Originally posted by mecrox If portability and airline carry-on are major concerns, have you looked at M43 and their 100-400mm zoom and 300mm prime lenses? They seem made for that particular scenario and could be used on the occasions when the big stuff is just too big for convenience.
I think wildlife photographers tend to gravitate towards bigger sensors over time. In good light micro four thirds is going to be fine, but unfortunately the light isn't always adequate to keep the iso low, particularly not when you are shooting with a long lens that is f5.6.
I also think reach is overstated by smaller sensor folks. A 400mm lens is a 400mm lens. The question is how much additional "reach" shooting it on a 20 megapixel micro four thirds sensor would give you over shooting it on a 24 megapixel APS-C sensor -- some, but it is not the additional 50 percent that some people seem to think it gives. The same is true for full frame versus APS-C. If you are shooting with a K-1, there is no additional reach gained by shooting with a 16 megapixel APS-C camera, while a 24 megapixel APS-C sensor does give some additional reach, once again it is no where near the 1.5 additional reach that the crop factor would indicate.
I like crop cameras. They have faster frame rates, tend to be smaller, and are cheaper for what you get, but they do have their limitations too.