Particularly now that we have the Z9, it's time to go back and make some points about autofocus and how it differs between the Nikon DSLRs and the Z System cameras. The Z9, in particular, starts to eradicate the last remaining DSLR advantages.
So let's examine a series of focus traits and see where we stand:
- Focus precision — A slightly mixed story. On static subjects, the Z cameras focus more precisely than the DSLRs, and particularly so as you move from f/4 lenses to faster lenses (especially true of f/1.4 and f/1.8 primes). That said, the geometry of the DSLR system's math is a little more precise, though the DSLR focus system doesn't always take advantage of that. With moving subjects, the DSLR cameras long held the advantage. At least until face/eye detect, and now the Z9's subject detect, came along. The Z9 I'd now say is slightly better than the D6 for many moving subjects, but those two cameras are both in a category that almost no other cameras match to start with. (No, a Sony A7 Mark IV doesn’t match a D6, as much as some will insist.)
- Focus speed — Nikon has always been dinged on focus speed. Technically, Nikon cameras do tend to think just a little more about where to put focus before doing so than do Canon cameras (and to lesser degree, also Sony). This gets talked about often as "focus acquisition" speed, though there are other variables in that general category (e.g. lens motor performance). Here, I see a lot of variability across cameras and lenses, even within Nikon's lineup. The D5/D6 distinguish themselves from the D500/D850 in getting to initial focus, probably because they have a faster, dedicated focus CPU to do the work. All the Nikon cameras, DSLR or mirrorless, do have a tendency to miss initial focus if the lens is far off the actual focus distance when you press the button (which is why I've always suggested keeping your lens pre-focused at about the distance you expect to be photographing). The Z9 is better at this than the Z6/Z7 cameras, but more like a D850 than a D6. In some cases, I can see the exact way the Z9 is thinking with subject recognition turned on: recognized human, now see upper body, now face, now eyes, let’s stick with the eyes. In good light with well distinguished subjects, that sequence is near instantaneous, but in poor light with partially disguised/turned subjects, not so much. In terms of the camera part of focus speed, the D6 and Z9 and nearly equal in most cases, though those cameras are ever so slightly slower to initial focus than the Canon R3/R5/R6/1DX. So slightly that it's extremely difficult to measure, but it's a real thing that's different between Canon and Nikon.
- Focus tracking — When Nikon added 3D-tracking to the DSLRs with color and pattern awareness, tracking objects—even ones that went outside the focus region briefly—became uncanny. Initial attempts on the Z cameras (called Subject tracking) didn't quite provide the same level of assurance. 3D-tracking on the Z9, however, brings mirrorless much closer to the DSLRs in this respect, though not perfectly so. I see "tracking drift" on occasions with the Z9 that I don't see with my D6, which either tracks or doesn't. Also, the CSM #A3 options have a real impact with some AF-area modes on the Z9, but not for 3D-tracking. Finally, on a Z9 if you start 3D-tracking on an out-of-focus subject, it doesn't track as well (probably because the color/pattern it noted aren't the same in focus as they are out of focus).
- Dynamic and Group focusing — Most Nikon users don't fully understand these two AF-area modes, so let me cut to the chase: mirrorless and DSLRs seem about equivalent with their Dynamic-area focus choices these days, but DSLR still has the advantage of Group focus. What's that advantage? A real guarantee of closest subject priority within the focus constraint box (and a wide choice of those boxes on a D6). Nikon has confused the closest subject priority on the Z system cameras: if a subject is found, it won’t use close focus as a priority. If a subject is found, it will usually use close focus as a priority, but not always (bright backgrounds are a distraction).
- Short axis focus discrimination — Another DSLR win, particularly on cameras such as the D6 where all focus detection is bidirectional (both horizontal and vertical axis covered equally). All of Nikon's mirrorless cameras use the old Nikon 1 system: rows of focus-aware photosites, and those rows are spread apart by 12 pixels. That means that focus information on the mirrorless cameras is mostly discriminating on the long axis. The problem is this: if you try to focus on part of a subject that doesn't have much long axis detail but has short axis detail, the Z cameras don't focus as well, if at all. The top DSLRs don't have this issue.
