Here are some simple calculations and experiments that show the 7D does give extra reach of about 45% over a 5D III. Theory tells us that in order to resolve two parallel lines, their images should be at least 2 pixels apart. On a 7D, which has a 4.3 µ pixel, that would be 8.6 µ apart and on a 5D III, with a 6.25 µ pixel, that would be 12.5 µ apart. For an object at a distance v, which is much further away than the focal length of the lens f, the size of the image is given by the size of the object times f/v. For the same lens on a 7D and the 5D III with a lens aperture wide enough so it is not diffraction limited (wider than f/6.9 for the 7D, 10.1 for the 5D III), the sizes of images on both sensors will be the same for the same distances but the image will span more pixels on the 7D. In order to cover two pixels, the object will be 1.45 times further away for the 7D. I did some experiments to test the resolution of the 7D fitted with an f/2.8 300mm II and 2xteleconverter to give f = 600mm and f/5.6. I photographed a feather whose barbs (middle right, below halfway) in the photo were approximately parallel lines separated by 0.27 mm or 0.31 mm (middle right, above halfway). At 5 m distance, nearly all the barbs, including others that were not parallel to the rows or columns of the sensors were very clearly resolved the calculated distances in the image were 16.1 and 18.7 µ, respectively, for the lower and upper sets. At 9 m separation, the barbs were still clearly resolvable, with calculated image sizes of 8.9 and 10.4 µ, just above the supposed limit for resolution of 8.6 µ. The barbs were just resolved at 12 m, with image sizes of 6.7 and 7.8 µ respectively. The images became unresolvable between 14 and 16 m. My calculations of height of image are reliable within 3 percent since the same calculations gave a pixel size of 4.4 µ for the 4.3 µ pixel. So, despite all its supposed limitations, the 7D resolves images at the closest theoretical limits. In order to have the clean separation seen at 9 m on a 7D, the feather would have to be at 6.2 m from the 5D. The transition of just being resolvable at 12 m from the 7D would be 8.3 m from the 5D, and the complete loss at 14-15 m from the 7D would be 9.6-10.7 m on the 5D. It doesn’t matter how superior the 5D III is than the 7D, the laws of optics and information theory dictate that at 9 m you can resolve the barbs of feathers on a 7 D but you see a blur with the 5D using a 600mm lens.
