In the most general sense, the process of image formation involves a succession of refracting or reflecting surfaces whose purpose is to alter the curvature of impingent wavefronts originating from some object scene. The character and arrangement of these surfaces determines the ultimate location, magnification, and fidelity of the image formed by the process. In designing an optical system, surface curvatures, their spacings, and the properties of the intervening media are adjusted to achieve the desired results. In practice, as design requirements become increasingly stringent, the availability of additional degrees of parameter freedom (read "surfaces") may be required; in theory, the media may be non-homogeneous, and there exist no constraints upon the geometrical form of the refracting/reflecting surfaces. In practice, we are limited in our creativity by our ability to execute all design parameters accurately, and by our ability to tailor refractive index gradients and surface shapes.