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Excerpt from Field Guide to Geometrical Optics
Spherical aberration causes the power or focal length of the system to vary with pupil radius.
The transverse aberration TA is the transverse ray error from the top of the pupil.
The longitudinal aberration LA is the distance from paraxial focus to marginal focus (where the real marginal ray crosses the axis). The longitudinal ray errors εZ for SA are quadratic with yP. The real marginal ray angle is U'.