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....Also 800mm f11 with no IS . I hope it's either very bright or you have a great set of legs and the subject is a statue
Adding a 1.4x on the 400 f5.6 makes it quite dark through the viewfinder...and AF slows down a bit even on the 1D4. I cannot even imagine how dark adding a 2x would make it! I concur with Ian. Shooting with an 800mm @ f11 would require mid-day sun AND high ISOs, and a static subject... That said, when I owned my 400mm f4 DO, I would add a 2x to it from time to time to get a shot which I could not get otherwise. Sharpness was acceptable, but the camera/lens combo still needed to be on a tripod.