My personal XP as a non pro doing from time to time portraiture with fast lenses and having participated as main (non pro and only photographer) in 2 weddings.
Concentrating on AF aspects...
My personal technique
I use my K3 with primes. DA15, DA21, FA31, FA77 and at time F135. I use AF-C, slow or med burst mode with back button AF and single point AF selection to get the focus where I want. I usually put the focus on the eyes for portraiture and get a few shots to ensure I do not get only the one where the subject has the eyes closed, or as a strange expression or the shot where the AF didn't work for some reason.
The practice allow to avoid all the focus/recompose issues that doesn't work at all for moving subjects and that is problematic for fast lens with shallow dof.
Pro of Pentax AF
(So K3, K3-II, KP and 645z that all use the same module)
This overall work quite well as long as you the photographer select the right AF point. You can get quite high keeper rate, in most conditions.
Cons of Pentax AF.
Remark: it isn't that bad, I put everything so you are aware. Remember that overall the Pentax AF is decent !
- The Pentax AF doesn't track well on X-Y. That's one of the reasons to select the AF point yourself.
- But... there no dedicated AF joystick or touch AF and the button to move the AF do other things depending of the mode. i always end up entering that mode when I don't want it and the camera then do not does what I want.
- You'll see that even if the subject isn't moving that it will go a bit back and forth at time, near the optimal focus and sometime that it'll go out of focus.
- There no eyes-AF implementation that does all the hard job for you for portraiture and that always nail the subject in focus without much effort. I didn't try that myself but is said by many to be a game changer for portraiture.
- This is a DSLR, while you'll AF generally in lower lighting conditions than other cameras, the AF has back/front focus qualibration issues and isn't as precise as what contrast detect can do. This is important for shallow dof. I would say highend mirrorless bodies of Sony for example would do a better job there.
- All the AF point are near the center area. On the K3 the coverred area is acceptable and enough to put the subject on one of the rule of third hot points. It fall a bit behind on K1 and on 645z this isn't the case at all.
Conclusion for AF
I can't really say for MF, didn't even own one and at least when 645z got out if was rumoured to be one of the best for AF.It say more of the limitations of others MF than as how great 645z is in absolute term, in particular for the price range... I don't know if the recent MF camera do better now, like the new mirrorles Fuji. Do your homework
For APSC and FF, I'd say Pentax implementation is decent but not the best. If you can afford it, in FF, I'd bet more on A9 or A7RII / A7RIII. Theses may not be at the level of Nikon D5 for AF, but mirrorless seems to be better for accurate AF (rather than ultimate speed for action) and the Sony do have Eyes AF that seems to be a game changer for your type of practice. Again do your homework, don't take what I say for granted.On my side, I thinking more and more my next camera will be a mirrorless because of theses reasons. I am not in a hurry through. A D5 or D500 may do as well but personnally I prefer to stay on the smaller/lighter side of things.
For skin tone and portrait look.
Personal but I found out that Capture One does really render portraits well. I am not expert there but the same photos with Capture one seem to look better. Portraiture photos. The rendering engine make for more 3D/pop. On the opposite I do find that m4/3 have difficulties with such subjects, in particular with their standard Jpeg. Fuji JPEG are nice if you like them but heavily processed and not necessarily suited to your liking... And their raw are more difficult to work with.For day to day processing I use DxO, and for portraiture, I often choose a rendering suited for that thanks to DxO film pack.
For the rest:
- FF get a benefit for high iso and the bigger sensor give more margins and help for subtle tones reproduction. I think it also quite significant that you can use an f/2.8 or f/4 zoom and still have great low light and subject separation.
- Honestly Pentax is still building its lens lineup and is a bit lacking in the area. If you go Pentax be sure before you buy that all the lenses you want are actually available. What is missing for example is a 24-105 f/4, a modern line up of fast prime and there no prime bellow 31mm or higher than 85mm for FF with AF and optimized for portraiture. It goes up to 100mm if you include the 100mm f/2.8 macro but f/2.8 isn't f/1.8 or f/1.4 and macro isn't optimal for portraiture. Tend to render analatically rather than artistically. Bad for portraiture.