I'm trawling this excellent web site and many others with the view of buying one of the Sigma 500mm offerings for wildlife. But after reading all the info and opinions, I'm beginning to wonder what use a long zoom is.
It would replace a 300mm f4 Nikkor and TC14 converter (420mm f5.6). Wide open (the only aperture that matters with wildlife), this combo has a relative resolution (MTU50) of 1847 in the centre and 1576 in the periphery. This translates to pin-sharp and sharp, soft appearance coming in at around 1400.
The Nikkor 80-400 f5.6 at 400mm wide open has 1781 in the centre and a woeful 1242 in the periphery (that surprised me). And the Sigma zooms are being compared to that benchmark. The prime lens with converter is sharper in the centre, and the zoom is picture-spoiling soft in the periphery. Do the Sigma 500's perform better at 400mm than the Zoom-Nikkor? Peripheral softness wide open seems to be an issue with the Sigmas, also.
The argument that the periphery doesn't matter with a tele because the subject is only in the centre seems for children! I would've thought that anyone caring about wildlife compositions would need a sharp periphery, and it's essential for landscapes.
The obvious advantage of the zoom is that you can back off quickly should the subject change or for a looser composition. By backing off a little with a zoom, you would have a slightly sharper 300mm lens of around f5, I suppose. With a few more seconds, I can remove the 1.4x converter and have a crackingly sharp and even 300mm f4 lens, that one review calls the closest to a perfect lens yet reviewed.
By adapting the 300mm prime to my Olympus 4/3 2nd body, I get a pin-sharp manual focus 600mm f4 image stabilised lens that weighs 1400 grams (and focusses closer than any of the zooms). My 300mm f4 and 1.4x, both ruggedly made, were purchased 2nd hand for less than the cost that the Sigmas are selling for 2nd hand.
I'm still interested in a long tele zoom; I guess I'm wanting to be convinced of their advantages:
Is the backing-off ability really valuable while working wildlife?
Does peripheral softness spoil your pictures, other than bullseyed shots?
And is anyone using a 1.4x converter with one and getting publication quality?