Autofocus precision

Processed to taste in Adobe Camera Raw. Nikon AF-S 85mm F1.8G.
ISO 64 | 1/200 sec | F2.8

Nikon says the D850 has the same autofocus system as both the D5 and D500, with 153 AF points (99 of which are cross-type), a dedicated AF processor and the ability to focus down to -4 EV with the central point. Indeed, we titled the D5 review with the phrase, 'Setting new standards,' due in no small part to the D5 having the best phase-detection autofocus system we'd ever tested.

So, does the D850 follow through and deliver D5 levels of autofocus performance? As it turns out, it's not so cut-and-dry.

Processed to taste in Adobe Camera Raw with sharpening and noise reduction left to default. Nikon AF-S 85mm F1.8G. Camera placed on tripod, single off-camera flash.
ISO 64 | 1/100 sec | F1.8

What good are 45.7 million pixels if your image is out of focus? Fortunately, it looks like the D850 exhibits excellent autofocus precision much of the time - focus errors are mostly linked more to the shortcomings of DSLR autofocus system design, which we'll get to in a moment.

For the image above, we shot ten images with a very patient and very still model, and de-focused the lens before each shot. We alternated between setting it to the minimum focus distance, and then to infinity, and then initiated autofocus with a point placed directly over her eye, using a single point in Single AF. This image is the softest example from that set, as it is very, very slightly front-focused. Indeed, you need to look at 100% to even really see this, and every image exhibited more than an acceptable level of sharpness.

We ran this test with both the 85mm F1.8G and 105mm F1.4G lenses at fairly close range with stationary models and had largely the same results - consistency and precision are sufficiently high as to be a non-issue.

Processed to taste in Adobe Camera Raw, sharpening and noise reduction left to default. Nikon AF-S 105mm F1.4E
ISO 64 | 1/200 sec | F1.4

That said, Nikon continues to lack a spot focus mode - if there's something other than your intended subject that's close to your AF point, you may find that you're missing focus. This happened in particular with attempting to lock on to a motorcyclists' eye through his helmet - the camera would focus on the helmet only, and in the end required manual focus adjustment.

After looking at AF precision with some close-up subjects, we took the 105mm F1.4G out for some wide-open, full-length images, and ran into some issues.

Processed to taste in Adobe Camera Raw. Nikon AF-S 105mm F1.4E
ISO 64 | 1/200 sec | F1.4

Click through on the above image to see how the Nikon back-focused despite the autofocus point being placed directly over the subjects face. At this particular distance, this happened repeatedly. Not wanting to AF fine tune a lens in the middle of a shoot (seemed rude), I switched into Live View, which uses the main image sensor and contrast detection to focus incredibly accurately, and all was well. Then I moved in closer and resumed shooting through the viewfinder, and lo and behold, the 105mm was tack sharp again.

However, repeated testing has shown both that lenses (including our 105mm F1.4) can require different fine tuning for different focus depths. Furthermore, we found the results change, based on whether the lens starts at close focus or infinity: a source of error that calibration can't address.

Processed to taste in Adobe Camera Raw. Nikon AF-S 105mm F1.4E
ISO 64 | 1/200 sec | F2

So it certainly seems that the D850's autofocus system is capable of extremely promising results. And though its operation isn't obvious and can be a little cumbersome, Nikon's Automatic AF Fine Tune feature makes it easy to calibrate your lenses at a given focal length and given distance-to-subject. But therein lies the issue; there is no provision to calibrate a lens at different focal lengths (for zooms), or different focus distances, or for anything but the central AF point. When you've got F1.4 primes and 45.7MP of resolution, a small calibration can be the difference between the perfect shot and a throwaway shot: the D850's resolution ends up highlighting a problem inherent to DSLRs.

So now that we've addressed some aspects of the D850's autofocus precision, let's take a look at how its autofocus tracking works.

Image processed to taste in Adobe Camera Raw. Nikon AF-S 105mm F1.4E.
ISO 64 | 1/200 sec | F5