There is not the same kind of space/market for a DX sports/action camera as it was in 2007 and 2016.
And this conclusion is based upon what evidence?
Back in 2007, both the D3 and D300 were 12MP, and those cameras had the highest pixel count on the Nikon lineup at the time. In 2016, both D5 and D500 were 20MP, while the D810 was 36MP but a slower camera. Therefore, DX had much denser pixels in those days. That changed a year and half later when Nikon introduced the D850, which has a similar pixel density as the D500 and essentially the same AF system. I know quite a few people dumped their D500, which was only a year or two old, in favor of the D850, although I wasn't one of those.
While the D500 and D850 shared AF tech, the D500 had the notable advantage of AF points covering most of the field of view. The D850's AF points were concentrated near the center of the frame. The D500 had better AF performance than the D850 as well as a faster burst rate and deeper buffer. For the dedicated bird and wildlife photographer on a budget, it was the clearly better choice.
Even if budget wasn't a limitation, if the D850 were focal length limited and its photos routinely required a significant crop, the D500 was objectively the better option. On the other hand, if one was filling the frame of the D850, it was an outstanding wildlife and bird camera.
Your definition of "space" is essentially "DX must have more pixel density than FX or there's no space." I don't believe that to be true.
In that case we clearly disagree, strongly.
If a DX body cannot provide significantly higher pixel density than FX, we might as well just crop from a Z8 or Z9. If such DX body has much higher pixel density, we are entering the diffraction zone.
Only when pixel peeping. When viewed in the whole - as most people do when looking at a photo - an image made with a higher-res sensor shows no more diffraction than one made with a lower-res sensor of the same format. If you're assuming the FX photo would be cropped to match the framing that lens would deliver on a DX body, we're talking same-format systems.
Moreover, I don't think Nikon can produce such DX Z body at the 2016 D500's $2000 price point. If it approach's the cost for a used/refurb Z8, it will be difficult to sell.
A true Z-mount D500 could easily be priced in the $2,200 to $2,500 range in today's market. While that's a price point Nikon is comfortable with when designing future products, I suspect their concern is that a Z900 would largely serve to compete with the Z6III and Z8.
So, while I would personally welcome an APS-C Z-mount D500, I doubt it's a good fit for Nikon's business model.
Of course, if Thom is right about what he's proposing in this recent article, we'll have an answer in few months. I'd welcome being wrong about this. While I wouldn't be a customer for a Z900, I loved the six years I had with my D500 and think it would find a dedicated audience of users amongst Z-mount photogs.
What f2.8 and f4 lenses are you referring to?
In the Z mount: 70-180mm, 70-200mm, 400mm, 600mm at the moment. We know of at least one 200mm f/2 coming. From the F mount, we have the 300mm, 120-300mm, and 180-400mm.
Those 600/4 TC and 400/2.8 TC that are well over $10K new? I wonder how many people can afford those. (I do know a few that have both; one frequents this forum.) Today, the popular telephoto lenses are the 180-600mm that is max f6.3 on the long end, 600/6.3 PF and to some extend 800/6.3 PF.
In wildlife and bird photography, a comparison of maximum f-stops across a wide range of focal lengths is not indicative of low-light performance. It's one thing if you need 200mm to fill the frame with your subjects. If that's the case, by all means, buy the fastest 200mm zoom or prime you can afford and enjoy the results.
But if you'd be pairing a 200mm f/2.8 or f/4 optic with TCs or cropping in post to match the framing a 400-600mm lens produces, that "fast" lens will ultimately result in noisy images that don't pass muster.
In wildlife and bird photography, we should be comparing entrance pupil diameters:
Lens.......................................Max. Ent. Pupil
600mm f/4....................................150mm
300-600mm f/4.............................150mm
400mm f/2.8.................................143mm
800mm f/5.6.................................143mm
800mm f/6.3.................................127mm
500mm f/4....................................125mm
300mm f/2.8.................................107mm
180/200-400mm f/4.....................100mm
400-800mm f/6.3-8......................100mm
600mm f/6.3...................................95mm
180/200-600mm f/5.6-6.3.............95mm
150-600mm f/5-6.3........................95mm
200-800mm f/6.3-9........................89mm
500mm f/5.6...................................89mm
200-500mm f/5.6...........................89mm
400mm f/4.5...................................89mm
150-400mm f/4.5...........................89mm
150-600mm f/5.6-8........................75mm
300mm f/4......................................75mm
100-400mm f/4.5-5.6.....................71mm
70-200mm f/2.8..............................71mm
100-400mm f/5.6-6.3.....................64mm
70-180mm f/2.8..............................64mm
70-300mm f/4.5-5.6.......................54mm
70/200mm f/4.................................50mm
The best wildlife/bird lens for a photographer is one with the focal length needed to fill the frame and the largest entrance pupil that's budget-friendly in a size that one can comfortably carry.
And those lenses are being used mostly on FX bodies, not DX, so divide the top focal length by 1.5 to see what would be DX equivalent. But frankly, 600mm f/6.3 on a DX body is only just into the diffraction zone, and providing a 900mm equivalency; I just don't see the potential users complaining.
A 600mm lens on a DX body is a great wildlife and bird combo. Hundreds of thousands of bird and wildlife enthusiasts paired APS-C DSLRs with the Tamron and Sigma 150-600mm zooms in the teens. Diffraction at f/6.3 to f/8 is not an issue.
For the newcomer to the hobby, used copies of those older bodies and lenses can be had for under $1,000 and would make great starter kits. If the Nikkor 600mm f/6.3 PF fits within a person's budget, that lens paired with a Z50II is an incredibly sharp and portable kit that performs very well in low light.
I should also point out that all these "the lenses aren't there for DX complaints" assume that no new lenses ever appear that solve part or all of the problem.
--
Bill Ferris Photography
https://billferrisphotography.pixieset.com/arizonaslittleserengeti/
