Misunderstandings always build up in threads because people come in at the end without reading the earlier posts. All of these images are for quick comparisons done under the normal conditions I, and many others, would use for amateur bird photography. The results are of use to me and for some others who may be similarly wondering whether to keep their 7D now they have a 5DIII or whether they should sell their old 7D to buy a new camera. The conditions are not what the perfect ones that TDP etc use for a scientific comparisons, but sub-optimal conditions used in every day life.
1. The title says exactly what is going on – cropped images for bird photography at 600mm for a 5DIII vs 7D. It is not a scientific survey of FF vs crop under all conditions for all lenses and all bodies. The 7D is not a representative of all crop cameras. Its sharpness will depend on the sensor’s characteristics - for example its AA filter. The comparison is done for heavy crops from both systems to the same size, not for landscapes etc.
2. I said at the beginning that the photos were not very good. These photos are for comparison purposes under the same set of conditions, not choosing the best photos taken on a particular camera under optimal conditions. They are not the highly selected best ones taken, of which one is proud - I have plenty of fine keepers from both cameras. So no snide comments please about how you would have discarded them immediately.
3. Regarding focus problems. The lenses have all been AFMA-ed with the bodies using both Focal and sloping ruler tests. There are issues with the AF of the 7D – many posters find that it is erratic. Motion blur is unlikely – the one of the grey (not blue) heron is representative of a group of shots that were all similar, taken at 1/1250 s and 4 stops of IS, with the camera still and resting on a ledge.
If I want to take a photo of the moon, then my 600mm on my 7D with a good tripod etc will out-resolve the same lens on my 5DIII. But, what I wanted to know the answer for is how much of the advantage of crop on the 7D is realised in 90% of what I take. The answer is that the 5DIII is better. But, the situation might change with a 7DII. And then a 5DIIII might reverse it.
First and foremost, thank you for the images and comparisons. Many (hopefully) of us understand that it is a quick test and as such, the images will not be perfect.... but then, that's kind of the purpose of the test.... if you had posted perfect images taken with both cameras the comparison would have been worthless.
I do not believe in absolute statements about this camera against that camera as there will always be specific cases to the contrary, but no matter how hard people argue those specific cases the generalities remain.
In general, the bigger the sensor the better the camera performs in low light.
In general, FF outperforms APS-C for wide angle photography.
In general, on distant object photography UNDER GOOD LIGHT APS-C will outperform FF
In general, FF is more tolerant of lens resolution limitations than APS-C
How it comes together for any individual is a mix of all those factors and a lot more.... but from what I can gather of your pictures they seem within what those rules of thumb would suggest.