I've a Pentax K-30 with the following lens:
Pentax-M 50mm 1.7 manual focus
Tamron 17-50mm 2.8 autofocus
Pentax 55-300mm 4-5.6 autofocus
Considering that :
Which should I use?
Use 50mm if you want to separate the person from the bacground, you can always crop the image to simulate the 70mm (at 8Mpix) or 90mm (at 5Mpix). To include enviroment use 17-50/2.8 at smaller focus distances. And take telezoom for larger distances.
You may found usefull to experiment with http://camerasim.com/apps/camera-simulator/
If you are going to make mostly portraits, a 50mm prime would fit you much better. Not only gives you a lower aperture, but the image quality is better with primes.
Don't worry about the "ideal" 70-90mm range, 50mm is a classic focal length for portraits.
It also depends on how much background do you want to keep.
With the Pentax prime or Tamron zoom lens, you'll get a better background separation, in addition to having more backdrop behind your subject. This will arguably be nicer as the bokeh effect will be more apparent (more concentric circles, soft focus etc.).
On the other hand, if you want to get closer to your subjects without being too literally in their face, this is where the longer focal length will come in handy. In addition, any prevalent pincushioning lens distortion tend to be more flattering too, as it makes subjects just that little bit slimmer...
I think that taking a head and shoulders picture with a 50mm lens means that you have to get too close for your subject's comfort and in addition risk some distortion.
I've always used 90mm prime lenses with as fast an apeture as my bank balance will permit. As usual with photography, it's a trade off - it's a great lens but I have to move around a good deal to recompose the shots. Obviously, with a zoom I wouldn't have to do this but really fast zooms aren't cheap.
The bottom line is that you should be fine with your zoom shooting at f5.6.
(I am teasing you:) You seem to be seeking a hard rule that you can apply without thinking, but there are no hard rules, certainly no One rule, and thinking is always helpful. :) It always depends, on the situation, and on what result you want.
70-90 mm is "ideal", if assuming a cropped APS sensor, and assuming a normal subject distance of at least 6 to 8 feet (for proper portrait perspective), and it is not a group shot. But even better, the 70-300 will also do say, 150 mm or 200mm, which from say around 15 or 18 feet (2x focal is 2x distance) is still ideal, still with proper perspective.
And (speaking about outdoors) if focused at this 15 feet, it will blur the dickens out of any background that is reasonably distant. Plus, as important, it sees a much more narrow view of the background, so that moving slightly can easily choose a better one.
IMO, beginners go for the f/1.8 lens, because they are inexpensive, and they heard they should, (do you really like f/1.8 portraits?), and because they normally stand too close for perspective anyway.
IMO, pros probably more often seek the longer lenses from the longer distances, which are not f/1.8 (another plus), and they do the job better, good perspective, and better background choice.
Pentax 50mm has the nicest bokeh and sharpness I've ever see...
I think you've answered your question. Stick with image quality over anything else. You can always make up for other deficiencies with skill and technique. Shoot for best image quality.
