Originally Posted by TCav
So the "cropability" of an 18MP image from the APS-C 7D is better than the "cropability" of a 12.8MP image from a FF 5D. But it's more a result of the FF vs. APS-C thing, not so much from the 18MP vs. 12.8MP thing.
It's a combination, as it depends on how many pixels will be left in the image after cropping. That's what I meant earlier about doing the maths. If I want a camera for taking birds in flight and with no camera/lens can I get close enough to fill the frame, then I'll get better results using the same lens on a 7D than even a 5D2. That's partly because of the larger effective focal length, but greatly due to the number of pixels left after cropping. That's why I'm getting a 7D to augment my 5Dc, and not simply swapping it for a 5D2.