neuroanatomist said:
That link isn't quite the comparison the OP was looking for. This one is -
70-200/2.8 II + 2x III vs. 300/4 + 1.4x II. Looks like the 300/4 + 1.4x has a slight edge across the frame - probably not enough of a difference to have any impact on real-world shots, so from a straight IQ standpoint, either would do fine.
Fixed the link for you (don't forget the =).
My own two cents - the 300mm f/4 versus the 70-200mm is perhaps not a straight comparison.
With both you are going to be limited to 400 or 480mm equivalent at the long end, and f/5.6 as the maximum aperture. This is "good enough" for some larger subjects at a distance but not for others (small birds basically require a 600mm lens on APS-C format, I've found, even if you have good stalking skills).
With the price of the 70-200mm, for almost exactly 1/3 more you can get the new Sigma 120-300mm f/2.8 OS (and the cost of the EF 2X is the same - if you "only" need roughly 480mm equivalence you could get a 1.4X extender and even save money if you go for the version II). When you lose the TC you still won't be able to frame as tightly as the 70-200mm but you will get a roughly 600mm f/5.6 lens - nothing to sneeze at!
The only downsides of the 120-300mm f/2.8 OS are, aside from price, are: Autofocus being iffy on the T1i (a cheap camera however), the actual angle of view being somewhat shorter than a "real" 300mm lens, and the high weight - it's double that of the 70-200mm, or about six and a half pounds (without the TC)! Another problem - the MFD of the bare 120-300mm is 1.5 meters at 120mm, and 2.5 meters at the 300mm setting, which limits how close you can get to your subject. For a ~600mm lens it's a pretty great minimum focus distance, however. All said, if you can get close enough to the subject you might do better with an EF 135mm f/2! Still, I use the Sigma 120-300mm f/2.8 combination all the time.
Mt Spokane Photography said:
I'd not use TC's for birds in flight unless they are slow or hovering. AF is slowed down too much. That will be a bigger issue than sharpness. If you need 400mm, get a 400mm lens and things will work out better.
This as well - the native 120-300mm OS is plenty fast but with a TC - it's still as accurate but tracking birds is rather hopeless. I can track passing jets overhead, but they move very predictably compared to birds. A big problem with the newer TCs slowing AF speed is that it takes a while just to find the subject - from 1 second (a bit rare) to three or four seconds. Grabbing the focus ring and cranking it to infinity before you start tracking might help a bit - it's too bad Sigma still doesn't include a focus limiter on even their EX lenses (though when looking at the distance scale on the 120-300mm I think it would be rather awkward to implement correctly).
