Spherical aberration is the most important of all primary aberrations, because it affects the whole field of a lens, including the vicinity of the optical axis. The name of this aberration comes from the fact that it is observed in most spherical surfaces, refracting or reflecting. The longitudinal magnification of the part of the optical system after the surface under consideration is implicitly included as a factor. One may interpret Delano’s expression as the longitudinal spherical aberration transferred from the preceding system to the final image plane, magnified by the longitudinal magnification of the whole optical system. The primary spherical aberration is obtained if the aperture is large enough to deviate from the paraxial approximation to produce spherical aberration, but small enough to avoid high-order terms. A plane mirror in front of a lens, located between the lens and its focus, reverses the traveling direction of the light, as well as the relative positions of the marginal and paraxial foci.