Saying that someone has 20 / 20 vision ( 6 / 6 in metric terms ) means that they can resolve a spatial pattern ( a letter of the alphabet , say ) in which each element subtends an angle of one minute of arc when viewed from a distance of 20 feet ( six metres ) .

假设某人有20/20的视界（6/6公制单位），意味着当站在20英尺（六米）的距离观看时，他能分辨那些最小组成元素对应于一分弧度角的空间图案（比如字母表中的一个字母）。