Taking an extreme example of what good AF needs to do the Nikon 500 f4 AFS has 3 inches total dof at 25 feet focus distance at f4 so total dof is less than 1% of focus distance.

• 举个比较极端的例子来说明一下AF功能需要达到什么样的准确度，Nikon500F4AFS在25尺的距离上有大约3英寸的景深，所以全部景深距离甚至不足对焦距离的1%25。