This paper presents a mathematical model of image degradation derived from empirical resolving power formulas that purports to clarify certain principles of high-resolution photography. The model shows degradation to be the operative agent in image formation and show how certain kinds of degradation become resolution limits that reduce an imaging system's sensitivity to subtle improvements in performance. It demonstrates that, to exploit the full resolution potential of an imaging system, photographers have the task not so much of increasing image quality as reducing degradation. They must know fully the origins of degradation and must control them all without exception. -