This Demonstration shows the sagittal, the tangential transversal third-order aberrations, and the corresponding spot diagram for an optical system, which result from Seidel aberrations. These are described more fully in Details.
Seidel or third-order aberrations are related to the fourth-order wavefront aberrations for a centered optical system. These are the monochromatic aberrations including spherical aberration, coma, astigmatism, field curvature, and distortion. The defocus and tilt terms are included in order to balance the effect of the aberrations has on the size of the geometrical point image formed, wherever this is possible. The f-number of the system helps to check the scaling effect has on the aberrated image spot size. The ray transverse aberrations and the spot diagram are useful because they exhibit the behavior of the optical system when forming images from point objects in different field positions.
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 W. T. Weltford, AberrationsinOpticalSystems, Bristol: Adam Hilger, 1986.
 V. N. Mahajan, Optical Imaging and Aberrations, Part I: Ray Geometrical Optics, Bellingham, WA: SPIE Press, 1998.