- Focus in low light — A tricky question (see also Focus precision, above). The DSLRs often seem to focus fast in low light, but certainly not precisely with truly fast aperture lenses. This has a lot to do with the outer image circle areas that influence the focus decision: the fast DSLR primes have a lot of spherical aberration and other traits that provide false clues to the focus system (and if you’re not using the fastest aperture, a lot of the DSLR primes have focus shift in them). The fast Z-mount lenses tend to have much better behaved outer image circle optics, and the on-sensor geometry (no mirrors and alignment) also favors them. A Z system camera with a fast lens set properly tends to focus in situations where my eyes can't. I can't really say that with my DSLRs. However, here's a caveat: the Z system cameras need to have the exposure at the focus point set right. Underexposure at the focus point makes them focus more poorly than the DSLRs.
- Focus region — Because of the DSLR phase detect system geometry, accurate focus is restricted to a narrower region of the frame on full frame cameras than on mirrorless cameras. The D6 expands that out to almost the area described by the rule of thirds points, which means that you're only covering about the central 11% of the frame. All the Z cameras cover about 90% of the frame, a decided advantage (assuming you can move the focus sensor to your subject fast enough ;~).
- Focus magnification — A clear mirrorless win. You can magnify the viewfinder instantly while composing with the Z cameras—at the expense of slowing the viewfinder frame rate down—but you can only magnify images you've already taken with the DSLR. (Technically, you could use a physical magnifier accessory on the viewfinder with a DSLR, but those are a clumsy option and not very practical except for perhaps, macro photography.) Mirrorless gets a bonus. On a Z9 you can both magnify the area in the viewfinder and use focus peaking simultaneously.
- Focus aperture — (Before we drop into this subject, I need to point out that most phase detect systems are "best" at around f/4, for reasons I won't go into here. The phase detect "sweet spot" tends to be from f/2.8 to f/5.6, with apertures outside that range starting to add complications that impact focus.) This is a clear win for mirrorless. DSLRs always focus at maximum aperture, which is fine until you start adding teleconverters to slow lenses, or try to use a really fast lens (see Focus precision, above). It's not so fine if the lens has any focus shift to it (different focus achieved at different apertures with the same focus element position). The Z cameras focus at the aperture you set up to f/5.6, so focus shift influencing your results would only happen at apertures above f/5.6. But the Z cameras also aren't filtering the light through a partially silvered mirror, so they also tend to work for focus at incredibly small apertures. Most DSLRs stop focusing around f/8 as a maximum aperture because enough light doesn’t get to the focus sensors. I've used f/16 lens (equivalent via teleconverters) and still had my Z camera acquire focus with no trouble (note that this intersects with low light, though; you can't expect f/16 to find focus with the light at -6EV ;~).
- DOF viewing — With DSLRs we had a DOF Preview function, but this dims the viewfinder incredibly in order to show you that, and that's distracting and problematic for some. The Z's give you precise DOF viewing (with Apply settings to live view set) up to f/5.6. Above that, you have to resort to a different tactic to see DOF, but you can see it in the viewfinder without dimming.
So, The Z's are doing quite well compared to the DSLRs post Z9 (and firmware updates for the other models). I look at things a bit differently than this point-by-point view of the world, though. I have a D6 (best Nikon DSLR) and a Z9 (best Nikon mirrorless). Do I feel one is clearly better than the other at focus? No, they're somewhat different, and I have to set and manage them differently. For the sports and wildlife photography that I mostly do, both cameras work exceptional well for me once I've got them set properly. For landscape and macro photography, I'm preferring the Z7 II to the D850 these days, mostly because of focus precision and focus magnification. Your mileage may vary.
