The 40 art, a reference lens for sure, is still a 40mm lens. It's not really a 35, and it's not really a 50. I find it nearly perfect for full length studio work of dancers, and optically it is as good as it gets.
It's performance characteristics are that it's quite sharp wide open, with essentially no field curvature and no astigmatism and that right there means it's a wonderful lens for those shooting astro, or even at stopped down apertures, who want a flat field and no astigmatism - remember that both field curvature and astigmatism (which are of course related) do NOT improve as you stop down. There is no other lens anywhere near this focal length that is as even in performance across the frame - and for some things, that's really important.
At the same time, there are other really good lenses that might approach what the 40 art does well, even if not quite matching/beatint it, but do something *else* better. As an example, even as a 40 art owner, I'm thinking strongly about the 50/1.2S in Z mount. Not because I want to have a battle between the lenses, but rather because the character of the lenses at the early apertures is DIFFERENT. Note carefully I did not say "better", I said different. The 40 art has fairly rapid falloff in terms of OOF transition and decent bokeh. The 50/1.2S seems to have a slower transition to OOF and perhaps better bokeh. One lens thus might excel where the other doesn't, and vice versa. It's not always just as simple as "lens a is absolutely better than lens b".
Note also that you pay for the 40 arts optical performance - it's huge and heavy, and I can't even begin to imagine that thing hanging on an FTX adapter. It lives on my D850 for studio work. For the Z bodies, I'm likely to want to deal with a native lens.
