(from the Latin aberratio — deviation), distortions and errors in the images formed by optical systems. Aberrations of optical systems show themselves in optical images that are not quite sharp, do not correspond exactly to the objects, or come out coloured. The following kinds of aberration of optical systems are the commonest: spherical aberration — a defect of the image in which light rays emitted by one point of the object, those that have passed close to the optical axis of the system and those that have passed through the parts remote from the main part of the system, are not gathered at a single point: coma — an aberration that arises when light rays pass obliquely through an optical system. If, on passing through an optical system, a spherical light wave is deformed in such a way that the beams of rays issuing from one point of the object do not intersect at a single point but lie along two mutually perpendicular segments at some distance from each other, such beams are called astigmatic, and the aberration itself astigmatism.
The aberration called distortion breaks the geometrical similarity between the object and its image. The aberrations of optical systems also include curvature of the image field. An optical system may have several kinds of aberration at once. They are eliminated according to the purpose of the system, which is often a difficult task. The aberrations of optical systems listed above are called geometrical. There is also chromatic aberration, which arises because the refractive index of optical media depends on the wavelength of light. Owing to the wave nature of light, imperfections of the images in optical systems also arise from the diffraction of light at diaphragms, lens mounts and the like. These cannot be removed in principle (although they can be reduced), but they usually affect the quality of the image less than the geometrical and chromatic aberrations do.
The aberration called distortion breaks the geometrical similarity between the object and its image. The aberrations of optical systems also include curvature of the image field. An optical system may have several kinds of aberration at once. They are eliminated according to the purpose of the system, which is often a difficult task. The aberrations of optical systems listed above are called geometrical. There is also chromatic aberration, which arises because the refractive index of optical media depends on the wavelength of light. Owing to the wave nature of light, imperfections of the images in optical systems also arise from the diffraction of light at diaphragms, lens mounts and the like. These cannot be removed in principle (although they can be reduced), but they usually affect the quality of the image less than the geometrical and chromatic aberrations do.