So, I got myself a used 2x TC, a Kenko Pz-AF 2x Teleplus MC7, which turns my Tamron into a 140-400 f/5.6. I did some test photos this weekend, here is a quick debrief:
- I was looking for "a 300mm-ish lens", now I know even 400mm is not so long.
- The IQ of the 70-200 + TC combo is better than what I get from my 100-300.
- Autofocus works. I got quite a lot of misses, but things got better after I dialed a -5 AF adjustment. I guess some fine tuning could get me better results still, but I found the focusing accurate enough after the adjustment.
- SR is an issue. I don't know if there is a way to override its focal length setting, but if there is it is off on my K-1, and I didn't find how to change it with a cursory search through the menus.
- The tripod collar of the 70-200 is not adapted to a K-1 behind a large converter. Reversing the collar improves things somewhat, but it's still not perfectly balanced.
- Would I get better results with a DFA 150-450, or even a DA* 300? Probably. Is it worth the extra money? Not in my case.
Here are a few test shots, all of them more or less heavily cropped, with 100% crops for illustration of what you can get with this combo. The butterfly is tonemapped, the others received only minor editing.