On FM there is a 'discussion' (actually not too moronic initially - sadly there is real Zeiss fanboy clique building over there) on the 18 that got VERY interesting when a chap who doesn't post often and who is (unbeknownst to most) a lens designer apparently popped up with some comments.
His view is that the 18 is likely not well corrected for lateral CA - based on the MTF charts.
As you'd expect, a lot of self-proclaimed experts got their knickers in a twist re: using MTF to judge CA, then oddly enough shut up real fast when it was pointed out by someone that the poster in question was a lens designer (for whom, no idea).
His comments (username brianc1959) on judging CA from MTF which I found interesting:
"When you stop down a lens all aberrations are reduced except for distortion and lateral color.
Distortion doesn't affect MTF. Lateral color does affect the MTF very strongly, but only the tangential curve in the outer parts of the field.
So if you look at a series of MTF curves for different apertures of a lens with lateral color you will see the sagittal curve improving much faster in the outer parts of the field than the tangential curve as you stop down.
If both the tangential and sagittal curves improve similarly as the f/# increases then its a sure bet that the lens has little or no lateral color. The MTF curves for the 21mm Distagon at f/2.8 and f/5.6 show this behavior.
There are caveats and complications to this explanation, but the basic idea is correct"