Originally posted by Nickrs Bearing in mind that even the best teleconverter (TC) [which I understand the Pentax 1.4x is] might compromise the ultimate performance of a prime lens then do we find that there is no significant IQ benefit of a prime with TC over a zoom lens?
I think that depends entirely on both the prime and zoom lenses as well as the circumstances....
One of our members here uses a Pentax DA*200 f/2.8 both with and without the 1.4x TC instead of the DA*60-250 f/4 he previously used for short-to-medium-range nature photography. For the distances he's photographing at, that combination gives him significant extra reach over the 60-250, which provides less and less magnification at close ranges due to focus breathing. It's also a more compact solution, if I remember correctly.
Another example is the DFA100 f/2.8 Macro - I believe some people are combining this with the 1.4x TC for even greater magnification in macro work.
For me personally, I guess I can see a potential benefit to using it with the DA50 f/1.8 to provide two very nice portraiture focal lengths in a reasonably fast one lens kit. I haven't tried it yet, though, and of course I could just carry the DA50 and DA70 together - it's not much more space!