Aberration, << `ab` uh RAY shuhn, >> in optics, is the failure of a lens or mirror to produce an image that is sharply focused and has the same proportions as the object being viewed. There are three main kinds of aberration: (1) point aberration, (2) distortion, and (3) chromatic aberration.
Point aberration
produces a fuzzy image by failing to focus light to a point. The three primary types of point aberration are (1) spherical aberration, (2) astigmatism, and (3) coma.
Spherical aberration occurs in a lens in which one or both sides are curved like a portion of the surface of a sphere. A spherical surface is easy and inexpensive to produce. As a result, a typical lens in a camera, a pair of binoculars, or a small telescope is a piece of glass with spherical surfaces. This kind of lens focuses light well enough for those applications.
To understand spherical aberration, imagine that parallel rays of light strike a lens. If there were no aberration, all the rays would come to a focus at a point on the other side of the lens. But because of aberration, rays that pass through different parts of the lens come to a focus at different points. All the points are located along the optical axis, an imaginary line through the center of the lens. Rays that pass through the lens near its center come to a focus at a point relatively far from the lens. Rays that pass through the lens near its edge come to a relatively close focus. An inward-curving spherical mirror produces spherical aberration in a similar way.
Aberrations are not necessarily a result of manufacturing errors. For example, even a lens with perfectly spherical sides could not focus all parallel rays that pass through it to a single point.
In astigmatism, light comes to a focus as a line rather than as a point. In coma, the light focuses in the shape of a cone. Astigmatism and coma make the image blurry at its edge but not at its center.
Distortion
results when a lens or mirror focuses light at an incorrect distance from the optical axis. The image will appear either stretched or compressed near the edges.
Chromatic aberration
occurs in lenses but not in mirrors. It occurs because a lens focuses light rays of different colors at different distances. The distance to the focal point of a ray of light depends upon the wavelength of the light. Wavelength is the distance between successive crests of a light wave. Each color has its own wavelength. From the longest wavelength to the shortest, the colors of visible light include red, orange, yellow, green, blue, and violet.
In longitudinal chromatic aberration, a lens focuses the different colors at different points on the optical axis. Violet light comes to a focus that is closest to the lens. Then comes the focal point for blue light, then the point for green, and so forth. In lateral chromatic aberration, focal points are also offset above and below the optical axis—red the least, violet the most.
