ABSTRACT
Spherical aberration affects the image over the whole field of view of a lens, including the optical axis and its vicinity. There are some other image defects or aberrations that affect only the image points off the optical axis. This chapter describes these aberrations, first studied during the 1850s by Ludwig von Seidel. For this reason, the primary or third-order aberrations are known as Seidel aberrations. It also describes the off-axis aberrations of centered systems. These are systems that have a common optical axis, where all centers of curvature of the optical surfaces lie on this optical axis. The chapter explains the off-axis monochromatic aberrations, but to help the reader in understanding them, it is convenient to introduce some basic concepts to explain why they appear. There are several possible ways of calculating these contributions. One is by using the results for a complete system, but taking the system as only one surface.
