Thank you for answer! Good point. And a very lovely image of short eared owl!
To measure AF speed, you could record the sound of AF mechanism, then measure the duration of sound. Once shooting white-tailed eagles in flight with sigma+TC1.4 i was frustrated with AF work, so i decided to measure speed decrease then using TC.
Also - may i ask a question? What factor will influence on sigma`s AF speed? I remember that in low-light conditions canon`s 400mm AF speed was slower, than in good light. There is no such effect on sigma. But sometimes it definitely work faster or slower. Sometimes slower in very good light. What a gizmo!
Considering CA, i think, i`ll agree that Canon is better, but this will not mean that it is impossible to take good pictures using sigma lens.
I think, that it is more correct to compare 100% crops to judge IQ, than resized images. For example, here is a good article about it:
http://www.juzaphoto.com/article.php?l=en&article=10
Here are crops from 400mm f5.6, at f5.6, default raw conversion settings, no sharpening, bird`s filling of frame is good, so, IMO, these photos will represent maybe not maximum resolution available, but somehow close. I tried to find photos taken in soft light, at overcast (more or less) weather. Yes, there are just test samples, nothing more.
Sorry - looks like this forum will not allow me to use HTML shortcuts, initially i wanted to post small preview images-links to original sized images, and already wrote and deleted appropriate img codes (quite simple actually).
At f5.6
http://i537.photobucket.com/albums/ff336/kirillso/40056_100.jpg
at f6.3
http://i537.photobucket.com/albums/ff336/kirillso/40063_100.jpg
From sigma 300mm, at f2.8
http://i537.photobucket.com/albums/ff336/kirillso/30028_100.jpg
at f8 things improves dramatically:
http://i537.photobucket.com/albums/ff336/kirillso/3008_100.jpg
with TC, 420mm at f4
http://i537.photobucket.com/albums/ff336/kirillso/4204_100.jpg
with TC, 420mm at f8 (sorry - i can`t find appropriate great tit`s shot, uragus`s feathers are softer than great tit`s - so it is bit unfair comparsion here, but, in general, it represents IQ)
http://i537.photobucket.com/albums/ff336/kirillso/4208_100.jpg
Off cause samples above will not represent all light conditions available. My overall impression is that canon does have more contrast and resolution, comparing to sigma with TC 1.4. Without TC sigma 300mm is not so far behind canon in terms of resolution, but canon is tad contrastier.
I loved the ability of canon`s 400mm prime to refocus from background to bird very quickly, and blazingly quick AF itself. It really helps then shooting BIF, for example, these:
http://i537.photobucket.com/albums/ff336/kirillso/sterna_alb5.jpg
http://i537.photobucket.com/albums/ff336/kirillso/e2.jpg
http://i537.photobucket.com/albums/ff336/kirillso/wte_anniv1.jpg
And just demonstration of AF speed, not IQ:
http://i537.photobucket.com/albums/ff336/kirillso/ZWBF5_res.jpg
If sigma loses focus on bird (my hand`s fault, actually|<|), it will take about 2-4 sec. to refocus again - a lot of time actually. Yes, i am using rear-focus technique (AF by pushing AF-on button, Arthur Morris described this method not so long ago). Also, IMO, sigma tends to hunt more. I also tried to take photos of goldeneyes in flight, using sigma with TC1.4. It is quite difficult then bird is filling entire frame.
So why i bothered with Sigma? Once i was laying in hide (don`t build narrow hides, it`s a true torture!
), trying to take photos of sandpipers at the beginning of sunrise. I used 400 5.6 lens, and shutter speeds were low, about 1/6...1/20 sec at ISO 400-800 (at higher ISOs there is a lots of noise in my camera, canon 40D). It was a magic light, but i was unable to take photo i wanted due blur of moving birds, so i deleted all photos. I needed a faster aperture. Since i was using a hide, a distance was not a problem, and a less focal length will not bring major inconvenience. Most of birds i met, were quite shy, even 600mm will not help. So, using hide is required to achieve minimal distances and natural behavior of birds. I decided, that 300mm f2.8 will be appropriate lens for me.
Sigma lens is good, but have it`s own weaknesses, even comparing to "lower level" canon 400 mm f5.6. It definitely have some strong sides, such as light-gathering ability and amazing background blur (i mean not bare strong blur, but beautiful pattern of blur). I`ll say that sigma is more appropriate for low contrast light conditions (overcast, also on sunrise and sunset) - there CA is not problem.
Anyway, nowadays i have a very little time to build hide and use sigma as i planned. Also, i forced to use f2.8 in manual focus, and i need to make focus-adjustment...unfortunately, there are no service centers of Sigma in radius of several thousands kilometers
For occasion shooting at good light, i think that canon is better - in terms of IQ and AF. For hide using, i think, sigma is better due aperture and close focus ability.