I sometimes use my D300 in multi focus zones mode and have a question. When I shoot a busy scene, the camera will display several focus zones that it is using to determine focus. How can it do this and why? Since the lens can only focus at one point, does it use the closest spot and the furthest one and stop down the lens to make sure both are in focus? If not, how does it determine which one to focus the lens on? Thanks much!!