Tamron SP 90mm f/2.5 Macro
OM-1 with mirror lockup
Vignetting = A-
Distortion = slight pincushion
Aperture Center Corner
f/2.5    C-     C
f/4      B-     B
f/5.6    B      B
f/8      B+     B+
f/11     A-     A-
f/16     A-     B+
f/22     B      B
f/32     B-     B-
Notes: Good detail in low contrast subject matter; at f/11 = slightly higher image rendering performance at 25% out from center of image versus a 90mm f/2 Zuiko.

Tamron SP 90mm f/2.5 Macro with Tamron SP 2X teleconverter
OM-1 with mirror lockup
Vignetting = A
Distortion = slight pincushion
Aperture   Center Corner
f/2.5 (5)  F      F
f/4 (8)    B-     C-
f/5.6 (11) B      C
f/8 (16)   B      C+
f/11 (22)  B      B-
f/16 (32)  B-     B-
f/22 (44)  C      C
f/32 (64)  D-     D-
Notes: Remarkable contrast, except at f/2.5, where exceeding low; clearly diffraction limited stopped down; little quality loss in center of image at wider apertures.