This is one in a series of posts on the Nikon Z7. You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series; just look for “Nikon Z6/7”.
In response to yesterday’s post, I got a question:
Are you suggesting that the autofocus accuracy of the Z7 is “off-the-charts” better than a D5?
If so, in what situations?
The short answers:
- In static or less-than-violent dynamic situations, with better-than-abysmal lighting.
The long answer follows.
By the dawn of the CaNikon DSLR age, autofocus (AF) reigned to such an extent that the SLRs manual focusing aids had either disappeared or become vestigial. Autofocus was fairly mature, and the early DSLRs were low-res by today’s standards, and AF worked just fine. As the DSLRs passed 16 MP, AF accuracy started to become iffy in some situations. But liveview came along and saved out bacon in static work where high accuracy was required. I’ve had three D3’s (counting the S versions). I always thought the AF was just fine. When I got the D4, with its fractional resolution increase, AF was no problem. But with the D800E? Not so much. Part of the problem with the D800E was that I was shooting static subjects with it, as opposed to what I was using the D4 on. A big part of the problem was the greater resolution of the D800E. But a big part of my dissatisfaction with the D800E AF was that I was by then putting my toe in the MILC world (we called mirrorless cameras EVIL in those days — it stood for electronic viewfinder interchangeable lens), and even the relatively primitive AF systems in those cameras afforded better accuracy on static subjects than DSLR AF.
Moving along to the situation in the last couple of years before the Z7 appeared, I found myself with a D5, a D850, and several Sony full frame MILCs, including the a7RIII and the a9. By then, the AF performance of the MILCs had come a long way, and, in spite of DSLR improvements in AF dynamics and low-light sensitivity the MILC/DSLR comparison s were striking. For static subjects, and even for those not exhibiting violent motion, the MILCs were head and shoulders above the DSLRs for AF accuracy. For sports and the like, the best Sony camera I’ve used is the a9, and the top DSLR is the D5. Performance in that environment is god-top-excellent in both cameras. Each has its strengths, but overall I’ve give the nod to the D5. I suspect that some of that superiority comes from the generally faster-focusing Nikon lenses (I use the Nikon big iron lenses for sports, and Sony only has one of those, the 400/2.8, which I had on order before I got the Z7, but have since decided not to get).
But this is mostly a post about static AF accuracy, and here’s a rather dramatic demonstration of DSLR/MILC differences:
What’s plotted is the errors in focusing for 16 shots at each f-stop, at the sensor plane with the same lens — the Nikon 58 mm f/1.4 — on both the Z7 and D850. Zero is perfect focusing, and front-focusing is indicated by negative numbers. The green dotted lines are for the D850, and the red solid ones for the Z7. Luminance sharpness is the criterion for the plane location calculation. Not only are the mean errors much smaller for the Z7 than for the D850, the scatter, as indicated by the distance between the thinner plus and minus one standard deviation lines is less with the Z7. Note that the red channel longitudinal chromatic aberration (LoCA) of the 58/1.4 pulls the luminance error upwards at f/1.4, as the camera is optimizing for the green channel.
Micrometers of focal plane error may not mean much to most photographers, but many technically adept ones relate to circles of confusion (CoCs) due to misfocusing:
I know that you can’t have negative CoC diameters, but I’m choosing to indicate front-focusing that way. The pixel pitch of both the D850 and the Z7 is 4.35 micrometers. The Z7 errors are mostly less than one pixel pitches. That’s pretty darned sharp. The D850 errors get well over 2 pixel pitches. Not exactly blurry, but not critically sharp.
Why the disparity?
MILCs in general, and the Z7 in particular, have a couple of arrows in their AF quiver that are denied to DSLRs.
First, they can focus at narrower apertures than wide open. This is actually more complex than it appears, since neither the phase-detection autofocus (PDAF) systems in MILCs nor the ones in DSLRs respond equally well to light rays from all parts of the lens, but the DSLRs need to have their lense wide open during focusing to keep the optical finder nice and bright. In a MILC, the finder brightness is whatever the camera firmware wants to make it, and the Z7 takes advantage of that by focusing at taking aperture for all f-stops wider than f/6.3. The DSLRs are subject to focus error due to lens focus shift upon stopping down (which is in turn a result of spherical aberration in the lens). The MILCs can finesse some of this by aperture management that is independent of finder brightness.
Second, MILCs can let the PDAF system get close to the right focus, and then trim the focus plane with a quick application of contrast-detection autofocus (CDAF). PDAF has a nasty little feature: as you get closer and closer to the right (or right by some internal logic) plane of focus, the PDAF signals get smaller and smaller, so noise in the system can throw the results off. CDAF has it’s own quirk: you can’t tell if the image is in focus except by defocusing it slightly, and then you’ve got to put it back where it was. Still, the CDAF trim phase produces more accurate results, and also has less scatter (as measured by the distance between the mean-plus-standard-deviation and the mean-minus-standard-deviation lines in the above graphs).
