I have just gotten my 2x III extender and wanted to test out focal lenght. I shot the first four with my 5D II, and then I wanted to see how the 640 effective focal length would be, and I shot the last with my wife's 600D. I notice some color differences. I have been thinking about getting a 7D as a secon camera, but my question is: Is the quality of the 7D versus the 600D large?