It is shown both theoretically and experimentally that diffraction effects due to the finite wavelength of light and the limited angular apertures of the lenses employed set a fundamental limit to the precision with which the radii of curvature of small surfaces can be determined optically. Furthermore, attempts to measure the curvature of smaller areas of surface are inevitably accompanied by a greater spread in the individual measurements. In the case of some typical designs of radiuscope and keratometer, this spread corresponds to about 0.2 D in the measurement of corneal power.