The effects of Seidel aberrations (primary defocus, spherical aberration, astigmatism, and coma) were simulated on four images using a digital image-processing system. The tolerances of the human visual system to different levels and combinations of the aberration types were determined by a forced-choice discrimination technique. The resulting threshold levels, expressed in units of wavelength, specify the changes in wave-front aberration that can be detected with some defined probability and represent just-noticeable differences (JND's) in image quality. The results are related to the corresponding Strehl intensity ratios and to the equivalent modulation transfer functions. The ultimate aim of the work is to link wave-front distortion to human visual discrimination, in order that meaningful methods of assessing visual image quality may be devised. The present investigation lays the foundations for further work, which, inter alia, will determine the effects of some higher-order aberrations.