I think I did well with this tc, these often bid up because they aren't made for PK any more AFAIK. There was only one other bidder. Very pleased with its LNIB condition on arrival.
According to the review page info, Sigma tried to restrict the compatibility of this tc to a selection of its own lenses. However apart from finding it a little reluctant to engage its locking pin in the adaptall PK mount, so far so good.
These are my first tests, Pentax K5, with my Adaptall 60B 300mm f2.8, compared against the adaptall 014F 1.4x tc, and the Kenko 1.4x tc I have had for a couple of years
All f2.8 (=f4 with tc). From top L: reference 60B, upsized 140%; 60B + 014F, 60B + kenko, 60B + sigma apo.
First tests indicate clear superiority for the sigma apo.
I am going to park the FD 400mm for a while and concentrate on using the 60B with the tc(s). That gives me a third of a stop advantage, f4 vs f4.5, more importantly the K5 has so much better dynamic range than the samsung NX20. My 60B was a bit of a battered ol' thing when I acquired it, though optically its good, and it suffered a serious mishap (ALWAYS! always fully close up double zip pulls to one end, if you don't the zip
will open up), exiting myLowe Slingshot and landing on its mount end with a thud. Fortunately no optical damage, but now the aperture ring is really stiff and my efforts to loosen it merely lost the clicks. And PKA mount won't click into AE position so I am limited to M mode - but thats ok, a couple of years ago I would have been more reliant on the auto of Av mode, but now my skills and juudgement of exposure have improved..
Quiet at the RSPB, this cormorant provided the only rather distant opportunity with the new tc. Big crop.