A general question. Which Canon lens would users recommend for use as a wildlife photography lens with my 5D Mk3? I favour the fast 400mm with a 2x extender but have been advised that the 500mm lens is a better idea as there is no loss of image quality(apparently 2x extenders do degrade the image a little) and there would be no problem retaining autofocus( I have an eye problem). What do members think?