The existence of this lens is only reason I'm shooting 645 format. The SMC-A manual focus version was at the time priced similarly to what top 21mm lenses in 35mm cost used, and seems every bit as sharp as anything I've got (and I've got some rather exotic 35mm lenses).
While I don't have a lab set up to test the actual resolution, 30+ years of eyeballing this stuff says it's macro sharp. When shot on film like Astia, Velvia or Ekta that can resolve 100 lp/mm the lens doesn't seem to be presenting much or any bottleneck. On film I see just amazing amounts of fine detail, quite astoundingly good. Lens is reasonably resistant to flare, too; I often include the sun and backlight subjects in the scene. And from what I see on film (have only the P645N) the lens is not going to be breaking a sweat to resolve the ~90 lp/mm that the smaller than FF P645D sensor.
One online comparison (16:9 site) against Zeiss Biogon/ Rollei/ Mamiya lenses of the same lengths has the Pentax SMC-FA version running neck and neck and even besting the field working apertures of f/8-16. That comparison is long in the tooth and was shot on a Canon 1Ds sensor and not film; my version (SMC-A) resolves a LOT better than the numbers 16:9 would seem to indicate. The MTF chart on PhotoDo is the only other site that I've found with test results. My copy of the lens suggests it's even better than the numbers would indicate.
The FA version became a cult-classic but may not work any better or even as well as the older SMC-A version, at least according to a P645D user who's exhaustively tested post over on the Pentax Forum. (Of course, the FA is autofocus, however, and could be bought new until just a couple of years ago).
The main drawback to this lens (or for that matter any of the very best that MF has to offer) is that you're going to need a better scanner than a Nikon Coolscan 8000/9000 to wring out all that detail off the best chromes or negs. 4K dpi just isn't going to scrape all that's there.
