Further to the last digital standard zoom shootout carried out by the ERPhotoReview, I think the author's last group test amongst six 50mm lenses is also worth mentioning:-
http://erphotoreview.com/wordpress/?p=243
This test is yet quite interesting as the Takumar 55/1.8, Pentax A 50/1.4 and Pentax F 50/1.7 each represents the well-established optical formula and design of each own, which have been long and widely used in various variants of the Pentax "standard" prime lenses (now the 50 is just a "short-tele" portrait on Pentax DSLRs, anyway).
It can be seen that the 50/1.4 actually performs the best, except its dominant CAs found (same case as the DA 16-45). Other than that, it performs very well optically for both image centre and "corners" (Well, the projected images are still cropped on APS-C bodies, right?).
Btw, how about the DA*55/1.4 when compared against the 50/1.4 then, it was found that the DA*55 perform even better, optically! Just see this:-
DA* 55/1.4 Vs FA 50/1.4 - A Measurebating Shootout
But yet very unfortunately, the DA*55 is using the SDM, which is found to be slow, inaccurate in AF, and proven to be unreliable! :-( Furthermore, DPR previously put really adverse comments about the inferior quality and control found of the DA*55 lens and units (they requested 3 different samples from Pentax UK (as told by the DPR reviewers) but the lenses they tested did have similar misalignment and inaccurate focusing issues). Too bad..