CONSTRUCTION OF PHOTO GRAPHIC LENSES The construction of technically good photographic lenses, of which some indication has been given in the preceding historical summary, depends largely upon the elimination of certain de fects of which the principal are (I) Spherical aberration, (2) Coma, (3) Astigmatism, (4) Curvature of field, (5) Distortion and (6) Chromatic aberration. Spherical aberration is due to the fact that a spherical surface does not bend or focus light to one exact point, in other words the rays passing through the edge of such a surface do not focus to the same point as those pass ing through the centre. It can be partly corrected in a single lens by the use of a diaphragm, and in combinations by varying the curves of the component lenses. Coma and astigmatism are func tions of spherical aberration, the first being a blur due to lateral spherical aberration of oblique rays. It is partly eliminated by the diaphragm. Astigmatism manifests itself in inability to focus accurately at the same time both vertical and horizontal lines near the margin of the field. It is partially cured by the use of a small diaphragm, and can be wholly eliminated by the selection of special glasses and the precise calculation of curves. Curvature of field is due to the failure of rays passed by a lens with a spheri cal surface to come to a focus on the same plane, the defect in creasing, like astigmatism, with the obliquity of the rays. Here, too, the diaphragm can be of great service. Distortion is of two kinds, "barrel" and "cushion," a square appearing in the former shape if photographed with a single meniscus lens with the con cave side outwards and the diaphragm placed in the front of the lens, and as a "cushion"—i.e., with concave sides—if photographed with a single lens convex side outwards and with the diaphragm behind. It is corrected naturally in symmetrical doublets, since the barrel distortion of one component compensates the cushion distortion of the other. Chromatic aberration is caused by the dispersion of the white light passing through the lens, and is due to the fact that the different coloured rays come to a focus at different distances from the visual focus, according to their wave lengths. It is corrected by compensating lenses of different re fractive powers. (See OPTICS.) In the general construction of
photographic lenses the defects which were enumerated above, together with a number of others, are neutralized by regulat ing the curves of the different positive and negative component lenses, the refractive and dispersive indices of the glasses used, and the distances of the refracting surface, in this man ner making the objective truly stigmatic or focussing to a point.
The performance of an adequately corrected lens depends chiefly upon its effective aperture or the diameter of the clear ray actually used in impressing the image on the plate. This diameter is not necessarily that of the diaphragm employed, but the latter is used for the purpose of mechanical regulation of aperture, i.e., reduction from "full" to the smallest possible. In early lenses the diaphragms or "stops" were of the Water house pattern, with thin pieces of metal with circular openings of different sizes which could be inserted in a slot in the lens mount. This pattern is still employed in portrait and other large lenses. A variation is the rotating stop, a circular plate containing a series of apertures which can be brought into position as desired. Most modern lenses are fitted with iris diaphragms which Messrs. R. and J. Beck claim to have been the first to apply to photo graphic lenses in 1880. These consist of thin leaves of metal or ebonite so arranged that by operating a ring to which they are affixed apertures of different diameters can be formed at will. Several systems of numbering diaphragm apertures are in use, the most common being the F number method in which the aperture marked is the focal length of the lens divided by the effective diameter of the stop. Thus an effective diaphragm open ing of z in. in the case of a lens of 8 in. focus would be f/16. In other words the aperture marked is a definite proportion of the focal length. Another system is the U.S., or Uniform Standard, proposed by the Royal Photographic Society in 1881 and adopted in the Kodaks. This takes f/4 as the standard, the markings corresponding to the F. numbers being as follows: f. values, f/4, f/5.6, f/8, f/11.3, f/16, f/22.6, f/32, f/45.2. U.S. Nos., I, 2, 4, 8, 16, 32, 64, 128.