Aberrations are imperfections in the optical image formed by a spherical lens (or optical mirror). There are five main aberrations:
1. Chromatic aberration. The refractive index of glass varies with wavelength. This results in different focal lengths and image magnifications for different colours.
2. Spherical aberration. Lenses with spherical surfaces have a shorter focal length at their periphery than at their centre.
3. Coma. The various circular zones of a lens produce an image of an off-axis point that is distorted radially into a comet shape known as a coma patch.
4. Curvature of field. The focal surface is not a plane but a bowl shape.
5. Astigmatism. Because a lens appears asymmetric to an off-axis beam, the image of a point is not a point but a short line radial from, or tangential to, the optic axis of the lens. These images are formed in different planes.
6. Distortion. The image magnification is greater towards the edges of the field (pincushion distortion) or less (barrel distortion) than at its centre.
These faults can be minimized by the choice of several glasses with different refractive indices, dispersions, and surface curvatures, and the judicious positioning of an aperture stop.
— Graham Saxby
Bibliography
- Ray, S., Applied Photographic Optics (3rd edn. 2002)
