Recently I saw more reviews and images featuring the new 200-400 f/4 with the internal T/C. I got so worked up, I started trying to figure out how to fund this dream lens. .... then I went on a snow geese photo shoot with my existing toys.
Background: I primarily shoot the 5D MkIII and use my 7D as a back-up. I own the 100-400, 400mm DO f/4 and a first generation 500mm f/4 as well as second generation 1.4x Teleconverter and a 2x TC.
At the bird shoot, I was very pleased with my shots from the 500 and the 500+1.4x using a tripod. I also shot with the 400 DO with and without the 1.4x TC and got a lot of really sharp shots. My plan had been to sell the 500mm and try to keep the 400 DO since it's so light and portable. If funds get tight, I'd put it for sale also to afford the 200-400. Now, I'm getting weak knees.
Help me! Other than the convenience of not changing lenses or having to install the TC, what am I gaining with the newer, extremely expensive glass that I can't already do? Would this be a wise investment change?

as good as it is I much need my 500 too... need both!
