I just bought a 105mm AIS micro and I noticed the focus "lock" knob. I know it's not supposed to lock fully but it should at least make the focussing a bit heavier right? Well, my focus lock knob doesn't feel like it does anything. The unlocked knob feeling is very heavy though. Can anyone give a comparison with the focus weight of this lens compared to say the 50mm AIS f1.4 or something? Is it supposed to be heavier or have I got a lemon? Thanks!!