It's time to talk about sample variation.
This is mostly triggered by a raft of reports of same on some recent Nikkor Z lenses, a couple of which I've verified. We've not really had a lot of sample variation in the higher-end Nikkors before, but I'm seeing some in a few lenses now, such as the 70-200mm f/2.8 S just released, which is disappointing.
The primary variation seems to be alignment. The way you test for this—at least large variations you don't want in your photos—is relatively simple with the 70-200mm:
- Mount the 70-200mm on a tripod using the lens collar.
- Take a photo with the camera in normal orientation.
- Loosen the lens collar and flip the camera 180° so that it is upside down, and take another photo.
Personally, I like doing this with detail on the diagonal that is the same distance from me for three focus distances: near close focus, 50-100 feet, and near infinity. Be sure your targets are parallel to the image sensor or else you've got a different problem ;~). The reason to use three targets is that focus elements move within the lens and their position might dictate what you actually see in side-to-side variation. I've seen a lens that was fine at normal and far distances that skewed when shooting close, and vice versa.
Both images you take at each distance should have (nearly) the same pattern of detail from center to corners. If you get clear blur only at the edge on one side that flips to the same blur on the other side between the two images, then there are alignment issues with the lens.
Here's where things get tricky. The most common thing that happens is someone discovers this, is that they send the lens into Nikon, Nikon tests it, and the report comes back "within tolerance."
So let's skip to something else that's been getting a fair amount of press lately, particularly from Lloyd Chambers: as sensor pixel counts go up and up, the mount/sensor alignment starts to become a critical issue, too. I don't know what Nikon's tolerances are on the 45mp cameras, but the reports of 61mp Sony A7R Mark IV's skewing 30 microns is definitely outside my tolerance range. Add in a lens alignment issue and things can get really bad, very quickly.
Now, I tend to shoot landscapes these days with the 19mm tilt-shift lens. I'm almost always using the Scheimpflug principle in controlling the the focus plane, so if my Z7 has a mount alignment issue it's likely not to be showing up unless I'm not controlling the tilt/shift correctly (image magnification with focus peaking is your savior here, as is slow, controlled work). Moreover, for most of my photography, a small alignment error probably isn't likely to show up. Certainly not on a 24mp camera.
But these things can be cumulative and destructive to pixel integrity. Imagine you have a lens alignment that's wrong and a mount alignment that's also wrong, and that they are wrong in the same direction. Yep: the gear may be "within tolerance" but the cumulative impact is certainly outside what you'd tolerate.
If camera makers are going to continue to pursue resolution gains through better optics and more megapixels—and I believe they should—we also need them to become more responsible in tightening and publishing their tolerances. Moreover, we need something better than a small choice of fixed sized shims that only deal with gross alignment issues, and we need the repair centers in each subsidiary ready to help a customer deal with cumulative mismatch issues.
Long-time readers know that one of the reasons why I'm not a fan of mount adapters or teleconverters is that the extra mount set introduces the potential for cumulative alignment disasters. When you mount an FTZ Adapter on the Z7 and a teleconverter on the long prime lens you've mounted on the adapter, we've got three mounts that all need to perfectly aligned, and unless you're good at winning lotteries, you don't have that.
While I'm picking on Nikon in this post, there isn't a camera or lens maker that's ahead of the game here. Simply put, it's time for them all to find a way to reduce manufacturing tolerances even more, let us know what those tolerances are, and truly work with customers to fix tolerance problems when they do occur. Nikon should lead the way.
