=Photographic Lens Characteristics.=—Whole volumes have been written on the photographic lens, and on the optical science utilized and indeed brought into being by its problems. Such works should be consulted by those who intend to make a serious study of the design of lenses for aerial use. No more can be attempted, no more indeed is relevant here, than an outline review of the chief characteristics and errors of photographic lenses, considering them with special reference to aerial needs. The modern photographic lens is, broadly speaking, a development of the simple convex or converging lens. Its function is the same: to form a real image of objects placed before it. But the difference in performance between the simple lens and the modern photographic objective is enormous. The simple lens forms a clear image only close to its axis, for light of a single color, and as long as its aperture is kept quite small as compared to the distance at which the image is formed. The photographic lens, on the other hand, is called upon to produce a clear image with light of a wide range of spectral composition, sharply defined over a flat surface of large area, and it must do this with an aperture that is large in comparison with the focal length, whereby the amount of light falling on the image surface shall be a maximum. This ideal is approximated to a really extraordinary degree by the scientific combination and arrangement of lens elements made from special kinds of glass in the best photographic lenses of the anastigmat type. The result is of necessity a set of compromises, whereby the outstanding errors are reduced to a size judged permissible in view of the work the lens is to do. These errors or _aberrations_ are briefly reviewed below, in order that the reader may readily grasp the terms in which the performance and tolerances in aerial lenses are described. [Illustration: FIG. 12.—Diagrammatic representation of spherical aberration.]