On more than one occasion, responsible engineering people have produced specifications for a lens system that are totally unachievable due to either diffraction effects, manufacturing limitations or both. This paper examines several typical examples of perfect and near perfect lenses, showing the magnitude of degradation that must be expected. Evaluation is accomplished first by examining the effect on energy distribution within the Airy Disc pattern. Then, in order to demonstrate the effect on images of extended objects, Modulation Transfer Function data is presented for the same condition. The data indicates that diffiaction at the system aperture, particularly where an obscuration is present, and minute residual wavefront imperfections ( X/4 OPD), will have an effect on the final system performance that must be taken into account. An important factor iri determining the significance of these errors on final system performance would be whether the optics are forming point images or extended images of real world objects.