I have a K1 and with the birth of a new family member I've really been doing a lot of 'portraiture'...mostly of the candid variety and also without flash. Available light generally indoors, are the elements I work with.
I've been using mostly prime lenses...my DFA 50 normal F 1.4, DFA, 50 Macro F 2.8, 70 Limited F 2.4 and 40 Limited F 2.8. The 40 and 70 are ASP-C lenses, but I use them anyways...my K1 on full frame...not cropped...and they are great...mostly I'm using F 2.4, F 2; 8 to about F 3.2...no complaints...I focus on the eye and click away.
My two 50's are full frame lenses...both normal 1.4 and macro 2.8 and they are sharp. The normal lens...usually F 2 to F 2.8...the Macro 50...F 2.8 to F 3.2. These are sharp lenses.
Now I'm considering getting a Pentax DFA 100 Marco F 2.8/ Versatile...ie; macro and sharp according to an old Popular Photography review of this lens when it first came out. I was thinking of a 77 Ltd...F 1.8 or maybe trying a third party 85mm F 1.4. The third party...Samyang I think is manual focus ...but.... my old eyes are what they used to be...they prefer auto focus.