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Really two quite different lenses due to AOV. D15 falls away steadily, ZE/F D21 is very even across the whole frame, but the D15 starts out very high at image centre. This works fine as the centre is often the furthest point in the composition (like the image above), the edges and corners closest with higher magnification to offset the decay in MTF.
A great shot, TS, great clarity and colour/contrast in difficult lighting - a Distagon hallmark. The low distortion is such a wide kind of fools the eye into thinking its FL is longer.
Yes, it it clear than just a few mm from the centre at 25 lp/mm the D21 Z* on FF will pull ahead from the D15 at 40 lp/mm on APS-C, at any aperture.
What interests me about the D15 on APS-C is the lower distortion (and to get rid of the moustache shape), and the possibility of even lower CA than the D21 Z* on FF. I have learned to appreciate absence of color aberrations more than highest possible MTF in the corners, as long as they don't fall into the "blurry" category like the D 2/25 Z* does in the extreme corners.