"What's a good telephoto lens for the Z50?"
This is a trickier question to answer than at first glance, as it would seem that you've got a plethora of choices. The Z50 body is small and light, though, so be forewarned that sticking a big, huge telephoto on it—for instance, the 400mm f/2.8 S—makes for some clear handling issues. You end up with a small handle on a big lens.
I've been of the opinion for some time that the most appropriate longer telephoto for the Z50 is the 300mm f/4 PF on an FTZ adapter. It's the right size and weight to keep handling from becoming an issue when using the combo hand held. You're effectively at 450mm, so you're awful close to what it would be like using a 500mm f/4 on a full frame camera. It's a nice combo that produces excellent images. You're also 75mm longer and a stop-and-a-third faster than using the kit 50-250mm f/4.5-6.3, which I think is enough of a difference to justify just on that alone. The 300mm f/4 PF is also better optically than the kit lens (it should be for the price differential). So win, win, win.
Beyond the 300mm f/4 PF, the choices get a little tougher to justify, typically because of handling issues. The other two options I've used that I've found acceptable, but a little unbalanced, are the 70-200mm f/2.8 S and the 100-400mm f/4.5-5.6 S. Both are about the same size and weight for packing (though the 100-400mm extends during zoom). The former gives you a fast 105-300mm equivalent, which is pretty nice for sideline sports, while the latter gives you 150-600mm, which is the DX equivalent to the upcoming 200-600mm. However, hand holding 600mm effective is not for the timid. And, of course, the Z50’s autofocus capabilities aren’t as good as the full frame Z’s.
Fortunately, all my above choices have lens-based VR, which is good because the Z50 doesn't have sensor-based VR.
What about the 400mm f/4.5? I'll put that lens in the same category as the 500mm f/5.6 PF: a bit too big and too much focal length most of the time. I'd tend to opt for the 100-400mm for the flexibility if you think you need 560mm effective focal length.
"I've got a slight scratch on the front element of my [Fill-in-the-Blank-Lens]. What do I do?"
Reduce your net worth by half the value of the lens on the used market.
Seriously. That's it. The primary issue you're going to deal with is that any disruption to the front coating on a lens reduces its value on the used market. Mainly because everyone thinks there's another issue that will arise: clear loss of image quality.
I've seen photos taken with a lens that was cracked in its front element that seemed fine. It takes a really big problem in the front element to show up as a quality issue back at the image sensor. If indeed your scratch is slight, I simply wouldn't worry about it from an optical standpoint. It's possible that some scratches might have an impact on flare or contrast in the area, but boy I've had a difficult time distinguishing that in results.
Note that some older exotic AF-S lenses actually had a protective glass element that can be removed (and replaced), mostly because replacing the big curved front element that started the true optical chain was expensive. Originally, those front elements that unscrewed were flat glass, but eventually Nikon moved to meniscus-type glass so as to reduce veiling flare. It's perfectly fine to photograph without either type of protective element removed from those lenses. Indeed, it's my preference if I'm not worried about things hitting the front element (like baseballs ;~). The 400mm f/2.8 TC VR S does not have this type of element, by the way, so be careful with that expensive lens.
“It seems like you test Nikon Z lenses with a Nikon Z7 or Z9. But I own a Z6. My question is: how to translate measurements on sharpness made using a 45mp sensor to 24mp sensor?"
Excellent question, but the answer I provide is not the answer you were probably expecting.
A lens doesn't change characteristics due to image sensor pixel count these days. A lens should present exactly the same light field to the focal plane of the sensor, regardless of pixel count. Where some people get confused has to do with what they will call "detail" or "resolution.” Technically, the resolution of a lens is not dependent upon the image sensor. How much resolution an image sensor can record might differ, though.
With Nikon in particular, a 24mp and 45mp image should render the same, particularly when output at the same size (no larger than the 24mp sensor can sustain, e.g. a 20” maximum size print at 300 dpi with no cropping). Nikon's been very careful with EXPEED and all its attributes to make it so. The difference between a Z6 and Z7 with a given lens is simply pixel count on a detail. In essence this is similar to the "pixels on subject" thing that caused many to use a DX camera instead of an FX, all else equal. A Z7 would have more pixels on the same-sized subject than a Z6. Whether you will use those extra pixels isn’t a given (at up to a 20” print without cropping, you’re not, but you would be if you crop). Technically, depending upon how you output, with no crop there might be a slight advantage to edge acuity for the higher pixel count sensor, but this isn’t due to the lens. Likewise, if you crop, you can print the subject larger with the Z7 than you can with the Z6, but again, the lenses quality isn’t changing.
Nothing in what I write about lens performance in my reviews really differs between a Z6 and Z7 in terms of lens quality. If a lens is edge to edge sharp, it should be so on both 24mp and 45mp sensors unless I write otherwise.
In the early DSLR days, there were some sensor-caused differences in how a lens characteristic was recorded by a sensor. For example, a D2H didn't have small enough pixels to actually record the Airy disc of diffraction; the disc always fell on just one pixel. So unusually, the D2H could be used at f/16 without seeing any drop in "lens performance” due to diffraction. Again, the largest Airy disc fell entirely on one pixel (or slightly more than one pixel, but because of the demosaic techniques used, that wouldn't cause any change in final pixels). But that was because the 4mp D2H wasn't recording something in the first place, while a 12mp D2X had enough resolution for the Airy disc to be recorded at smaller apertures.
A 45mp sensor does resolve the Airy disc somewhat better than a 24mp sensor. Generally, the rule of thumb I use for figuring out the aperture where diffraction begins to have an edge softening effect—not a lens effect—is when the Airy disc becomes larger than the diagonal across two diagonal pixels (stated another way: 2x the diagonal of an individual pixel). Unfortunately, diffraction varies with spectral wave length, as well as how a Bayer demosaic is performed, as the demosaic uses neighboring pixels to form full RGB data. The exact math gets way beyond what I’m comfortable putting on a Web site like this (or taking the time to check that I expressed it right ;~), and different demosaic algorithms produce slightly different results, so the math probably isn’t useful to you unless you’re developing a demosaic. Still, note what I wrote: not a lens effect.
Thus, when I write that a lens is “excellent” in the center and “very good” in the (absolute) corners at a specific aperture, that would apply no matter which Z FX camera you put the lens on. I suppose there could be an image sensor in the future that might change that (e.g. Foveon-like layers work a bit differently than Bayer), but not at the moment.
“When will Nikon create an adapter to use Z lenses on the DSLRs?”
Asked before and answered before. But the question still keeps coming, mostly because people like me keep writing that the same lens specification in its Z wrapping is better than the the one in the F-mount.
Thing is, it can’t be done (at least not without adding lens elements, which distorts the outcome). Z mount lenses are designed to be mounted closer to the image sensor than a DSLR lens. You can’t just move them outward on a DSLR without changing what they do. In particular, they won’t focus on the image sensor any more ;~). The inverse is also true: DSLR lenses have a longer distance between their rear mount and the image sensor, so you can just “add a tube” with the proper Z-bayonet and use those DSLR lenses on a mirrorless camera.
You probably could build a “relay” adapter that put the Z lens in the DSLR position and then uses a complex optical addition to “relay” the focus plane to where it needs to be. But that would cost a lot to create, and it would likely impact the final image quality.
So just get used to using only film SLR and DSLR lenses on your DSLRs. Over a hundred million were made (including third parties), and there’s a flood of really good ones on the used market right now for excellent prices.
