« on: September 27, 2014, 03:52:44 PM »
One advantage that the 7D II can offer that I don't think the 5D III or 1D X could ever really compete with is the ability to get long equivalent focal length at a faster max aperture...allowing the use of more than just the central cluster of AF points at the much slower f/8. The 7D II could achieve an effective 1344mm focal length (FoV equivalent) at f/5.6. At best, FF cameras can achieve 1200mm f/8. The added AF power the 7D II can offer at a very long effective focal length is intriguing.
That's true if your output requires more than the 7-8.6 MP you'd get by shooting the FF at 840mm f/5.6 and cropping to the APS-C FoV. Granted, the 20 MP APS-C allows deeper cropping, but in most cases the IQ will suffer from atmospheric impact at distances which would require such deep cropping.
It depends on what your shooting. As I said, my primary use case for 1200mm is not full body birds...it's bird headshots. The distance doesn't increase, only the amount of detail you are resolving on a smaller area of your subject. Similarly, when it comes to the smaller shorebirds, like a Least Sandpiper. Were I could get away with using 840mm for larger ones, 1200mm is useful for smaller ones. Again, distance does not increase.
Therefor, atmospheric effects are not an issue. The primary issue is getting pixels on target, for a subject that is filling a good portion of the frame. The 7D II will put more pixels (and hopefully BETTER pixels, at least than any other crop camera) on a bird headshot at 840mm f/5.6 than the 5D III at 1200mm f/8. I'd expect the IQ to INCREASE with the 7D II, assuming Canon isn't just playing games, and their high ISO has really improved by about a stop.
Don't forget songbirds! Little sparrow-sized passerines need a lot of focal length, even at moderate distances. It's rare they'll let you get close - especially the best-looking ones (in my experience).