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.