(single-coated)

OM-1 with mirror lockup

Vignetting = B @ f/3.5, A- @ f/5.6, A @ f/8 and beyond
Distortion = slight waveforming with slight to moderate barrel
Aperture Center Corner
f/3.5    B-     C+
f/5.6    A-     B
f/8      A      A-
f/11     A-     A-
f/16     A+     A-
Notes: Contrasty with high resolution.

OM-4T with mirror and diaphragm prefire
Aperture Center Corner
f/3.5    B      C+
f/5.6    A-     B+
f/8      A      A
f/11     B+     A-
f/16     A-     B+
Notes: Same lens as above test.