Zeiss 21mm f/2.8 Distagon vs Nikon 20mm f/1.8G: Mt Conness Glacier
The Zeiss ZF.2/ZE 21mm f/2.8 Distagon has long reigned as the champ in the ~21mm range for its high brilliance and strong control of color aberrations.
In Guide to Zeiss:
Zeiss 21mm f/2.8 Distagon vs Nikon 20mm f/1.8G: Mt Conness Glacier
Aperture series through f/16 with large crops and extensive performance analysis.
The Nikon 20/1.8G aperture series will be posted in the review of the Nikon 20mm f/1.8G soon.