The most fully transparent of all the multiply exposed, deceptively transparent photographs in Matthew Porter’s show “High Difference”—a WWI-era style of camouflage that aimed to dissemble and disorient rather than conceal, “High Difference” would also be a good name for the kind of easy-spirited conceptual double cross that seems to be the fairest flower of our world-weary age—is Field, a color print nearly 5 feet high.