The quality of an image is evaluated by an observer by a subjective comparison with image impressions stored and remembered more or less distinctly. A photographic print for example, is judged by its gray-scale, sharpness or definition; and its graininess. The subjective ranking of "poor, good or excellent" depends on the size of the print and the viewing distance, and may change substantially when an excellent print is available for a direct comparison. A study of images on an objective basis has but one purpose: to determine objective criteria which will correlate with visual observations. It is evident that the characteristics of the visual system must be included in an objective evaluation of image quality.