Autofocus Tracking

The Nikon D810's phase-detect AF module does a great job at initially acquiring focus quickly, and refocusing on an approaching or receding subject, even in continuous bursts. We call this depth-tracking, and good performance in this regard is pretty much expected of DSLRs today, with camera body performance so good that the speeds of focusing motors in lenses become the limiting factor. 

More interesting, and more of a differentiator these days, is the ability of the camera to automatically track subjects as they move laterally in the frame, something we call subject-tracking. Traditionally, DSLRs have used distance information from the phase-detect sensors to understand where your subject is and where it's moved to, but this was never a very accurate or reliable way to subject tracking compared to actual, image-based scene analysis (see our video on depth- vs. image-based subject tracking). That's where metering sensors come in: while color-aware metering sensors have been used for some time now to expose your scene properly, Nikon started using its low resolution 1,005-pixel RGB metering sensor to 'see' the scene and understand your subject back with the D3. With the introduction of the D4, though, this sensor saw a nearly 100x resolution increase to 91,000 pixels. That sensor has trickled down to the D810, and it works wonders: finding faces or understanding the subject you initiated focus on to track it no matter where it moves to within the frame, something Nikon calls '3D tracking'. 

What the 91k-pixel RGB metering sensor 'sees', allowing for face detection and subject tracking. We scaled an image down to 213 x 142 pixels, then blew it back up for ease of visualization, then overlaid the D810's AF grid. 213 x 142 x 3 (for R, G, B) = 90,738, or approx. 91k-pixels. If there are 91,000 pixels each for any color channel, then the sensor actually sees approx.. 3x the resolution of what we're showing here. Click image for a clearer view

We can't emphasize enough how magical 3D tracking is: once you get used to trusting the camera to follow your subject for you, you may be hard-pressed to go back to using a camera with a lesser subject tracking system. Focus hit-rates and compositional freedom sky-rocket, particularly for moving subjects, when using 3D tracking. See this video on how 3D tracking helps focus on moving subjects, or aids the 'focus and recompose' technique by refocusing if you or your subject move in relation to one another, also avoiding the focus plane shift inherent to the focus-and-recompose technique.

Let's take a look at its performance.

Telephoto AF-C during bursts

With a Nikon 70-200mm F4 VR lens, we shot a continuous burst of images at 5 fps using AF-C '3D tracking', initiating focus on the our rider's shirt, and having the camera both (1) track her by selecting appropriate AF points as she moved within the frame, and (2) refocus on her during the burst. Only 3 out of the 12 frames here are critically unsharp, and even then, sometimes because it was distracted by the splashing water. The nearly 75% hit rate is quite good, considering we're conflating subject tracking on top of simple depth-tracking, while shooting a burst.

The frames that do not show an AF point indicate that focus did not lock, so the responsiveness of the D810 could certainly be better. But we got similar performance from a Canon 5DS R when shooting a similar, telephoto subject. While we've - unsurprisingly - found pro-level D4S or D5 cameras to be better at 3D tracking during continuous bursts, the D810 does a respectable job here, and actually can 3D track on par with the D4S when it's not shooting - something we'll look at next.

3D tracking: so accurate it can track an eye...

Although 3D tracking can struggle during bursts, it's incredibly accurate, and fast, prior to taking a shot. This means that you can track a subject you've initiated focus on, even as it moves or as you recompose, and when you're ready to fire the shutter, the subject will already be in focus. 3D tracking is so accurate it even tracked the eye of our mannequin successfully (you do have to specify where the eye is by initiating focus point on it with your selected AF point):

Compare this to a Canon 5DS R, which just cannot, with any high degree of accuracy, stick to the eye of the mannequin, and is easily distracted by other objects in the frame, even other parts of the mannequin's face. Interestingly, the 5DS R does just fine when tracking distant, telephoto subjects well-isolated in depth, which leads us to believe its subject tracking system relies largely on distance information from its phase-detect sensor to subject track (the old way of doing things). despite its high resolution metering sensor. The Nikon, on the other hand, is more robust: capable of tracking both telephoto and nearer subjects, with the latter capability seriously benefitting the fast, wide prime photography that is a staple of photojournalism, candid portraiture, wedding, and toddler photography. 

Real-world 3D tracking of a toddler

3D tracking has real-world benefits for tracking of erratic subjects, so that they're critically focused at the moment the shutter is fired. That's how we used 3D tracking in our test below: we initiated focus on the toddler's eye, then kept the shutter half-way pressed for this entire series of shots, firing off the shutter at random intervals. As long as we didn't release our finger completely from the shutter (always kept it at least half-pressed), the camera remembered that we'd initiated focus on the eye, and almost always kept either that eye, or the other eye, in focus. That's the 91k-pixel RGB metering sensor and scene analysis system at work: it has enough resolution to understand the eye is what I want in focus (since I initiated AF over it), though not necessarily enough resolution to always distinguish one eye from the other.

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Compare this almost perfect performance to the Canon 5DS' ~30% hit-rate with its equivalent of '3D tracking': ITR. The Canon fares far worse; in fact, you just can't rely on a Canon DSLR to shoot in this manner. Both the D810 and 5DS R were shot with on-brand 35mm primes, at F1.8.

Real-world d9 Dynamic AF performance

We also tested out a more traditional focus method: keep your AF point or cluster of points generally over your subject of interest, reframing as necessary. We used d9, in case our framing was a little off and the eye wasn't quite under the selected point - sports and action photographers often use this mode so that they're not required to keep one point perfectly centered over a moving subject. Again, we kept the shutter button always half-depressed, firing off the shutter at random intervals (this was not a continuous burst).

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Again, the D810 has a nearly perfect hit-rate. At F1.8. Compare this to the Canon 5DS' ~50% hit-rate in the same test with its comparable 'Expand AF Area: Surround' mode.* You can just trust Nikon's subject tracking system to not get confused, and generally help you nail your subject whether you use 3D tracking of the Dynamic AF area modes in AF-C. This is very confidence inspiring when you're shooting erratic subjects or wide open. Subject tracking performance can drop during bursts, but that can be said of nearly all cameras, and if burst performance with subject tracking is critical to you, you should pick up a D5.


* Disclaimer: We note that there's a possibility that Canon's default acceleration/deceleration rates may not be optimized for this type of shooting. However, considering the 5DS' general lack of accuracy when it comes to subject tracking close subjects that aren't well-isolated in distance - like the eye of a face - we don't have much confidence in the area modes outside of single point AI Servo compared to equivalent modes on modern Nikon DSLRs. That's not to say, however, that the 1DX series aren't top-notch sports cameras - we intend to put the 1DX II's AF system to the test in our upcoming review.