What is the most important for portraiture is to get the light, composition and mood right.
Speaking of focal length to use, you have to understand that there very different kind of portraits: from full body with some environnement to basically a narrow face shoot. On FF this would mean from 35 to 200mm, roughly. But it will depend a lot what you want to express in that portrait, were do you work (indoor vs outdoor), the kind of shots (candids vs studio) or subject (child, man, woman, eldery people...)
I tend to like tight portrait and for that on APSC I find that the FA77 is great asset, even indoors. And when I can't go near enough to the subject for candids, I like a lot the FA135. The nearest remplacement for that on FF would be 135 and 200mm.
Some people on the opposite would prefer, still on APSC FA31 and DA*55. That quite a different view. And that would translate to 50 and 85mm on FF.
I believe that an accomplished photographer know when each focal length will add something to the photo. That this flat face would benefit of a shorter focal length, or that big nose a longer one. That it would add a lot to the picture to get this portrait of the person in her shop or house, with the environement so go all wide or on the contrary concentrate on the deep eyes and go quite narrow.
So if we are speaking primes, you'll likely want 2-3 lenses and that why zoom are so conveniant. On APSC it can be said that f/2.8 is a bit limiting bokeh and low light wise. But with an FF, I don't think that's an issue anymore. As such a 70-200 on FF would do wonder. You don't have to get the Pentax one that is very expensive. You can get the tamron. Anyway portraiture is not that often about perfect sharpness of skin pores in the corners.
If you find theses 70-200 too big, and I can really understand that, I feel that myself, then you may want to have several primes. F/FA 135 are quite innexpensive (300-350€ used I think were I live) and do fantastic portraits while keeping a low profile. The many 50 or 55mm out there are quite small and light. The FA50 f/1.7 is maybe 150€. In addition to a 77 or 85mm lens, that would be quite usefull.
Last edited by Nicolas06; 05-15-2016 at 05:48 AM.