I think I have shown very clearly in the below link that my copy of this lens is sharp down to the last corner pixel both wide open and at f/8. While it is possible to see some faint CA in the extreme corners, this is something I would describe as "almost non-existant" without any second thoughts.
RE: The current Zeiss 18mm: I had the lens for a few days, and my only objection(and the reason I returned it) was that on my D3 cameras, it showed too much illumination falloff in the outer areas of the full frame for my taste. I found that it had to be stopped down to f/8 or so in order to minimize the falloff. At the time, the Nikon 14-24 f/2.8, with it's telecentric, reduced falloff design appealed to me, so that's what I got. However, if I ever find a reasonably priced used ZF 18, I may buy it, because it had superb sharpness rendering for an 18mm. Digi-Lloyd also has mentioned a kind of zonal color-banding issue on certain full frame cameras with both the ZF 21mm and ZF 18mm.
