The performance of a lens or protective shield can be described in terms of degradation of contrast, “contrast transfer” or “modulation transfer”—produced by that lens or shield for specific angular resolutions. The performance so described can be directly related to the seeing requirements of the wearer. The use of a modulation transfer specification is essential in applying tolerances to zones, scratches and haze in contact lenses and other plastic materials. Techniques, problems, and experimental results are discussed.