I see nothing wrong with your question. In short, just how handy is the lens.
I know that on a specification list there's only a small weight loss for the 800 over my 600mm L f4 IS (not the new II version, which will be lighter), but somehow the balance and the perceived weight seems much better. I intend to sell the 600 if I become completely satisfied with the 800. I also will be selling my old version 1 400mm L f2.8 as it is also very heavy. My 1D Mark III and 1Ds Mark III will probably go on the block, too, since the 5D2 does what I want for full-frame.
Of late, I've had more problems with my back and I decided to try to lighten the load.
I'm already happy with how much closer I can get with just the 800mm lens. After making some exposure adjustments and more test shots, I'll try the two TC IIIs on it. The refurbished 1D4 seems pretty damn good, especially at 1600 and 3200 ISO.
This one was on auto ISO at 1250, program mode at 1/320s @ f5.6. Really, I'm getting a tingle up my leg!
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