To assess the importance of various objectively measured imperfections in visual optical and electro-optical devices it is necessary to appreciate the interactions of the eye/brain system. This system contains not only optical transmission components but also a complex sequence of series and parallel neural processes. Early parts of visual image processing are now sufficiently well understood to be able to develop fairly comprehensive models. Such models need to acknowledge, amongst other things, that diffraction, aberrations and refractive inhomogeneities all play a part in retinal imagery, that the retinal image is rarely in perfect focus due to accommodation errors, that the visual system is inherently noisy, that visual performance varies progressively across the retina and that the eye senses primarily changes in retinal illumination in space and time. The basic stages of early visual processing are briefly described. Models are then discussed for describ-ing the display/observer interface factors which influence retinal image formation, the threshold performance functions for fovea' vision, suprathreshold perception of image fidelity and rudimentary search processes.