the Sigma isn't a variable aperture lens. it's a constant f5.6. the Sigma 800/5.6 prime is better than the 300-800 zoom and is not as sharp as my Nikkor 600/4 AFS-II with the Nikkor TC-E14II extender. the 800/5.6 is slower than the Nikkor to focus too. i ended up selling my Sigma 800/5.6 after testing the two side by side.
Herb...
Edited by HerbChong on Jun 04, 2007 at 04:11 PM GMT (Reason: fix typo and add info)
For what it is worth. In my film days I simultaneously owned both the Canon 800mm F5.6L and the Nikon 800mm F5.6 ED. I originally owned the Canon because it was the only 800mm F5.6 lens available as the nikon 800mm F5.6 came along several years later. I thought the canon was good but when I did side by side film tests the nikon was the clear winner. It wasn't even close. The nikon was quite superior wide open, stopped down, at infinity, at the closest focusing distance. The canon was lighter. And the rear knob focusing was a little easier to use. But that was about it for the canon. Image quality wise it just wasn't in the same class as the nikon.
