Thanks Chris.
I agonized over which ultra wide lens to buy for months looked at the 16 - 35 canon and the Zeiss 18 and 21s but all had their problems , the zeiss has good shrpness but terrible vignetting and the canons need to be stopped down for good sharpness across the field so it left the Nikon which has great optics but is heavy , can't use filters and requires an expensive adapter to use it on the Canon.
Nothings perfect but the Nikon is as close as I could find.