Changes in retinal image quality with focus for an aberration-free eye with a 5-mm pupil are discussed in terms of both geometrical and physical optics. Further modulation transfer function curves illustrate the effect of spherical aberration. These theoretical results are compared with analogous experimental data obtained at different wavelengths with quasi-monochromatic illumination, using a double-pass photo-electric, scanning instrument. The comparison shows that the approximations of geometrical optics predict ocular performance quite well, provided that the errors of focus are reasonably large (greater than or equal to 0.5 DS for a 5-mm entrance pupil). Possible sources of error in the experimental measurements are indicated.