In geometrical optics, the deviation of the behavior of optical systems from first-order theory, or paraxial theory, which occur even for light that is quasimonochromatic.

Paraxial theory results from the approximation of sin(θ) by just θ for small θ. This corresponds to taking just the first term in the Taylor Series expansion of θ:

sin(θ) = θ - θ3/3! + θ5/5! - θ7/7! + ...
If we take the first two terms, then we get a third-order theory. Differences between the first and third order theories result in the primary aberrations (also called Seidel aberrations). They are: In addition, deviations of first-order theory from higher-order approximations are called higher-order aberrations.

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