I am trying to decide on my next lens purchases. I am a bird and wildlife photographer who does landscapes & macro on the side. I currently have the EF 16-35mm f/2.8 L II for my landscape work, 100mm f/2.8 for my macro work, and the 100-400mm f/4.5-5.6 L IS for my primary photography. I currently use the 7D, however depending on when the 7D II is announced/released, or how the high MP camera pans out, I may get a 5D III.
I ran into the limitations of the 100-400 some time ago, and am ready to move on to bigger, better things. I have been renting Canon's new line of telephoto lenses. For my primary work, my heart is pretty dead set on the EF 600mm f/4 L IS II along with the EF 1.4x TC III (and probably 2x TC III with the 5D III + f/8 firmware) for the bird photography. However before I spend that kind of money, I wanted to figure out if there may be something shorter that I could use for birds in flight and wildlife. I rented the 300mm f/2.8 L II a few months back, and the quality is simply unbelievable. It blew me away. With 1.4x and 2x teleconverters it extends right up to 600mm f/5.6, which is good for a lot of things, including wildlife at a comfortable distance with 300m and 420mm and bird photography at 600mm in good light (although that f/5.6 aperture fails to handle morning and evening or overcast photography very well.)
If I do pick up the 600mm f/4 L II, that will burn up my budget and then some. I'm sure that lens will do well for some wildlife photography, however it won't be all that handholdable (although Canon's weight savings are an amazing achievement), and that focal length could make it difficult to get some wildlife shots for the less shy wildlife...and we have quite a bit of that here in Colorado...deer will get within feet of you at times. I've wondered whether picking up the EF 70-200mm f/2.8 L IS II would be a worth-while replacement for the 100-400, and capable of producing quality shots. According to my calculations, the aperture, despite being a relative f/2.8, is actually the same size entrance pupil as the 100-400 (200/2.8 = 71.43, 400/5.6 = 71.43). The smallish entrance pupil on the 100-400 has not really done much to produce that nice, high quality, creamy boke. It never comes close to the quality of the 300mm 500mm, or 600mm Mark II telephoto lenses. Additionally, the 100-400 is quite soft at 400mm until you stop down to f/7.1, at which point it sharpens up a bit, but is still visibly poor compared to the sharpness of any of those same telephoto lenses.
Is the boke and sharpness of the 70-200 f/2.8 II closer to the caliber of Canon's new Mark II telephoto lenses? How does it fare with the 1.4x TC? Is the 280mm focal length with the 1.4x TC a good enough replacement for a 300mm prime?
Or, to get the kind of quality I'm looking for, do I really just have to knuckle down and get both the 300mm f/2.8 L II and 600mm f/4 L II?
Thanks for any help!