The Nikon D810 features the company's Multi-CAM 3500FX phase-detect AF module, a 51-point unit with 15 central cross-type sensors, 11 of which will continue to operate when the effective maximum aperture of a lens is just F8. The AF system cross-references distance information from its phase-detect module with color and pattern recognition from its scene analysis engine, which uses a 91,000-pixel RGB metering sensor to understand your subject and track it no matter where it moves to within the frame ('3D tracking'). The scene analysis system can even find, and track, faces if they're not too small in the frame.  

Nikon 3D Tracking. I used 3D tracking here to initiate focus on the eye. Despite the baby then being bounced forward and left, the camera tracked the eye and refocused quickly enough for me to fire off this single shot, in focus at F1.4. Shot with a Sigma 35mm Art lens. 

There are several AF point selection modes, most of which will be immediately familiar to existing Nikon shooters. These range from single point AF where you specify a single point, up to the Auto Area mode where the camera chooses from any of its 51 points (and focuses on faces or the next closest object). New to the D810 is Group Area AF (introduced in the D4S), where you place a cluster of 5 points (in a diamond shape) anywhere in the frame, and the camera gives equal priority to any one of those points, focusing on the point registering the closest subject. In AF-C mode you gain several modes between these extremes: the Dynamic Area AF modes (which consider a patch of 9, 21 or 51 points, if the subject strays away from your specified point), and 3D Tracking, which continually shifts the AF point as needed to follow the initially-specified subject.

You can cycle through the available AF point modes by holding down the AF button at the center of the AF/MF switch to the lower left of the lens mount and turning the front command dial (turning the rear dial switches between AF-S and AF-C). Custom Menu option a11 lets you disable any of the AF point modes you don't use, to make it faster to cycle through the ones you do.

AF point coverage

Nikon D810 
Canon EOS 5DS R
Sony a7R II

The Nikon D810 offers more frame coverage than its smaller sibling, the D750, while offering roughly similar coverage to Canon's 5DS/R. Its trumped by the massive frame coverage of the Sony a7R II's phase-detect points (in green), though. 

Other AF settings

Although the D810 inherits much of its AF system from Nikon's pro-sports D4S, it remains relatively easy to set up, with little in the way of configuration required. We really like the fact that the camera just tends to work 'out of the box' across a wide variety of focus scenarios, without requiring one to fiddle with hundreds of permutations of AF settings. There's only really one menu setting: Custom Menu a3 - Focus tracking with Lock-on that defines how persistently the camera sticks at the same approximate focus distance, rather than jumping to a nearer or more distant subject. 

On the whole, this setting (which defaults to 3) only needs to be changed if the situation you're shooting requires the camera to regularly ignore closer objects (in which case select 4 or 5), or if a significant change of focus distance to a different subject is likely to be needed quickly (in which case choose 1 or 2). Beyond that, you can choose whether capturing the moment or capturing precise focus is more important to you by biasing toward release or focus priority, respectively.

AF in low and challenging light

Cross-type sensitivity

Only the 15 central AF points on the D810 are cross-type points, which ended up sometimes being an issue in real-world usage. Comparatively, the Canon 5DS/R has 41 total cross-type points, many of which are relatively peripheral, and Nikon's own D5 and D500's sport 99 peripheral and central cross-type points. Wider areas of cross-type coverage reduce the risk of hunting for non-central subjects, by increasing the chances of locking focus particularly on low contrast subjects in low light, or in backlit situations, where subject detail is often difficult to lock on to. The video below demonstrates how the lack of cross-type points on the D750's similar AF module may be limiting in challenging situations where subject contrast is low.

Cross-type vs. non-cross-type AF points. This video illustrates how cross and non-cross-type points behave in challenging light. We start with the center point, which is cross-type, and then switch to an outer point, which is not. The non-cross-type off-center AF point struggles to focus on a vertical line (it's horizontally sensitive); meanwhile, the off-center point placed over a cross-hair with both vertical and horizontal detail has no problem locking focus. Turn the lights up, though, and all points work just fine. Focus is intentionally thrown off between each shot.

Another thing to note is that the center AF point locks focus considerably faster than the off-center AF point, despite both points focusing on a cross-hair. This is likely due to the center point being cross-type: it's more definitive at locking on to focus as it has more detail to evaluate.

