Top store - wish we had such stores here in Australia.
What I meant by far field was somewhere between infinity and long range where the master lens IQ resolves less (if that makes sense). My logic is that the TC may actually prove better than the master lens in these situations. I might go out and try this idea now using the F*300 at photograph objects at about 50-100m. I have a laser range finder so I will know distances. Will report back soon.
---------- Post added 12-30-15 at 06:28 PM ----------
I was intersted to see if there was a range where the TC was superior to image crops of the master lens (F*300mm).
I tested complex objects at 36m, 62m, 69m and 93m. Tripod + SR off + K3 + f6.3 for all images. Shutter speed varied due to TC influence.
Results are as follows.
F*300mm + TC @ 36m (f6.3 1/200)
F*300mm @ 36m (f6.3 1/500)
F*300mm + TC @ 62m (f6.3 1/400)
F*300mm @ 62m (f6.3 1/1000)
F*300mm + TC @ 69m (f6.3 1/400)
F*300mm @ 69m (f6.3 1/1000)
F*300mm + TC @ 93m (f6.3 1/250)
F*300mm @ 93m (f6.3 1/500)
---------- Post added 12-30-15 at 06:34 PM ----------
All in all, the F*300 probably wins still at 'far field' but the differences seem to be mostly contrast related. A little PP tweek to ramp up contrast and also a tiny week in the sharpness department and I imagine the only difference would be colours (sigma TC dulls colours a tiny little bit). Again, a little week in the colour department and I suspect that difference can also be accounted for.
So, based on this rapid comparison I find no reason to NOT use the TC if one is less inclined to PP/ crop images. The extra reach does sacrifice IQ but perhaps only 5% or there about. The above images are quite please imho.
The Sigma 1.4X Tele Converter is definitely a good TC when matched with the F*300mm