Originally posted by kangamel What I am wondering is if there is going to be much more noticeable distortion from a 28mm prime that is not present from a 35mm when capturing a face at close distance.
One other thing you can do is look up the specific 28mm and 35mm lens you are thinking about on review sites and see their measured distortion.
There is also a difference between distortion and perspective. Basically, to have your subject fill the frame, you will have to come much closer with the 28mm lens, which emphasizes the perspective and makes the person look even more round.
tl;dr: yes, 28mm is noticeably wider and worse for portraits than 35mm, which itself isn't perfect for such usage.
Anyway, Pentax isn't selling a 28mm lens right now (only zooms that have that range), so the DA 35mm f2.4 and FA 35mm f2.0 are the only choices. But both are a great lens. Sharp, fast AF, good low light performance. But! For closeup portraits 35mm is still not very flattering. It makes faces rounder. Especially with children, because they are small and you have to come closer. 35mm is good for photos where the hole person is in the photograph and some of the environment. Honestly, for actual portraits I would suggest DA 50mm f1.8 or DA* 55mm if the budget permits (top notch portrait lens). The DA 50mm f1.8 is very cheap now, too, under $200. Used its even cheaper. Or the Pentax M 50mm f1.7, which can be found for under $60, if you don't mind using a manual lens.
Simply switch camera to portrait mode or Av mode (f-number down between f1.8 or f2.8) and you will have nice portraits.