For completeness I should add that if money is no object, the combination of a DA*300 f4 and DA 1.4x TC has a number of advantages:
- 1/3 stop faster (300mm f4, or 420mm f5.6 with the TC)
- quiet and possibly more accurate autofocus (not necessarily faster)
- quick shift (manual focus override) (The FA has a clutch to switch to MF which is pretty good for a lens of that vintage, but not as good as QS.)
- more recent coatings
- Weather resistant, both lens and TC
- best quality TC
- EXIF data records correctly
- SR based on the real equivalent focal length
Downsides:
- Cost: about $US1100 for the lens and $500 for the TC. The lens is a little cheaper used; the TC is very difficult to find. (Compare that with about $850 for the F or FA 300 f4.5 + a decent third party TC)
- Some people who have used both argue the image quality from the F/FA lenses is better (probably splitting hairs)
- SDM autofocus is known to fail at times, and the repair may be uneconomic (but the lens can be converted to screw-drive AF). Many of the screw-driven F/FA lenses are still going strong after 25-30 years; you wouldn't expect many SDM lenses to last that long.
- DA weighs about 100g or so more
- A little bulkier
Neutral:
DA*300 uses 77mm filters. same as the DA 12-24, for example
F/FA*300 use 67mm filters, which are a little cheaper. Same size on a number of DA* lenses, and some others like Tamron 17-50.
(This is an indication of the smaller diameter of the F/FA* lenses.)
Theoretically, a third party TC with pz contacts (e.g. Tamron/Kenko pz) will drive SDM autofocus, but people who have tried it report that the AF doesn't work well, or reliably. That's why if you get the DA*300, you should really get the Pentax DA TC.
One more lateral thought, Jack. Since you have a Sony A7ii (24mp?) have you looked at other long tele options for that instead? The 24mp sensor is an advantage for photographing wildlife, where cropping is the norm. Or replace both the Sony and the K-50 with a K-3/K-3ii/K-P, or even a K-1, if you are not heavily invested in Sony gear.
Last edited by Des; 03-01-2017 at 04:11 PM.