It may be old, but it is tiny, fast and a beauty.

Dale108 wrote:

Agree completely; the FA 50 is one of my favorites on the K1.

More agreement here.  I picked mine up second hand in 2003 or 2004 and it's been my most used lens on APS-C for studio work ever since; though since getting the K1 a few weeks ago the 77 is getting more use  (that's just the perfect angle of view / working distance).  I was out shooting (normal photography - not rigorous testing) with the FA 50 on the K1 yesterday and was very happy indeed with the results - the pixel pitch of the K1 is the same as the K5 so the centre part of the image is basically what I'd get on the K5 but at the apertures I was using you can't see any fall off in quality near the edges.   It's not just compact but versatile, e.g. as a macro lens when combined with extension tubes (or reverse mounted).

The OP (and title) isn't blind to this lens's downsides: it came out in 1991, and its widely believed that Pentax made a final batch a few years ago and when those are gone they're gone. The optical design is older - AIUI the FA is optically the same as the A (with the addition of autofocus mechanism) and the only update to the A was the addition of the electronics for the camera to control the aperture, it may well be that the glass is unchanged from the first SMC Takumar 50 f/1.4 which came out it in the early 1970s - which in turn added coatings to an established combination of elements. (50mm Takumars command decent prices)
 If Ricoh add an in-lens motor and the latest HD coating  this 1960's design will be good for decades yet.