(multi-coated)

OM-1 with mirror lockup
Vignetting = A- at f/2.8
Distortion = none
Aperture Center Corner
f/2.8    B-     B-
f/4      B      B
f/5.6    B+     B+
f/8      A-     B+
f/11     A-     A-
f/16     B+     B
f/22     B+     B
Notes: Contrast loss at f/16 and f/22. No apparent contrast difference versus single-coated version, but sharper overall.

OM-4T with mirror and diaphragm prefire
Vignetting = A- at f/2.8, A thereafter
Distortion = slight pincushion (none?)
Aperture Center Corner
f/2.8*   D      C
f/2.8    C-     B-
f/4      B      B
f/5.6    B+     B+
f/8      A-     B+
f/11     A-     A-
f/16     B+     B
f/22     B+     B
Notes: * = with a Canon 1A filter, which exhibits low contrast. Moderately low contrast at f/2.8 without filter, as well as f/4 and f/16-22, moderate contrast at f/8-11, moderately high contrast at f/5.6.