John,
more focal length is generally better for birds if it's sharp enough,
the sigma lens is a touch soft at 400-500mm. Here's a comparison test
for the two lenses featuring 100% crops from a test target:
http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=113&Camera=453&Sample=0&FLI=7&API=2&LensComp=683&CameraComp=453&SampleComp=0&FLIComp=5&APIComp=0
<http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=113&Camera=453&Sample=0&FLI=7&API=2&LensComp=683&CameraComp=453&SampleComp=0&FLIComp=5&APIComp=0>
bear in mind this is a severe test and is pixel level view at 100% and
an actual image won't look quite as bad, but the Canon lens is clearly
much sharper and will also AF better, the Sigma is f6.3 at the long end
and the AF is a bit slow because of that, in fact it lies to the camera
as Canon bodies (except 1 series) do not allow AF at slower than
f5.6. The quality of the Canon is such that you can crop the image
down to the same framing as the sigma at 500mm and still be ahead on
image quality.
I shoot with a Canon 300mm f4 IS + 1.4x and it is also a lens I think
you should consider, it will be 420mm FL with a 1.4x, and at that it
focuses down to 1.5m for a magnification of 0.37x. Add a 25mm extension
tube and if gives near 0.5x (half life size) at a distance of about
900mm, making it an excellent long macro suitable for dragonflies,
butterflies, reptiles, larger wildflowers as well as being a good bird
lens. The IS allows you to handhold under those conditions and still
get very sharp results. I have had zooms in the past and I've found
for nature work it's at max zoom and wanting more 99% of the time. You
can select and compare image quality with other lenses from the drop
down menus above, you can also look at the effect of stopping down and
removing/adding tele converters.
You can see some results from the 300mm f4/1.4x combo on my website,
look mostly in the insect galleries at the butterflies/dragonflies,
there's also a few bird images, though I mostly use a Canon 500mm f4 for
birds, but that's an entirely different beast. Note that the smaller
damselflies, skippers and grass blues and the like were taken with a
200mm macro lens as they are very much smaller insects. The 300mm f4
will also produce good images with the latest 2x converter, but AF ( a
bit slow but usable) is only available when using a 1 series body. This
image was with the 300mm f4 + 1.4x:
http://www.aus-natural.com/Birds/Australia/Fairy%20Wrens/slides/Superb%20Fairy%20Wren%206.html
My website is here: http://www.aus-natural.com/index_gallery.html
regards,
Chris Ross
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