The optics of image magnification are derived. Necessary criteria for contact lens dispensing are explained and well-known approximations are demonstrated, along with their inherent limitations. The image size with spectacle lenses is contrasted with that of the corresponding contact lens, and the simple relationship between the two is demonstrated. Thus, no tables of contact lens image magnification are necessary for the clinical practitioner. Visual acuity as commonly expressed is not an exact measurement, but rather indicates that the patient has visual acuity equal to or better than the line read, but poorer than the next smaller line on the chart. It is shown how contact lens/spectacle interchange may be of significant social and legal benefit to selected groups of patients.