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What's the real world impact of using aperture lenses of slower than f4 for portrait shooting or edge af on the 5d3/1dx? This really bugs me is that I have this 70-300L/4-5.6 around, and it seems even my 60d's edge af is "better" for this lens because all the outer points on the 5d3 will be only horizontal if I zoom in more than 100mm and the aperture goes beyond f4...