Originally posted by pentaxus Some teleconverters are better than others. A four element converter reduces sharpness more than six or seven element converters. Takumar K mount converters are four element converters. Pentax converters are higher quality. The lens you use it on makes a difference. I used my Pentax 1.4x on my 85mm f/2 and got good results but putting it on my 85mm FA 1.4 was ugly. I also used the 1.4x on my 300mm and got good results. My Pentax 2X always seemed to be not quite as good as I wanted, but better than a four element. In other words, you will need to do testing.
Personally, I don't recommend TCs for less than DA* equivalent glass or on focal lengths less than 70mm. And I only recommend the Pentax HD DA 1.4 or Pentax F 1.7x AF Adapter,
Too many people say they don't work well for me to issue any blanket statements about all TCs. I've tried my TCs on my worst lens an M 135 Vivitar 2.8, and they just make the badness (softness coupled with CA and purple fringing) 1.4 and 1.7 times more visible.
But on my DFA 100 macro, DA* 60-250, DA* 200 2.8, and Tamron SP AF 300 2.8 they work great. If you are using lower quality glass, all bets are off.
TCs don't help you get the most out of average glass, TCs help you get the most out of high quality glass, glass that out-resolves your sensor by a considerable margin that can still crank out the lw/ph even when enlarged.