The auto-focus (AF) system in the 1D X has been overhauled. Instead of using a single AF sensor, the 1D X collects data from the AF sensor, the auto-exposure (AE) sensors, an AF correction light-source detection sensor and, with certain lenses, a panning detection gyro sensor. Auto-focus points have increased from 45 points (39 cross-type) on the 1D Mark IV to 61 points (41 cross-type, the number will vary depending on the lens you're using). These AF points' area of coverage has remained more or less the same. Compared to the 1D Mark IV which could focus in dim environments equal to EV -1, Canon claims that the 1D X can focus in even darker conditions to EV -2.
The 1D X AF system also comes with something completely new that Canon calls EOS iTR AF, a mouthful that stands for EOS intelligent Tracking and Recognition AF (which yet another mouthful). In short, instead of using only contrast-detection to hunt for focus, the 1D X can now detect colors and faces, and use this additional information to inform auto-focus as well as tracking focus.
EOS iTR AF can be switched off in the menu, but we found that it does make a noticeable difference. With it switched on, the 1D X does a better job of automatically finding faces, and also a better job of tracking AF (more on how the 1D X's particular brand of tracking AF works below).
The shooting menu and AF area selection mode has been carried over from the EOS 7D and the 5D Mark III. Auto-focus can be configured in the menu according to six different use cases:
You can also go in and fine-tune three parameters inside each use case; tracking sensitivity, locked on timing and responsiveness. This all means you have a boatload of custom AF settings you can choose from, which is a great help if you know your subject and how they're likely to behave. We shot a couple of netball matches and found that switching AF use cases does make a difference in how the AF performs, but it's definitely a learning curve to find what works best for what you're shooting.
There are six AF area selection modes to choose from:
They're enough to cover you from determining AF for still, as well as moving subjects, and similar to what's come before in the 5D Mark III. What we'd like to point out is how differently the 1D X handles that last one, 61-point Automatic Selection AF. When you're shooting with that area selection mode and One-Shot AF mode (for focusing only once), the 61-point AF will automatically select a focus point out of all possible 61 points.
When switched to AI Servo AF (for continuous auto-focus) however, you'll need to select a focus point and lock focus first (akin to shooting with single-point AF) and the camera will then track that subject even as it or the camera moves, similar to 3D-tracking. The mechanism itself works well, when the camera locks on to a subject it tends to stay locked onto the subject even if it's moving. And like 3D-tracking it works quickly with the focusing and re-composing method of shooting (the single AF point can be shifted away from center).
It's when you encounter specific use cases that this kind of focusing can handicap you. When shooting sports where the action moves across a space quickly, like netball or basketball, by the time you swing your camera to where the action is happening you might not have time to find your subject, lock focus and then re-compose the shot. You certainly won't have time to shift a single-point across the screen to where your subject is if you want to skip the re-compose part. The worse case scenario is if you shift the camera to the action and the subjects move out of center (or where you've set the AF point), in which case firing the shutter results in beautifully focused images of the background, instead of the moving people.
This behavior is similar to the 5D Mark III, but with that camera you could set 61-point Auto Selection AF with AI Focus AF, which would switch the camera between One-Shot and Servo AF while preserving 61-point Auto AF where the camera will automatically select a focus point out of all 61 points.
While it's best to be able to find your subject by yourself so you get the correct AF lock, sometimes what you're shooting moves so fast that you just don't have the time to do so, even when using Zone AF. We wish that the camera could have kept the option to use true 61-point Automatic AF in both One-Shot as well as AI Servo AF, just like the 5D Mark III.
The 1D X's AF presents somewhat of a mixed bag. In good light, the AF is snappy and accurate at finding and locking on to subjects. Plus EOS iTR AF does seem to have the ability to detect and place emphasis on faces in a shot.
However, we found that our 1D X had a tendency to get confused in low-light, high-action environments. It would lock and track focus admirably, and then inexplicably lose focus, either racking out to infinity or suddenly focusing into the background (shooting with the 70-200mm f/4L USM and the 70-200mm f/2.8L IS II USM lenses, in AI Servo AF with the drive mode set to High-speed Continuous Shooting, and AI Servo 1st & 2nd image priority set to 'focus').
In this sequence below for example, the 1D X nailed focus on the main and obvious subject in the shots prior (not shown), and then oddly lost focus for the last two shots, racking out to infinity for no discernible reason.
While losing focus in a couple of shots out of a burst is not uncommon when shooting quick-moving subjects, we couldn't understand this particular behavior with our 1D X - especially with subjects that are so obvious. Also because it seems that the majority of AF reviews on the web are mostly positive. It turns out however, that the 1D X does have a certain issue with AF tracking in low-light. According to a Canonrumors.com forum user, Canon Professional Services Australia says that while the 1D X can focus at EV -2, that is only good up to shooting at 10 frames per second. At 12 fps the focusing ability drops to EV -0.5:
On the first issue of AF tracking in low light. Everything you found was correct and the camera needs to be dropped back to 10fps to AF in the lowest light conditions. At 10fps the camera can focus fast and effectively @ -2 Ev Level but at 12fps the Ev Level is changed to -0.5 Ev. This is because of the light reduction when the mirror moves at that speed. The camera does this automatically. This is what you were experiencing under the conditions of the cat walk. This is comparable to all professional 35mm products out there on the market.
Canon Singapore has confirmed with us that the above statement is accurate, and we think it's worth pointing out that we couldn't find the same information in the 1D X's instruction manual, nor spotted it in any of the marketing material we've seen thus far.
In that particular thread, other users also report the same problem as we've found (and some report having no problems at all). The solution seems to be restricting the High-speed Continuous Shooting mode to only 10 frames per second, at which AI Servo AF does much better. But in our own tests, dropping the shooting speed to Low-speed Continuous Shooting (approx. 3 shots per second) didn't make much of a difference. The only change that improved AF was switching from AI Servo to One Shot AF.