_ANASTIGMATIC LENSES.
The group of anastigmats may he roughly divided into two classes, the unsymmetrical and the symmetrical, the latter being often described as the universal. Some of the various com binations employed in different pat terns of unsymmetrical aliastigmats are mat is symmetrical, or nearly so, and its combinations can be used separately. It is even more perfectly corrected than the nnsymmetrical anastigmat, and, as its name indicates, may he used for almost shown in Fig. 509. .,ks a rule, the two components of these lenses cannot be used separately, as in the rapid rectilinear type. The modern anastigmat represents the highest point of lens manufacture at any branch of photographic work. Figs. 5111 and 511 show two patterns of the Beck-Steinheil orthostigmat (anastigmat), and Fig. 512 indicates how a single com bination may be used separately. The
present attained. Astigmatism is prac tically eliminated, the other corrections being also remarkably successful. It gives high rapidity and splendid quality of definition, but, as a consequence, a certain amount of depth of focus has inevitably to be sacrificed. The universal anastig six components of a Universal orthostig mat arc shown uncemented in Fig. 513. The Goerz double anastigmat, another justly celebrated objective of the uniN er sal type, is illustrated by Figs. 511 and 515. It possesses a flat field, exquisite de finition and remarkable covering power.
traitnre. Negatives produced by such lenses have the further advantage of re quiring less retoucliing, on accbunt of their greater softness of• definition. A large aperture is more necessary- with a