That was really interesting, Woody! The discussion touched upon several issues I've been thinking about with regard to the implications of the move to mirrorless.
I should state that at present I'm primarily using F-mount cameras and lenses (supplemented with a μ4/3 system or a Sony A6000 when I want a compact camera). I expect to expand into the Z system at some point, probably when the second-generation cameras come out. I'm holding back on buying a few short-focal-length (=retrofocus) F-mount lenses I had in mind because I may eventually be using primarily Z-mount lenses. I'll always be using some old F-mount lenses - long telephotos, other specialty lenses, or classics.
Part of evaluating the long-term benefits of one system over the other is the question of lens quality. For the Z system that's partly a matter of what Nikon does and partly a matter of what third-party manufacturers such as Sigma might do if they enter this new market. The latter consideration is early and speculative, but I don't want to invest thousands of dollars in lenses that then turn out to be significantly surpassed within a short time.
We have some early results from Nikon. They have definitely taken advantage of the excellent characteristics of the Z mount to gain more optical freedom in their designs. This is shown by these first lenses having very good to excellent results out to the edges of the frame.
Then the question comes up as to whether Sigma (and other third-party lens manufacturers) might also produce lenses that take advantage of not just the short flange distance but also the wide opening of the Z mount. However, a related consideration is that third parties might choose to produce optical designs that would fit all full-frame mirrorless mounts, to save on design and manufacturing costs.
That would mean that the most restrictive full-frame mirrorless mount characteristics would constrain these optical designs. The L and R mounts with their 20mm register distance would provide one major limit and the Sony FE mount with its 43.6 mm throat diameter would provide the other important limit.
Note that the throat diameter of the FE mount is actually slightly smaller than that of the F mount (44 mm), yet it's located much closer to the sensor. This would eliminate one major advantage of the Z mount, and I have to wonder if it could actually create a small disadvantage. At least such lenses could still use non-retrofocus designs which have certain advantages. Overall, this approach could put third-party mirrorless lenses at a distinct disadvantage relative to native lenses on the Z mount.
Alternately, Sigma and/or others such as Zeiss might choose to produce two sets of optical designs, one for the FE mount, and the other for the Z, R, and L mounts given that the latter mounts offer greater optical design freedom. For these mounts this would mean a register distance of 20mm, and a throat diameter of 48.8 mm due to the L mount. This scenario seems unlikely due to the much greater R&D cost.
It looks like the decision by Sony to use the crop-sensor E mount at the basis for their full-frame mirrorless mount may ultimately have effects on people who don't even use Sony cameras or lenses!
