Lenses such as the 20-35mm FA originate from the film era, and therefore the light rays may well be quite oblique peripherally. This certainly seems to be the case as the lens performs well ...

"The PENTAX smc P-FA J 18-35mm F4.0-5.6 AL is an ideal entry-level zoom lens designed to be used with fully automatic cameras. Its lightweight design makes it easy to handle and maneuver.