Originally posted by Reportage okay thanks..
looks like the bigma since with a 1.5x tele can get 750mm which is very serious range for around the same money as a DA* 300mm.
With a 1.5x tc you get a 750/11! And than you have to close the aperture, first to improve IQ and secondly to get some depth of field. I would say, that the DA300 (Which I don't have, only other 300mm lenses) and the 50-500 are complimentary lenses. Palmors photographs with the 300+tc are exceptional, but I personally try to use my lenses without a tc, to get max. performance (except the Tamron 300/2.8, which works very nicely with a tc). So I do compromise, when I choose a zoom over a prime, because of its convenience and versatility.
What lens is best for you, depends on your applications and preferred style of shooting. If you use the longer end (400-500mm) mainly, the Bigma would be a sensible choice. If you prefer the medium focal length, the 300mm will probably be a better investment, as it is smaller, more light weight and faster.
If you are into birding or shy wildlife a lens can hardly be too long, so that there is a slight advantage with the Bigma. You just have to be aware, that there is a noticeable contrast loss, increasingly between app. 420mm and 500mm fl, which you should correct in post-processing. (But the sharpness is still there.)
If you use the Bigma with a tc, go for the matched Sigma Apo 1.4x-tc, which can be used with the Bigma from 100mm fl onwards (there is a lock on the lens, which prevents useage of shorter fls, because that might damage the rear element or the tc's protruding front element. But I personally would not use it with a tc.
Ben