The D80 manual requires me to lock the aperture ring of a lens at the minimum aperture. But the ring on this18-135/3.5-5.6 Camera Lens does not lock at all. There is no click and the ring can be rotated clock- or counterclock wise by about 150 degrees without any effort. I don't know what to make of it and the D80 Manual referrs me to the lens manual which I do not have. I dont even know what that ring is for. SOS, please!.