When I was considering high end spotters a couple of years ago, a dealer who is into birding did a lot of research for me. She quoted someone at Swarovski as saying the difference between HD and non-HD versions was seeing individual bricks in a wall at 200 yards vs. seeing the texture of the bricks. I did not get either Swarovski (cost), and have no way to verify what I was told, but I do have confidence in this dealer, who seemed to offer honest opinions and did not try to hard-sell me any particular brand or scope.
OTOH, I know a serious competitive shooter who is a professional photographer who says HD glass offers no advantage in monochromatic contrast situations like shooting at black bullseyes. That seems to confirm that it's main advantage is discerning color contrasts, as in photography.
Paul
