Originally posted by rvannatta Mechanically there is no issue with an AF converter.---You just need something with a Cam that connects through. None of the Pentax ones do that.
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One possibility: the currrent pentax roadmap includes an SDM compatible converter it it may also have a drive shaft for non SDM lenses. I"ll doubt however if you will ever get a converter
for those long lenses better than the dedicated converters made only for them which is effectively what the 1.4x-L and 2x-L converters are.
There
is a mechanical issue with teleconverters using the screw drive system. By adding two more couplings and additional bearings to support the shaft, there is a lot more mechanical play and friction in the system. This makes the AF hunt back and forth a lot more, and makes the AF slower and noisier. I had a Tamron 1.4X AF converter for a short while, and I used it mainly with a Pentax FA 135mm f2.8 lens. It did work and I could get correct AF, but the AF performance was very slow and noisy. Also, it added so much drag that manual focusing was almost impossible (but that isn't an issue with QuickShift lenses).
I hope that the upcoming SDM converter does
not have the screwdriver shaft. That would give terrible performance. One benefit of SDM and the electrical contacts for AF is that there is no loss of AF performance with a tele-converter (other than the loss of light for the AF sensor).
And I agree that dedicated converters generally have much lkess impact on image quality than generic converters designed tyo work with any lens.