Helevitia said:
Just to be clear, I am not saying the 7D is a bad camera, or that there are not very good reasons to choose one over a FF camera, all I was addressing was the farcical claim of a 48.8% reach advantage.
It is not farcical that the 7D has about 50% more reach than a 6D with the same lens. I have written this before, but it is buried in another thread. Here are some simple calculations and experiments that show the 7D does give extra reach of about 50% over a 6D. 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 micron pixel, that would be 8.6 micron apart and on a 6D, with a 6.54 micron pixel, that would be 13.08 micron 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 6D with a lens aperture wide enough so it is not diffraction limited (wider than f/6.9 for the 7D, 10.5 for the 6D), 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 1.5 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 micron, 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 micron, just above the supposed limit for resolution of 8.6 micron. The barbs were just resolved at 12 m, with image sizes of 6.7 and 7.8 micron 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 micron for the 4.3 micron 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 5.9 m from the 6D. The transition of just being resolvable at 12 m from the 7D would be 7.9 m from the 6D, and the complete loss at 14-15 m from the 7D would be 9.2-10.2 m on the 5D. It doesn’t matter how superior the 6D 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 7D but you see a blur with the 6D using a 600mm lens.
Theory dictates and experiments confirm the extra reach. Of course, if you use a crappy lens, like a Sigma zoom at 500 mm, you won’t get near the theoretical resolution, as you won’t if you are down a coal mine lit by a candle. But, the good Canon lenses at wide aperture are not limiting for resolution in decent light, it is the pixel density that is limiting in resolution.
Why is the extra reach important? You need it to capture high detail at far distances. My bird photography friends with their 1Ds and 5Ds need 50% longer lenses to capture the same detail as can be got on an 18 megapixel crop sensor. But, in dim light where extreme resolution is not essential, the guys with the 1Ds win.
