"How bad is the [Fill_in_Z_Camera_Model] in low light autofocus?"
This question is impossible to answer without putting the answer into some sort of context. Moreover, camera settings and even firmware updates make a difference to my answer.
Consider the current Nikon cameras specifications (set without Low-light adjustments and using the viewfinder):
- -2 EV — Z5, Z7, Z50
- -3 EV — D750, D780, D7500, Z7 II
- -3.5 EV — Z6
- -4 EV — D500, D5, D850
- -4.5 EV — D6, Z6 II
0 EV is f/1 at 1 second, basically. EV is the exposure value, which is really a stand-in for the "amount of light getting to the image plane."
Even within one of those settings I tend to find some differences in performance. Remember, the DSLRs focus with the lens wide open, but the mirrorless cameras focus at the set aperture up through f/5.6. So if you're stopping down, the mirrorless cameras tend to lag the DSLRs just a bit in really low light (though in really low light you're not likely to be stopping down). On the other hand, if you're using the maximum aperture, the mirrorless cameras tend to be slightly more accurate in focus, all else equal.
Usually when I find someone talking about how bad the autofocus is in low light, there's something else involved. Often it's contrast. In particular, the Z System cameras so far are only sensitive to detail on the long axis. Sometimes just tilting the camera off horizontal slightly is enough to get good focus in conditions that are challenging. The DSLRs tend to look at detail on both axes; indeed, the D6 does so across all of its focus sensors, which makes it better than the Z6 II at the same low light level.
At other times the problem is underexposure. The EV numbers reported above apply to essentially a "correct" exposure. When you start underexposing, you're lowing the absolute EV. You also have to think of "exposure" as meaning the area where focus is attempted. It should be obvious that if you're trying to focus on black in low light you'll have a harder time than if you're trying to focus on white (and again, you need contrast in either).
There's little doubt in my mind that the Z DX cameras and the Z5 are the worst of the bunch in terms of focusing in truly low light. That said, my office is generally dark (to keep reflections off the display) and I happen to have a Z50 sitting on my desk, so I just did a quick-and-dirty test (at ISO 100, with the default settings in place). At about 2 seconds and f/3.5 (proper exposure) I started to have some mild hunting issues on a decent contrast subject. It wasn't many years ago when DSLRs couldn't attain focus in such situations, which gets me back to the context comment: how you view the focus performance of the Z cameras is likely to be influenced by what you were doing before using one. Using a D6 and moving to a Z5? Yeah, I can see exactly how you might feel that you're missing something in low light. Going from a D300 to a Z6 II? I suspect you'll have the opposite observation.
It also isn't as if there aren't things you can do in low light to help the autofocus system. In the extreme, turn Low-light AF On, turn Apply Settings to Live View Off, and boost your ISO temporarily to achieve focus. Some suggest using AF-S with Pinpoint AF-area mode, but I don't think you usually need to go that far (that last setting will force contrast detect to be used as the final focus positioning, and will slow the focus system down, though make it highly accurate).
I guess the real question is this: how often do you need to rely upon the autofocus system making quick work of finding focus in really low light? There's little doubt that the Z6 II is the best choice for you if that's something you are constantly encountering. However, in pragmatic usage, even with a Z50 I find it rare that I'm struggling with focus.
If you're struggling, in the order you should do it: (1) set a correct exposure; (2) make sure you're focusing on a contrasty area; (3) rotate the camera slightly to help find some short axis detail; and (4) invoke the Low-Light AF/Apply Settings to Live View/ISO help. Also note that larger autofocus area is better than smaller autofocus area, all the above being equal.
"I'm not getting great results with the 50-250mm lens. What's wrong?"
So let me tell you about something I'm still investigating. In general, I'm not having issues with the sensor-VR cameras. However, with the 50-250mm lens-based VR on the Z50 and Zfc bodies, things are a little different. Sometimes everything seems to work perfectly, sometimes I get slightly blurry results with a clear motion impact. (I don't see this problem with the 16-50mm, but then again 50mm isn't 250mm, either.)
I believe this to could be an interaction effect of the timing between the lens-VR and camera-focus systems. If VR is working hard before focus is achieved, I'm seeing worse results than if the focus is first dead on and then the VR system starts trying to correct camera handling. Small vibrations seem to create this problem more so than simple camera handling movements. For example, I see the problem when in a moving vehicle more than I do when I'm standing on solid ground (hmm, wonder what happens during an earthquake? ;~). Likewise, I see the problem when using the gripless Zfc a bit more than I do with the substantially-gripped Z50.
I first noticed this issue when photographing from a plane trying to isolate a ground detail. I kept getting sub-par (but still somewhat usable) results. When I examined these problem images versus the better ones taken with the same lens in the same session, I noted a small motion in the problematic ones. But also a small missed focus. You probably won't notice this inconsistency unless you're pixel peeping, because the difference is small, but at this point I'm convinced that there's something I need to pay attention to and understand. I just haven't 100% worked out how to avoid it. It appears that focus needs to be prioritized over VR, which means that stabbing at the shutter release totally spontaneously at 250mm produces worse results than first ensuring that I have proper focus and am doing my best to keep the camera stable while doing so.
"Why is there so much used Z camera/lens inventory available?"
