Tough to imagine any of these being a "bad" choice. But that is part of what makes the decision so difficult. In other threads, one of your themes has always been the "packability" of the 100-400L, meaning that it compresses and both packs away nicely and travels well. With that in mind, and I am sure you have thought about this, another option would be a 500 mm II and use it primarily with a 1.4x III on the 1Dx. This would give you the option of 500 mm f/4, 700 mm f/5.6 (very comparable to your current 640 f/5.6), and 1,000 mm f/8 (AF issue). If I were having this debate (and hopefully I will in a couple of years), it would come down to this and the 600 mm. But, in another thread, Rick has pointed out that the 600 mm with the 2x III loses some resolution. So, with the 600 mm you have a larger, less packable/hand holdable (how often will it need to be attached to a tripod?) supertele giving you 600 f/4, 840 f/5.6, and 1,200 f/8 (slight hit to IQ plus AF issue with 1Dx) vs a slightly more packable/hand holdable 500 mm.Originally Posted by neuroanatomist
While everyone always wants longer, if I was in your shoes, I would probably be thinking of getting the 500 and using it with the 1Dx and 1.4 III (700 f/5.6) as the primary option, but keeping the 7D for those times when you want even more (800 mm, 1,120 mm, and 1,600 mm f/8-usable AF) reach. It terms of being able to go out and have fun shooting, that seems to be the most flexible approach.
Of course, if you want to become a serious birder and plan to head out with a tripod a good portion of the time, that could easily tip the scales back to the 600 mm.
I like the idea of heading down to B&H (would Hunts have the supertele's?). Good luck...
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