ABERRATION. This is a term much used in Optics. When a pencil of light is refracted through a lens, or reflected from a mirror, it happens in certain cases that the directions of the refracted or re flected rays do not all pass accurately through a focus. This error is called " aberration," a term which must not be confounded with " deviation," q. v.
There are many kinds of aberration, but those with which the photographer is most concerned are called " spherical aberration," and " chromatic aberration." I. Spherical zlberration.—This is a term exclusively employ( d denote the aberration produced by the reflection or refraction pencil at a spherical surface. Thus, in Fig. 1.— Let A B be a single convex lens, and Q A B a pencil of light incident upon it, proceeding from a luminous point Q. The pencil, after refraction through the lens, will not form a cone of light in which all the rays come to a common focus, but an effect will take place which it is important clearly to understand. In the first place, the emergent pencil is 'symmetrical with respect to an axis F 8, which axis produced passes through the centre 0 of the posterior spherical surface A S B. The refracted rays which emerge from the immediate neighbourhood of the point S forma small pencil, which may be considered as having a focus F, called the "geometrical focus." The outer rays of the emergent pencil will cut the line S F at points c, d, b, as shown in the figure ; the distances F c, F d, F b, increasing as the distance from S of the point of emergence of a ray increases. F 6 is therefore called the aberration of the ray Q B, F d of the ray Q D, F c of the ray Q C, and so on.
When F S is large compared with S A, the aberration F d is pro portional to the square of S A.
It is impossible to construct a single lens with spherical surfaces, so that the pencils shall be entirely free from aberration; but by combining two or more lenses, made of different kinds of glass, or of the same kind of glass, spherical aberration may be to a great extent, although not entirely, corrected. Such compound lenses, or combinations, are said to be " aplanatic," q. v.
There are certain forms of reflecting and refracting surfaces and lenses in which a particular pencil is reflected or refracted without aberration. The only case with which the photographer is concerned is that of the parabolic mirror. All rays which proceed from luminous point in the focus of the mirror are reflected in directions parallel to its axis.
The methods usually employed for correcting spherical aberration in photographic lenses are described in the article " Lens."
It is a common error to confound spherical aberration with curva ture of the image. Curvature of the image may exist where there is no spherical aberration, and vice verad. See "Lens." The nearest approach to a correct focus which can be obtained with a lens, when a large aperture is employed, is called the "least circle of aberration." It will be seen in Fig. 2, that if the various rays of a refracted pencil are produced through the axis, there will be a certain posi tion, m, of a circular area through which they all pass, in which the rliameter of that circle will be the least possible. If F a is the aberration of the pencil, the distance of this least circle of aberration f-nm F is three-fourths of F a ; and if the aperture of the lens A B is small compared with its focal length, C F, the cliameter of the least circle of aberration is proportional to the cube of the diameter of the part of the lens employed.
TI. Chronuztie ilberration.—The light which proceeds from the sun, and most luminous bodies, is found to be heterogeneous, that is, composed of different kind.s of light, of different degrees of re frangibility. If a ray of such light is refracted through a prism or lens, it will be decomposed into its constituent rays ; and if it be a direct pencil passing through a lens, there will be formed a system of emergent pencils, corresponding to the different kinds of light of which the incident pencil is composed. Sunshine is found to be composed of light of seven different colours, viz., red, orange, yellow, green, blue, indigo, violet, arranged in the order of their refrangibility, red being the least, and violet the most refrangible. VVIen, therefore, a pencil of sunshine is refracted through a convex lens, the foci of the coloured pencils are arranged along the axis of the lens in the order of refrangibility. If we designate these foci by the letters r, o, y, g, b, v, (r being farthest from, and v nearest to, the lens), the distance between r and v is called the " chromatic aberration" of the pencil.
It is found that, by combining n lenses made of n different kinds of glass, according to a certain formula, n different coloured foci may be unite,d in the same point on the axis. When two or more foci are thus united, the lens or combination of lenses is said to be " achromatic," or corrected for chromatic aberration. In such optical instruments as are not intended to be used in photography, it is customary to combine the focus of the green rays with that of the orange ; but in photographic lenses the violet rays should be combined with the yellow. See "Spectrum."