What I meant Duncan is that a normal 50mm lens is not optically designed to work at it's best at macro distances so you will not get the flat field a true macro lens will give and no doubt there are other optical corrections missing at these distances but I agree for normal use like close up of flowers and what have you there is no discernible difference. Convenience and versatility are the main differences, as you say there is no new glass in the optical train. Are the tubes you are looking at auto tubes Alex? Much easier to use if they are.
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