The usefulness of the geometrical-optical treatment of frequency response is considered. With this treatment calculations have been made, using numerical integration, for several cases of defect of focus, primary and secondary spherical aberration. The results are compared with those obtained from diffraction treatments of the same cases. Provided the wave front aberration exceeds about 1 wavelength, it appears that geometrical optics will give an accuracy in the calculated frequency response of about 5% of its value for zero frequency, over the range of frequencies encountered in many photographic objectives.