Choose a fast 50mm, any fast 50mm...well almost any...
Which one depends on how "manual" you want to go. There is a huge advantage in keeping auto-aperture actuation (limited to K-mount) so my top three might include any mix of:
- Pentax-K, M, or A 50/1.4
- Pentax-K 55/1.8 or M or A 50/1.7
- KMZ Helios 44K-4 58/2.0
Special consideration might be given to the Helios, in that it comes packaged with soft corners and unique bokeh. Hint: There is no shame in shooting the DA 50/1.8.
Going full-manual (focus and aperture), meaning M42, opens huge doors. Of those on my shelf, I would suggest:
- Super-Takumar 55/1.8
- Auto Rikenon 50/1.8
- Auto Rikenon 55/1.4
- KMZ Helios 44M 58/2.0
Yes...I know...that makes four...
In case anyone is wondering, the FOV for 50mm on APS-C is the same as 75mm on 35mm FF and right in the range of a traditional portrait lens. What that buys you is adequate working distance to avoid spooking the subject and also less tendency for perspective distortion of features.
Steve