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. The farther this point is from the optical axis, the larger the aberration is. In this chapter, we will describe 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.