The video clip above shows the Nikon D750's non-cross-type points having difficulty focusing on vertical lines in the dark (-2 to -3 EV). While the central cross-type points focus quickly and decisively, the outer points - sensitive to horizontal detail - hunt. That doesn't mean these outer points can't focus on vertical detail at all, it's just that the thing you're focusing on has to at least have some horizontal component to it. And the thing is - most real world objects do have some horizontal detail; however, In the dark, or when there's just generally a lack of subject contrast, all detail is harder to 'see', so the more axes along which an AF point is 'looking' for detail, the more decisive focus will be.

In the real-world, we did note better performance of the central 15 AF points across a wide variety of scenarios, with off-center points hunting in strongly backlit scenarios, for example. This is particularly a shame because we tended to use the off-center points more on the D810 (than a comparable Canon) due to its extremely intelligent subject (3D) tracking system for non-central subjects - rather than 'focus and recompose' using the center point.  

Low light focus

Despite being formally rated to -2EV (only one stop less than the Nikon D750), in real-world use, we were surprised to find the D810 failing more often than it should in low light. As part of the Sony a7R II review we compared the performance of the Nikon D750, Canon EOS 5DS and the Sony in extreme low light. The D810 simply couldn't keep up in this company. It even falls well behind its predecessor, the D800/E, which we found to focus down to -3 to -4 EV in our lab (it was under-rated). 

Surprised by our real-world results, we did some controlled testing using a tungsten bulb. We slowly lowered light levels on our subject until each camera tested below was no longer able to focus with the focus point under study. We then, in very minute increments, increased light levels until the camera was once again able to focus. This light level indicated the lowest level of light the AF system is able to focus in. In the examples below, the darker the photo, the lower the 'extinction' light level - the lowest light level at which the camera focused (indicated in parentheses). We used a scientific light meter, and F1.4 on-brand primes.

Nikon D810 (-1 EV) Nikon D750 (-2.3 EV, in some cases -2.7 EV)
Canon EOS 7D Mark II (-2.7 EV) Canon EOS 5D Mark III (-3 EV)

The D810's center point gave up at around -1 EV, while the D750 hovered around -2.5 EV* (-3 EV for a high-contrast cross-hair target, though), and the Canon DSLRs performed best: focusing down to nearly -3 EV. The Canon's also felt more confident at locking focus, likely a testament to their dual cross-type design that 'searches' for detail along four axes using f/2.8 and brighter lenses. Next, we look at off-center performance: 

Nikon D810 (-1 EV)
Canon EOS 7D Mark II (-2EV)
Nikon D750 (-2.7 EV)

Again, the D810 remains substantially behind its competition, with the Nikon D750 pulling ahead of all the other cameras in this test. The 7D Mark II falls slightly behind the performance of its lower-light-rated center AF point, only focusing down to roughly -2 EV.

Does this translate to real-world usage? Yes: when out shooting, we found the D810's autofocus will give up in very low light circumstances, particularly when there's little contrast for it to pick up on. While these circumstances may be rare for your type of shooting, we found it to be a nuisance at dim wedding receptions, or romantic candle-lit dinners, particularly considering we didn't have these issues with a Canon 5DS/R, nor a Sony a7R II as long as we had an F1.4 lens attached to it. Furthermore, quite often we found focus to be quite far off (back or front-focused) in low light, despite meticulous calibration under daylight - we can't help but wonder if accuracy (and/or precision) also suffers under low light.

Something to keep in mind is that AF systems rated to lower light levels don't only offer benefits at the lower threshold; rather, their increased sensitivity can come in handy even in slightly brighter, yet still low light, situations where subject contrast may be lacking. We don't always or even often, shoot between -1 and -3 EV, but when we do shoot in low light (even 0 or 1 EV), a system that performs down to -3 EV will typically focus quicker, and more confidently than a system that fails at -1 EV.

Live view autofocus

Live view autofocus and its options are much more simplistic, since the camera can only use contrast-detection to assess focus. This means performance is slower and much more lens dependent, since most Nikon AF lenses were designed for phase-detection AF, which requires a very different sequence of driving the lens. The camera offers four AF point modes: normal, wide, subject-tracking and face priority.

* If you're wondering why even the D750 and 7D Mark II didn't quite make it down to their rated -3 EV, it's likely because we're focusing on a lower contrast subject here. Ratings are typically based on idealized scenarios, such as high contrast targets - which doesn't always translate literally into practice.