Sampling and updating, basically. Let's take them individually.
Nkon's had some excellent pricing at times that has allowed a DSLR user to see whether "mirrorless is ready yet, or not." A lot of folk apparently didn't like what they found, though I suspect it was more that they didn't take the time to learn how to optimize their use of a mirrorless camera. These "not ready yet" users then sold the camera they sampled. (The ongoing supply chain issues have prices mostly returning to list price on new gear, so such sampling has reduced for the moment.)
I also keep hearing from folk who sampled a Z camera and then decided that it didn't take their photography forward enough to justify the money spent, so they dump it on the used market trying to recoup as much as possible. This has been particularly true for the Z50. Somewhat less true for the Z5. Even less true for a Z6, and it seems that the Z7 is probably the body least prone to this sample-and-dump business.
Personally, it took some time for me to get fully settled into my Z's, but they've pretty much replaced my DSLRs for most of my photography, with the D500/D6 being still somewhat better at certain tasks, so these are the DSLRs I use most often now.
The other interesting thing that happened is that a lot of early Z6 and Z7 owners ended up making a quick upgrade to a Z6 II or Z7 II. I was a little surprised at this—and I suspect so was Nikon—but it seems that the II changes were just enough to have folk get out their credit cards again. With a number of these folk, it was the USB Power Delivery that pushed them over the edge, as the II cameras suddenly became great streamers during the pandemic. Others pointed to the dual card slots (?!$#?), and still others pointed to the real vertical grip and the new "no overlays" display function. Whatever. Enough of you found reasons to make a quick second generation update, and that left a lot of first generation models on the used market (I should note that this same thing happened with the Sony A7 and A7R when the Mark II models appeared).
With lenses, I've noticed something slightly different: people opting for some of the higher-end lenses and then discovering that the gains are smallish, because the kit and other f/4 lenses are already really good. I've seen more f/1.8 and f/2.8 lenses come back onto the used market than I would have expected, given their high quality. This trend is not nearly as visible as the used body trend, but definitely can be measured.
"Will there be a Z8?"
I suspect so, but likely not any time soon. There's the simple issue of "what is a Z8?" If it's a body with an integrated vertical grip, then a Z8 pretty much has to be the studio camera to the Z9's speed camera. Which implies what, a 80-100mp image sensor? If it's a single grip body, then the Z8 has to be different from a Z7 III, which is itself likely to be a fairly high specified body in its own right. I suppose once again the distinction would have to come mostly in the image sensor. Another new sensor puts a lot of stress on Nikon's sensor group and all the downstream image quality work. Nikon's already juggling five image sensors, and probably needs at least three more to fill all the camera gaps.
Right now I think it's more important to imagine what the Z7 III will be than what a Z8 will be. If you can define the Z7 III, then you know what design space is left between it and the Z9.
My best guess is that Nikon is prioritizing the following models in their mirrorless lineup and we'd see them sooner than a Z8: Z50 II, Z90, Z5 II, Z6 III, and Z7 III. That's a lot of cameras to get honed just right. And plenty of product space to distinguish Nikon's engineering from the competitors'. Moreover, there's likely two new image sensors just in that group alone.
So I'm not worrying about what a Z8 is and what hole it will fill. It'll happen when it happens.
"Why haven't you reported about Nikon's two f/1.2 zoom lens patents?"
I'm trying to reign in the speculation some, basically. We have plenty to talk about with just the existing products and some known pending ones. Sprinkle in a few clear gaps that need filling, and that's more than enough to write full time about.
For those that aren't aware, Nikon some time ago (January 2019) patented a 35-50mm f/1.2 and 50-70mm f/1.2 zoom design. The Japanese patent application appeared in August of this year. Recently, a Japanese Web site discovered the Japanese patent and published the information, and that has now spread through to global photography sites.
My take on these two lenses is NOCT-like: Nikon was experimenting with what they could achieve optically in their new, far less restrictive lens mount. Would such lenses actually appear? Well, maybe, but I'd be shocked if they appeared before Nikon had filled in most of their known gaps and needs. Why? Because these fast zooms are not trivial undertakings. They'd be large—at least as big as the 70-200mm f/2.8—expensive, have a lot of complex glass with difficult polishing in them, and because of the narrow focal range, not particularly more useful than the 50mm and 85mm f/1.2. From a system standpoint, they wouldn't add a lot.
You might note a theme in the answers to these last two questions: yes, Nikon could do a lot of things in the future and they could be unique and intriguing. However, Nikon has bigger fish to fry at the moment. Nikon needs to completely shore up the existing Z System lineup, fill some more critical gaps, and keep the iteration pace up so as to stay in the leapfrog contest with Canon and Sony. Z8 and f/1.2 zooms are more in the halo product category than in the useful category, and Nikon needs to double down with the useful products right now.
The good news is that patents and leaked information shows that Nikon's engineering teams are back to trying to find the boundaries of what can be done, and very well could turn what they're doing into viable products, if needed. Reality, however, is that Nikon needs to sell a lot of US$1000-2500 cameras and even more US$500-2500 lenses. That's Job One in Tokyo. Speculation about Job Two (or more likely Job Three or Job Four) isn't going to help them much.
