Buy the Sony 400mm f/4.5: http://SDP.io/S400f45 Buy the Sony 600mm f/6.3: http://SDP.io/S600f63 Sony's new 600mm f/6.3 and 400mm f/4.5 GM primes weigh less than a bag of sugar — but should you buy them instead of the 200-600mm zoom? I'm Tony Northrup, and I put Sony's two brand-new ultralight supe...