(multi-coated)
average of two samples; OM-1 with mirror lockup

Vignetting = C
Distortion = very slight barrel
Aperture Center Corner
f/2.8    B      B-/C+
f/4      B/B+   B
f/5.6    B+     B+
f/8      B+/A-  B+/A-
f/11     B+/A-  B+
f/16     B+/A-  B+/A-
f/22     B/B+   B
Notes: Moderately high contrast.

OM-4T with mirror and aperture prefire; same lens as OM-1 based test
Aperture Center Corner
f/2.8    B-     C+
f/4      B+     B-
f/5.6    A-     A-
f/8      A-     A-
f/11     B+     B+
f/16     B      B
f/22     B      B
Notes: Moderately low contrast images at f/2.8, f/16 and f/22; moderate contrast images at f/4 to f/11.