With the vector plane-wave spectrum and stationary phase method, a rigorous vector diffraction model of an aplanatic system when the polarized point source is at an arbitrary location on the optical axis is presented. The computer simulation is used to discuss in detail the effects of various angular semiapertures on the object and image sides on the resolution. Results show that angular semiapertures on the object and image sides have an obvious effect on the resolution and image fields, which indicates that the classical Wolf theory [Proc. R. Soc. London, Ser. A253, 358 (1959)] cannot be applied to the study of imaging properties of an aplanatic system when the point source is not located at infinity in the direction of the axis.