TexPhoto said:The 300mm f4 IS is ...So sharp you can crop to the view of a 400mm and have a better photo than with the 100-400mm
TexPhoto said:The 300mm f4 IS is an awsome lens I think you should consider. You loose the zoom, and you won't reach 400m, but you'll have alot of reach and super sharp photos. So sharp you can crop to the view of a 400mm and have a better photo than with the 100-400mm or some combination of lenses and tele-converters.
The thing you have to consider with any teleconverter is will it produce a sharper photo than simply shooting with the same lens and then just cropping?
neuroanatomist said:TexPhoto said:The 300mm f4 IS is ...So sharp you can crop to the view of a 400mm and have a better photo than with the 100-400mm
The 300/4 is a little sharper, but not that much sharper - comparing the 300/4 wide open to the 100-400 @ 400mm wide open, they deliver about the same resolution in both center and borders. The 300/4 stopped down to f/5.6 is a bit sharper in the centers, but about the same at the borders.
I agree that the 300/4 is a great lens, but really, it comes down to the focal length you need/want. I wouldn't get a lens planning to crop most of the resulting images. Also worth considering the intended subject(s) - for example, if you're shooting birds, most likely even 400mm will on APS-C will require some cropping, 300mm would require more and a correspondingly greater hit on IQ (which is why I sold my 300/4 and bought a 100-400 instead). 400mm is a break-point - a longer lens will cost you significantly more (or you'll take a hit on IQ with less expensive 3rd party lenses).

TexPhoto said:I don't have a 100-400 to compare to my 300 f4, but the resolution tool over at the TheDigitalPicture tool show the 300 to be much sharper than the 100-400 at 5.6, center mid and corner. And the 70-200 2.8 looks really sad at 200, and 280, and 400 (with teleconverters) And the 300 still looks slightly better at 420mm (with a 1.4X) than the 100-400 at 400 (both at 5.6)