Image space F/# is the ratio of the paraxial effective focal length calculated at infinite conjugates over the paraxial entrance pupil diameter. Note that infinite conjugates are used to define this quantity even if the lens is not used at infinite conjugates. 

由以上信息可知：像空间F#是系统的有效焦距与入瞳直径的比值，而这两个参数都是以近轴光学为基础的。 

而且，这种F#不会因为共轭状态的改变而有任何变化，很显然，MTF 的衍射极限不会以此为准。