In order to assess the importance of the instrumentally measured quality of components of a visual imaging system one must attempt to take due account of the effect of the eye's own quality on the viewed image. Recent studies in a number of research institutions have shed considerable light on the detailed workings of the eye so that it is now possible, at least tentatively, to describe the performance of the visual system in terms of a series of quality functions. These various components of visual quality are discussed and their interaction with the quality factors of optical components external to the eye are considered. Finally some of the more popular M.T.F.-based figures of merit are considered in terms of the combined performance of an optical system and the eye.