Both Pentax 645 35mm lenses suffer from moderate field curvature: rather than ‘projecting’ a flat plane of focus, it is broadly concave from the camera’s viewpoint – shifting toward near corners. If shooting a brick wall with pot plants in either corner of the frame, you’ll find that it you focus on bricks in Zone A, the bricks in Zone A will be out of focus, but the pot plants will be sharp.
The A’s field curvature is more serious, because it’s non-uniform. At wide apertures you observe the plane of focus shift in wide ripples. However, it’s hard to tell because in Zone C and D, nothing is really ever sharp with the A. The FA’s less troublesome curvature is therefore easier to spot. Neither lens is as bad as the ‘what-the-hell-is-happening-here?!’ field curvature of the Pentax A 645 55/2.8 which (to both its discredit and it’s shame) has corners so sharp the curvature it’s brutally obvious at wide apertures.
The Olympus 35/2.8 shift has rather less field curvature: it’s almost gone at f5.6 and irrelevant by f8. As is customary when discussing these matters, we observe the mantra: sufficiently stopped down, field curvature is not a problem. And once again, we applaud the work Olympus was doing in the late 1970s.