It is the function of a lens to collect light from a point on the object and to focus that light to a corresponding point (a conjugate point) on the image. In nearly every case the lens will fail at this task, in that there will be some residual error in the precision with which the lens collects, refracts, and focuses that light. Rather than a true point image, the lens will produce a blur circle, i.e., a spot. It is the function of the lens designer to ensure that this spot size is sufficiently small to allow the lens to produce the required resolution, or image quality.
These errors in the lens's ability to form a perfect image are referred to as lens aberrations. There are seven primary aberrations that must be considered when a lens system is being designed or evaluated. By understanding the basic characteristics of these seven primary aberrations, the optical engineer will be better equipped to specify and evaluate the image quality of a lens or an optical system. The following paragraphs will describe the seven primary aberrations and discuss the more important characteristics of each.
Online access to SPIE eBooks is limited to subscribing institutions.