Optical aberration

An optical aberration is a deviation of an optical system’s image from an ideal image, caused by imperfections in how it focuses or transmits light.

Spherical aberration: An aberration in which rays passing through different zones of a spherical optical surface focus at different distances. It is a common geometric aberration caused by spherical surfaces failing to bring marginal and paraxial rays together.
Geometrical optics: The model of light as rays that reflect and refract at optical surfaces. Aberration analysis often traces rays through a system to compare actual and ideal image positions.
Achromatic lens: A lens assembly designed to bring two or more wavelengths to a common focus. It reduces longitudinal chromatic aberration in imaging systems.
Diffraction limit: The finite resolution imposed by the wave nature of light passing through an aperture. It limits image sharpness even in an aberration-free optical system.
Coma: An aberration that makes off-axis point sources appear comet-shaped because their rays form differently sized images. It shows how an otherwise sharp point becomes asymmetric away from the optical axis.
Gaussian optics: The first-order theory of image formation by paraxial rays near an optical axis. It defines the idealized image that aberration analysis measures deviations from.
Apochromat: An optical system corrected to bring three wavelengths to a common focus and to control other aberrations. It extends chromatic correction beyond the two-color design of an achromatic lens.
Airy disk: The central bright region in the diffraction pattern of a point source viewed through a circular aperture. It is the ideal diffraction blur, distinct from aberration-induced image errors.
Astigmatism (optics): An aberration in which perpendicular sections of an optical system focus at different distances. Its separated line foci demonstrate unequal focusing in different orientations.
Paraxial approximation: An approximation that treats rays near an optical axis as making small angles with it. Many aberrations appear as departures from the predictions of paraxial imaging.

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