The physical behaviour of an incoherent-light illuminated optical imaging system is investigated. The result obtained is that there is a coherent region in the reference spherical surface of entrance or emergent pupil which is corresponding to a certain spatial frequency. The author explained the physical characteristics of incoherent optical transfer function sufficently, and suggested a possibility which may vary the transfer function curves of the system, thus permits a normal optical imaging system to have the ability of processing input signals in the frequency region.