This paper is concerned with the theoretical description of the radiation field of parabolic mirror collimators. It is shown, by means of a series of approximations of the integral form of Maxwell's equations, that the physical optics and aperture methods lead to identical determinations of the electromagnetic field in a large portion of the Fresnel and Fraunhofer regions. Comparison with results obtained from the rigorous theory reveals that the aperature method accurately describes the field in the Fresnel region of cylindrical parabolic mirror collimators whose aperture diameters exceed approximately five wavelengths. Calculations based on a rigorous theory indicate that small-scale variations in the Fresnel field are essentially independent of the f/D ratio and that apodization effects in the Fraunhofer region are enhanced by decreasing the f/D ratio.