I just got a 2nd hand nikkor 18-200mm lens. However, when i switch to the A/M mode (autofocus with manual priorty), I still need to adjust to focus ring until the right focus before it allow me to shoot or whenever i want to take photos. Is this how this lens work or is there something wrong with my lens? I find it quite troublesome especially when i need to focus and shoot fast as the focusing will take some time and have to refocus again for the next pic. Thanks.