Am I the only one that prefers birding with a 5DIII over a 7D? I had both and there was just more detail available in the 5DIII then the 7D which negated the crop advantage of the 7D completely.
Well, the "complete" negation argument is just plain false
. From a spatial resolution standpoint, you would have to have a 47.6mp FF sensor to be able to crop out the center 18mp to match a 7D (both of which would have a 116lp/mm native spatial resolution (a bit less if you factor in the low pass filter)). Assuming you had a 47.6mp FF sensor, using the same lens on both cameras, if you fill the frame on the 7D with say a 600/4, then you could use the same lens on the FF and crop out the center 18mp and end up with an identical photo. Only then
could you actually claim "complete negation" of the 7D's resolution advantage. The 5D III is actually at a disadvantage spatially (80lp/mm), which is the very reason why you need a longer lens (960mm) to get the same "framing" as you might with the 7D and a shorter lens (600mm).
If you are lucky enough to own an 800mm lens, the 5D III will probably serve you well...but you would be stuck with 800mm. You couldn't slap on a teleconverter and get more (since you would then be at f/8, and could no longer AF). You could use a 600mm f/4 with a 1.4x TC and get 840mm, but your still short of that 960mm you would need to actually fully negate the 7D's resolution advantage. And thats nothing to speak of the additional advantage of the 7D with the same 600/4 + 1.4x TC, which gives you an effective 1344mm. One way or another, the 7D has a reach advantage you just can't match with the 5D III, or for that matter even the 36.3mp D800.