K-3 + Sig 50-500mm + 1.4X (4 imgs)
I made a quick check using the K-3 + Sigma 50-500/4.5~6.3 APO DG OS HSM (at 500mm) + Pentax 1.4X HD AW teleconverter. Note #1: CDAF does not work worth a damn with this rig. Note #2: I am certain that a Tamron-F 1.4X Pz_AF or a Kenko 1.5X Pz_AF would work almost as well.
(1) - The focal length and aperture is not corrected to the camera.
(2) - Although I am showing images that I took handheld, using the OS on the lens set to position #1, I suggest you use a tripod.
(3) - It is best to manually pre-focus the lens to near focus before trying to autofocus. Else just use Catch-in-Focusing (CIF).
(4) - It appears to me that aperture selection of f/8, f/9, or f/10 is best.
(5) - IMO the images are quite usable. Make of them what you will. YMMV.
Ron - 'We don't have time to go take pics this afternoon Carl.'
Carl - 'What do you mean? It will only take 1/1000s.'
'Keep your eyes looking forward. However, glance back now and then to see where you've come from. It will put a smile on your face.' ~ brandrx