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| 100-400 better than 800L for BIF |
Reno, I did test 7D with three lenses last week on head-on incoming flights: 300 f/2.8 IS, 85L and 300 f/4. I used two different AF point selection schemes: center point with expansion, and area AF. In all cases, 7D was significantly inferior in performance to my 1DMkIIN and 1DsMkII.
Joshua: As far as I can tell, 800L is handicapped for BIF photography by four factors: (1) huge defocus range which makes AF response sluggish unless the focusing distance is in the ballpark to start with, (2) significant inertial mass of the lens which makes it "resist" fast changes in swing and aim , (3) "looking straw" effect and (4) focusing distance limiter threshold values.
Naturally, the physical exertion of handholding compounds the problem.