DSLRs do have an advantage in really dim light, since their PDAF systems can have sensors that grab a lot of photons, while the MILCs have to steal their PDAF sensors from pixels that would otherwise be employed simply to capture images. DSLRs have AF trimming to try to help things out. Unfortunately, AF trimming is stored in the D5 and D850 as one setting per lens, regardless of f-stop, subject distance, and, in the case of zooms, focal length. So any such setting has to be a compromise. The 58 mm f/1.4 in the above test was optimized at the chosen target/subject distance at f/1.8. It has a lot of focus shift, but it is not alone in that regard in the Nikon lens stable.
So, for static AF, the MILCs are the clear winners in this contest. It’s not just the Z7; I’ve seen the same kind of thing with the a7x and a9 cameras, and with the Fuji GFX 50S, which doesn’t even have PDAF and has to do all its autofocusing with CDAF.
As an aside, it has not escaped me that the question at the top of this post concerned the D5, but that I responded with information about the D850. That’s the quantitative information that I have readily at hand. When I was testing the D5, I had not yet created the sophisticated focus accuracy tests that produced the results in the above graphs. I find the D5 and the D850 to be pretty much peas in a pod for static accuracy, although at a pixel level the errors affect the D850 more because of its increased resolution. I think the D5 is somewhat better than the D850 in dynamic situations.
This is one of the major reasons why I’ve embraced MILCs. That and I’m never doing the “AF fine tune” nonsense again.
You’re referring to a notion of abysmal lighting. At what kind of exposure would AF errors in the Z7 start to become signficantly greater than the results shown above (especially when focusing at smaller apertures)?
Also, I’m surprised you are including moving subjects in your claim. No doubt about the measurements you present, but those are with a static, well-lit, contrasty target, probably at base ISO; what makes you think this level of accuracy will (would?) hold up with (non-violently-)moving subjects under bad lighting at high ISOs? Especially considering the Z7 will be focusing at apertures as small as f/5.6 under these conditions?
(And I’m not disputing your claim, mind you—I’m just surprised by it. But surely you have good reasons for it, which I’d love to hear.)
I just have anecdotal results to justify that opinion. Hard to test, at least for me. Here’s something you may find of interest, although it does not talk about the Z7 specifically, and emphasizes the situations where AF errors happen:
I’ve has my Z7 for a year now and it has horrible horrible focusing problems in low light compared with my old D800, and I’m now thinking of going back to a DSLR model like the D850. By low light problems I’m referring to shooting event photography of people indoors during the day or night. Night is typically worse, and of course brighter rooms are better than dim rooms, but I even have problems in fairly well lit rooms. Imagine a few people standing in front of you waiting for their photos To be taken at an event and they are waiting, and waiting, and waiting, and waiting for the shutter to go off, but your camera keeps searching and searching and searching for a focal point. Embarrassing. Regardless if in focus auto select, or manual point auto focus, I can keep pointing the sensor at different contrast areas in the frame and frequently get nothing. It’s embarrassing to keep people waiting, and sometimes I need to fake as if I got the shot (by holding down the autofocus lock button on a blurry shot) just so they can be released from their perpetual waiting zone. Sometimes the only place I can get enough contrast to focus on is the top of their hair where a ceiling light might be illuminating which makes my focal point slightly off (which is only slightly better than totally off, and equally embarrassing to show a client a slightly blurry photo). I’m frustrated and hope someday Nikon’s mirrorless focusing can be good enough to not ruin tons of my photos!
I very much appreciate your blog, making your analysis results available to the interested public creates a highly valuable source of information, especially given far too many other internet and youtube contributions are often skin-deep, and redundant.
Reading your articles regarding the Z series AF, may I ask you
1. Can we assume that FTZ-adapted lenses are always operated with PDAF only?
2. Speaking of native Z-mount lenses, are they always operated with PDAF + CDAF trim, or are there excpetions with PDAF only?
3. Regarding accuracy, did you notice improments after the firmware updates?
4. I read from another of your articles that the Z6 AF accuracy is (much) better than the Z7s, did I understand correctly?
I recently rented the Z7 with FTZ-adapter and 24-70 f/4 kit lens, firmware3, I did not notice any focus hunting at all with an adapted 300mm nor the native zoom lens. Though the 300mm was subjectively better with the Z7 than with the D7100 I used to own, I was still not overwhelmed with the AF precision.
Thanks and best regards
No. I don’t know whether that’s true or not.
I don’t know.
No. The two cameras have similar focusing accuracy.
Thanks a lot, Jim, also for your reply on the other topic.
Assuming the CDAF trimming makes up for a big portion of the higher accuracy, and we don’t know if/when this trimming is applied, then we cannot assume much more than that accuracy is higher under your specific test conditions, used lenses, distances etc.
If not even you know, then I think I can stop searching the web :-), buy one of the Zs and make my own long term experiences (need to do this anyway)
Do wonder if the z7 af accuracy stayed the same over time from 2018 to now 2020 giving the upgraded firmware. Improved performance vs af accuracy?