Conclusion: The Pentax SMC-FA 100mm f/2.8 macro is an very good lens without significant weaknesses. The resolution level is very high (although not exceedingly so) from f/2.8 all the way down to f/22 (nominal, the effective aperture is probably larger). Distortions are absent and CAs are not field-relevant. Same goes for vignetting. Thanks to a metal body and tight tolerances the build quality of this old lens is excellent.