Both seem to be moderately well liked, and since they are at least half as good as the Nikon 200-400 while being 1/5th the price, I'm curious to hear if anyone has used them , and what they thought of them.
I've never owned the Tamron (But shoot with a friends Tamron).
For me :
Bigmas built quality is better,also slightly sharper on the long end and the contrast is also slightly better.HSM is silent&more accurate+little less CA
On the other hand Tamron is lighter&more managable(hand held 1/250-1/500),a interesting future : Detachable filter effect control (Never need it but maybe usefull for some others)
Both lenses comes with tripod collars (Again Sigmas collar is slightly better)
My vote goes for the Bigma.
@500mm handheld wide open Bigma+D2X/jpeg http://homepage.mac.com/Merts/bt4.jpg
i currently use the Tamron 200-500 and i used to have the 50-500 in Pentax mount. i sold the Sigma because although it was sharp, i hated the bokeh. i find the Tamron with very good sharpness and better bokeh. the drawback is that the Tamron uses screw drive focusing while the Sigma uses HSM, the equivalent of AF-S.
We have teh 170-500 at my paper, and I can't stand using it...its too slow for anything we do (even baseball in the bright daylight) and the quality is sub-par. I'm def. not a fan. I'd rather use my Nikon 80-200 2.8 and crop in anyday.
