A Dictionary of Photography (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: A PHOTOGRAPHIC DICTIONARY, Aberration. (Lat., ab, from, errare, to wander). This is a term much used in Optics. When a pencil of light suffers either refraction or reflection at the surface of any medium, it generally happens that the directions of the refracted or reflected rays do not all pass accurately through a point, or focus. This error is called " aberration," when the pencil is a direct one. The term, however, is not applied to those cases of confusion which occur in refraction through a lens, when both the incident and refracted pencils are oblique. It will be understood that we speak now of " spherical aberration;" that is to say, of the aberration which is produced entirely by the spherical form of the surface of the medium; the term spherical including the case of refraction at a plane surface, since a plane may be considered as a sphere of infinite radius. Fig. 1 will serve to explain a common case of spherical aberration in a large pencil refracted through a lens, the incidence at the front surface being oblique, and the emergence at the posterior surface being direct. 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 S, which axis produced passes through the centre 0 of the posterior spherical surface A S B. The refracted rays which emerge from1 the immediate neighbourhood of the point S form a small pencil, which may be considered as having a focus F, called the " geometrical focus." The outer rays o...