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With the 5D, you cannot do any microfocus adjustment yourself that you can do with the 5D Mark II. I am not sure which one you get. Either way, you need to make sure that the AF tolerances of both, individually and to each other are there. Then you can evaluate the AF performance of both using the center and also try the outer AF points. Under good lighting conditions, the outer AF points are not that bad. Pay attention to, look and use contrasty targets. I may sound like a broken record but you have to give any of the AF point a try until you know how low the lighting conditions can go before you resort to the center AF point only. Have fun!