Single lenses lack the power of producing a straight image of a straight object; the image will have the curve of the lens through which the light passes to form it; a double convex lens will give a greater curve than a plano-convex. This is called spherical aberration.
The main object to be considered in the manufacture of a lens for photographic purposes is to produce one with the least spherical and chromatic aberration.
Spherical aberration is overcome to a great extent by the use (in connection with the double convex) of a meniscus lens.
Chromatic aberration is overcome by the use of two glasses of unequal density in forming one lens; thus p90 the front lens of the portrait combination is composed of a double convex of crown glass and plano-concave or meniscus of flint glass, which two glasses are sealed together with Canada balsam.
The forms of lenses which are corrected for chromatic and spherical aberration will be seen in Fig. 5.
FIG. 5.
These lenses are termed achromatic, and, although each is formed of two kinds of glass, they are sealed together so as to be practically one lens.
Every manufacturer of portrait or view lenses, uses the six forms shown in the diagram (Fig. 2), in some manner peculiar to himself, but of the six, four will be found in every combination in general use, varied in radii, construction and dimension, according to the use for which they are intended.
Formerly the photographer's choice of lenses was restricted to two combinations, the double combination for portraits and the single for views. There have of late years been invented a great variety of lenses, among which and in the order of invention, probably are Petzval's Orthoscopic, Harrison's Globe, Ross's Doublet, Darlot's Wide Angle and Rectilinear Hemispherical, Steinheil's Aplanatic, Voightlander's Euryscope, p91 and greatest of all, Dallmeyer's Patent Portrait, [Wide Angle] and [Rapid Rectilinear Lenses].