My latest acquisition - The Pentax SMCP-K 55mm f/1.8
I recently tested the SMCP-K 55mm f/1.8 on my K5IIs against the SMCP-K 50mm f/1.2 and the hell of it - the A series 50mm f/1.7 and the results go like this:
the 50mm f/1.2 and 55mm f/1.8 are remarkably sharp at their widest apertures in terms of detail they are basically identical. The 55mm f/1.8 suffers from PF far more than the 50mm f/1.2 does : the A series 50mm f/1.7 is sharp at the centre, although it suffers from noticeable astigmatism at the edges of the frame - it is important to note that the two K series lenses do not have visible astigmatism.
At f/5.6 the K55mm f/1.8 and the 50mm f/1.7 are
slightly sharper in the centre than the 50mm f/1.2 - however at the edges of the frame the 50mm f/1.2 is sharper than the slower lenses, the A series 50mm f/1.7 still suffers from astigmatism but it isn't as noticeable as it is at wider apertures*.
The only thing the A series 50mm f/1.7 had as an advantage was lower amounts of vignetting - in this regard the 55mm f/1.8 is only
marginally better than the 50mm f/1.2. The Physical construction fo the K series lenses simply beats the A series lens hands down - I'll admit i'm prejudiced when it comes to plastic lenses, I always prefer metal construction with aperture rings that have positive click stops.
*Astigmatism cannot be eliminated by simply stopping the lens down, using smaller apertures merely hides it - and if you look closely you can see its effect in OOF regions at small apertures.