Nikon Nikkor 20mm f/2.8 AIS
Nikon F w/ mirror lockup and diaphragm prefire

Vignetting = B- @ f/2.8, A- @ f/4, A thereafter
Distortion = waveforming with slight barrel tendency

Aperture Center Corner
f/2.8    B+     C+
f/4      A-     B-
f/5.6    A      A
f/8      A      A+
f/11     A      A
f/16     A-     A-
f/22     B      B

Notes: Moderately high contrast at f/2.8 to f/5.6, high contrast at f/8 to f/22. Optimum aperture f/8, where corner performance was exceptional for an ultra wide angle lens.