Spherical Aberration Most photographic lenses are composed of elements with spherical surfaces. Such elements are relatively easy to manufacture, but their shape is not ideal for the formation of a sharp image. Spherical aberration (SA) is an image imperfection that is due to the spherical lens shape. Fig. 1 illustrates the aberration for a single, positive element. Light that hits the lens close to the optical axis is focused at position c. The light that traverses the margins of the lens comes to a focus at a position acloser to the lens. In this manner the focus position depends on the zone of the lens that is considered. When the marginal focus is closer to the lens than the axial focus, such as exhibited by the positive element in Fig. 1, one speaks of undercorrected spherical aberration. Conversely, when the marginal focus is located beyond the axial fo cus the lens is said to suffer from overcorrected spherical aberration. The image of a point formed by a lens with SA is usually a bright dot surrounded by a halo of light. The effect o f SA on an extended image is to s often the contrast and to blur its details [1]. Spherical aberration is uniform over the field, in the sense that the longitudinal focus difference between the lens margins and center does not depend on the obliquity of the incident light. Figure 1. Illustration of undercorrected spherical aberration. The margins of the lens have a shorter focal length than the center.
