Portrait Lenses.—At a very early period in the practice of the art the necessity for a rapid flatness of field, and the presence of an air space between the flint and crown elements of the back combination, gives perfect correction for spherical aberration. The only important modi fication in the design of the portrait lens was patented by J. H. Dallmeyer in 1886, improved covering power being obtained together with less liability to flare. This lens is shown in diagram L. In the Dallmeyer lens the position of the crown and flint elements is reversed, the crown element being a meniscus instead of a " crossed " double convex lens. These lenses are fitted in an adjustable cell so that the distance between them can be varied at will ; when placed closely together, sharp definition is obtained, while by separating them more or less " softness " results. The front combination of both types of portrait lens is frequently used alone, either for landscapes or for portraits, especially large heads. Small lenses of this form were used for rapid landscape work in the earlier days of the art, but at the present time they are rarely used except by naturalists for night photography.
Doublet Lenses.—The want of rapidity and rectilinearity of the single lens and the bulk and cost of the portrait lens, caused attention to be directed to the production of lenses for copying, outdoor work, etc., which should be free from these defects. One of the earliest of these was the Orthoscope or Orthographic lens of Petzval lens was felt by portrait photographers ; and in 1841, Andrew Ross constructed a double combination lens J for Henry Collen. This consisted of two cemented combinations, one being placed at each end of a tube to which central diaphragms were fitted. In the same year Thomas Davidson produced a symmetrical lens for portraiture, this being composed of two similar combinations, each composed of a plano-convex crown cemented to a piano (made by Voigtlander in 1859). For this lens great claims as to flatness of field, rectilinearity and other virtues were made, although actually it possessed but little advantage over the single landscape lens. It had a greater equivalent focal length than the distance between any part of the lens and the focusing screen, or, in other words, it was to a certain extent a telephoto lens, the back combination being of the negative form. The orthoscope was succeeded, in 1864, by several doublets of various rapidities, but all somewhat similar in type, designed by Thomas Ross (see M), and these enjoyed considerable popularity for many years, some being still in use. The next great stride towards perfecting the photographic lens was made in i866 when Dallmeyer, in London, and Steinheil, in Munich, almost simultaneously issued lenses of the type which is so well known under the name of Rapid Rectilinear (see N), the original uncorrected form of Steinheil being designed for wide-angle work only. This lens, working at an intensity of f/8 or more, gave perfectly rectilinear images, having a fairly flat field and being free from flare or " ghost," and achieved immediate success. Lenses on similar lines, but slower in
action, were constructed for wide-angle work, and no considerable improvement was made until the Zeiss and Goerz anastigmats appeared on the field in the early nineties of the nineteenth century. Rectilinear lenses were made by j. H. Dallmeyer with an intensity as high as f/3, while other forms, known as the " euryscope," "extra rapid rectilinears" or "universal sym metrical " had intensities of about f/6. The wide-angle rectilinears or portable symmetricals usually had a maximum aperture of 1/16. An "actinic doublet" was achromatised for actinic effect ; but modern doublets are achromatised for both the visual and actinic effects.
Anastigmatic Lenses. — Lenses of greatly improved design were introduced in 1890 by Carl Zeiss, of Jena, the first model being an apocliromatic triplet, this being followed by double combination which was greatly superior to all lenses then existing. Improvements have followed in rapid succession. The Series VII (see B) may be considered as being the most useful type of the Zeis anastigmats, being truly universal in its character. The single lenses are perfectly corrected for astigmatism, spherical aberration and curvature of field, and are practically rectilinear. Their intensity is f/12.5. Used in combination to form doublets they retain all their good qualities with the addition of greatly increased covering power, angles of 8o° to 9o° being obtained, while the intensity varies from f/6.3 to f/8, according to whether lenses of similar or dissimilar focal lengths are combined. In 1893 an excellent series of double anastigmats was introduced by Goerz with an intensity of f/7.7 afterwards increased to f/6.8. Each combination is com posed of three glasses cemented together. The single combinations may be used for landscapes, but do not work at the same large aperture as the Zeiss lenses. The Goerz " Pantar " lens, recently introduced, corresponds in rapidity to the Zeiss Series Vila. Other opticians have placed on the market lenses corresponding to the Goerz model under various names.
The foregoing may be considered as the principal groups of lens types, but many other forms are described under separate headings.
In process work, it is essential that the lens should give perfect definition uniformly all over the plate, that it should be free from distortion and astigmatism, and give even illumination ; and that it should be colour-corrected. For line reproduction it is not necessary to work with a large aperture, and therefore by stopping down extreme sharpness is obtained. In half tone large apertures are not permissible on account of the action of the ruled screen. For the same reason the focus must not be too short ; an equivalent focus of about 18 inches is usually considered best for a 15-inch by 12-inch plate. In colour reproduction without the ruled screen interposed large apertures are desirable to counteract the considerable length ening of exposure due to the action of the colour filters. Process lenses have to be fitted with Waterhouse diaphragm slot in order to permit of variously shaped stops being used.