Hi John
I echo Chris Ross’ comments about the Canon 300/4L IS and TCs for birds and other wildlife. Why? I used the same lens for over four years and can promise you that you would hardly ever need a focal length less than 400mm for birds. Unless you are shooting in a zoo or some tame birds at your local duck pond.
I have used the 300/4L IS with the Canon EF 1.4x and 2x converters and got excellent results. Even with stacked converters. However, a lot of it also has to do with shooting technique and the longer focal lengths with more TC stacks become more difficult at first (at least).
I never considered using the Sigma zoom due to the reports of softness at the long end. However, it will depend on what you want personally. If you want the best quality images, then go Canon (even the 100-400 zoom, though I wouldn’t choose that for the reasons mentioned in the first paragraph).
Here are a few sample images taken with that lens or lens plus Canon TC combo.
Full Frame with stacked 1.4x and 2x converters (manual focus with anything other than a 1D body – this was a 30D) http://amatteroflight.com/gallery2/v/ausbirds/falcons/kestrel/Australian-Kestrel_9290.jpg.html
300mm and 1.4x http://amatteroflight.com/gallery2/v/ausbirds/falcons/kestrel/Australian-Kestrel_9687.jpg.html
300mm and 2x (manual focus with anything other than a 1D body – this was a 30D) http://amatteroflight.com/gallery2/v/ausbirds/falcons/brwnflcn/Brown-Falcon_0113.jpg.html
300mm and 1.4x http://amatteroflight.com/gallery2/v/ausbirds/hawkowls/powerful/20110312_9999_24.jpg.html
Full Frame with 300mm and 2x, manual focus, hand held, 1/125th of a second shutter speed. So image stabilization works big time! http://amatteroflight.com/gallery2/v/DAMSELFLIES_DRAGONFLIES/Dragonfly_3774.jpg.html
300mm hand held, full frame shot. At minimum focus distance of five feet http://amatteroflight.com/gallery2/v/OTHER/SNAKES/easternbrown/_AGL3476.jpg.html
Sorry for the image overload. I would definitely go Canon!
Kind regards Akos
