I just got my 60mm Micro, and I have a couple of questions. 1. Do I want to keep the aperture lock off or on? 2. I cannot get the aperture to stay at 2.8, I get 2.8 and focus, and it goes to 3.2.I can play with it and it comes back to 3.2. is this normal? 3. I was in Aperture Priority, is this an issue with this lens, and if so, what should I use? Thanks.