it’s something that has so far been lacking in Movie mode. That makes life difficult for people shooting video. It can also put photographers who bought their EOS cameras to shoot stills with off playing around with the video function. Dual Pixel CMOS autofocus is Canon’s response to this.

To understand why autofocus in Live View or Movie mode lags behind the camera’s performance while shooting stills, we need to look at how autofocus works in an EOS SLR. The autofocus sensor is located in the base of the camera. The mirror that reflects light passing through the lens up into the pentaprism has a sub-mirror that deflects part of that light down to the autofocus sensor.

This diagram shows the path of light through the camera:

EOS 70D autofocus

When you press the shutter button to take a photo, the mirror flips up to allow the light through to the camera’s sensor. With the mirror out of the way, light is cut off from the autofocus sensor. That’s not an issue if you’ve already focused on the subject and the subject is still. But it may be a problem if the subject is moving, which is why your camera includes an AI Servo mode. In this mode, the camera calculates the likely movement of the subject between the time the mirror flips up and the photo is taken (measured in fractions of a second) and adjusts the focus of the lens accordingly.

This diagram shows the path of light through the camera with the mirror flipped up:

EOS 70D autofocus

This arrangement dates from when autofocus was first included in SLR cameras. It works fine if you’re shooting stills while looking through the viewfinder. But in Live View or Movie mode, the autofocus sensor is rendered useless. The mirror stays in the up position and the autofocus sensor, starved of light, can’t function. That’s why early cameras with Live View, such as the EOS 40D, only had manual focus in Live View. Later models introduced autofocus in Live View, but until now the performance hasn’t been great.

I mentioned earlier that one of the attractions of using EOS SLRs for shooting video is the shallow depth-of-field. Now, it’s hard enough focusing accurately while looking through the viewfinder using cross-type AF sensors, especially if you are shooting a portrait and focusing on the model’s eye (I wrote more about that here). It’s even more difficult, if not impossible, to do this while looking at the scene on the camera’s LCD screen in Live View or Movie mode. That’s why movie makers use accessories such as loupes or monitors, to make this task easier. And it’s why Canon is developing technology such as Dual Pixel CMOS autofocus.

How Dual Pixel CMOS autofocus works

Up until the EOS 70D, every digital EOS camera uses a single photodiode to capture light for each pixel on the sensor. That means an 18 megapixel sensor like that found on the EOS 60D contains around 18 million photodiodes.

The EOS 70D, with a 20.2 megapixel sensor, has two photo diodes for every pixel. There are 40.3 million photodiodes on the EOS 70D’s sensor – more than any other EOS camera.

When shooting stills, each pair is read together, and acts as a single photodiode.

But when for focusing purposes, the camera reads each photodiode in a pair separately. One photodiode looks towards the left side of the lens, and the other towards the right. The camera compares the readout from both and, if the subject isn’t in focus, use the phase-difference between the two parallax images to calculate how far to drive the lens to obtain accurate focus.

If you’d like to learn more about how phase-difference autofocus works, DP Review has an excellent article about it here.

A few more things about Dual Pixel CMOS autofocus :

  • It covers the central 80% of the camera’s sensor. It can only focus on a subject within this area. This diagram shows the area covered by Dual Pixel CMOS autofocus, compared to the 19 AF points available when shooting stills:

EOS 70D autofocus

  • It works at apertures of f11 or greater. At smaller apertures the camera will not enable Dual Pixel CMOS autofocus.
  • It works with face detection to keep moving subjects in focus. In Face+Tracking mode, the camera automatically detects faces and keeps focused on the face even if the subject is moving.
  • In Flexizone Multi mode the camera provides you with 31 autofocus points to choose from. You can select a single AF point or one of nine zones of grouped AF points.
  • There are 103 Canon lenses that support Dual Pixel CMOS autofocus, including the entire current range. Some older lenses don’t. There’s a list of Canon lenses with limited compatibility here. At the moment Canon haven’t released any information about third-party lens compatibility.

Conclusion

According to Canon Dual Pixel CMOS autofocus requires no extra processing power and is fast and accurate. It’s also good news if you’re a casual user interested in making movies, but have been previously put off by the difficulties of autofocus.

How good the system is remains to be seen and will become apparent as the first user reviews and reports for the new camera come in. But according to the ones that I’ve read so far, it seems to work extremely well. It’s probable that Canon will implement Dual Pixel CMOS autofocus on all future models. It may also mean that a new camera in the EOS M range will come soon, as that model has been criticised in reviews for sluggish autofocus performance.

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2 Responses to “Understanding EOS Autofocus: The EOS 70D”

  1. Mike says:

    Hi Andrew,

    An informative and easy to understand post on the technology in the 70D. Before recently upgrading my DSLR I was struggling with whether or buy a 7D or wait for the 70D. Mainly out of the need to replace my existing troublesome DLSR, and partly out of impatience after months of rumours about the 70D, I went with the 7D.

    The 70D looks a great camera but having understood its features and functionality I made the right decision in going with the 7D. I am, as you say, a stills photographer who shoots through the viewfinder, I have no interest in video whatsoever. I was interested to read that the Dual Pixel CMOS only works with apertures f11 or greater. I tend to work with f11 or smaller so, even if I did use Live View, the Dual Pixel CMOS wouldn’t operate.

    As a stills photographer I’ve noticed how Canon seem to be concentrating on developing the video side of their DSLRs. If that continues on future models it may influence buying decisions if and when I come round to replacing my 7D.

  2. Lyn says:

    Although I’ll never use Live View, I suppose the 70D is also better for photographing birds?
    And secondly I hope Canon will soon release information about compatibility with third-party lensen and the 70D.

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