I try to use a sigma 18-50 at f2.8 to take macro shots but they always come out slightly out of focus. It looks somewhat noisy too, but iso is low at around 320. Do I need a specific macro lens to do macro shots? What specifically does a 50mm macro lens do that my 50mm lens doesn't? Also do those extenders work? I don't take enough macro shots to buy a new lens really.