This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
By the C. P. Goerz American Optical Company.
764. The introduction of anastigmat lenses may certainly be considered the most notable step in advance in the field of photographic optics made during the last twenty years. In this instance, as in the case of all scientific progress, the improvements are not the work of one man, but result from the joint labors of an army of scientists, each member of whom contributed his share toward the achievement of progress.
765. We feel proud of the right to rank ourselves among the first workers in the vast field of research that was opened to manufacturers of optical instruments by the advent of the now famous "Jena glass."
766. Duly impressed by the advantages gained by the use of symmetrical components in photographic objectives, we made a special study of the application of symmetry in the construction of anastigmatic lenses, and succeeded in producing and bringing upon the market, in 1893, the Goerz Double Anastigmat Lens, the first anastigmat of symmetrical construction. This became at once, and remains today, the standard of comparison for all anastigmatic lens.
767. The improvement in lenses, keeping pace with the improvements in the manufacture of plates and films, made possible the application of photography to a great many purposes where formerly its services were unknown. The art of photography is today intimately interwoven with the entire industrial and commercial activity of the world, and no event of any importance to mankind takes place, even in the most remote corner of the country, without being photographically recorded and heralded to the millions. Modern anastigmat lenses made this possible.
768. The main points of improvement and advantage possessed by anastigmatic lenses over the older rectilinear type, is the power to give sharp definition at large aperture, and over a larger image surface than the older lenses could show. Additional advantages are gained by the close and compact build of the anastigmat as compared to the rectilinear lens. Besides increasing the field of the image, thereby making them useful as wide-angle lenses, the normal image is very evenly illuminated, a point in which the rectilinear type leaves much to be desired.
769. Although it may sometimes be advantageous to use lenses of comparatively long focal length, considering the required size of the image, it is an undeniable advantage not to be compelled to use such a long focal length in order to obtain satisfactory definition. In the case of the rectilinear lenses, we have no choice, but with the anastigmatic lenses we can give first consideration to any other condition and select the focal length imposed thereby, knowing that the covering power will always be sufficient.
770. In all Goerz Lenses, the astigmatism is completely corrected, with the result that even at full aperture the image is as sharp at the edge as it is in the center.
771. The curvature of the field is eliminated within an angle of 72 degrees - i. e., that part of the image which is comprised within that angle is absolutely flat. The definition and depth are the same in all parts of the field.
772. The co-existence of these two essential qualities - perfect anastigmatism and flatness of field - gives these lenses supremacy over all other existing types.
775. They are free from internal reflections and the images produced are accordingly brilliant and free from Hare.
776. The glasses employed in the construction of Goerz Lenses are so chosen as to reduce the secondary chromatic aberration to an inappreciable minimum. Apochromatic correction is therefore well nigh perfect.
777. The symmetrical arrangement, which is a characteristic of these lenses, permits the use of the single combinations of either type, but this use has been considered as of secondary importance in all series except in the "Pantar" double anastigmats, which are specially designed as convertible lenses, and have thus been calculated to work with perfect sharpness of definition at the full aperture of the single elements (F. 12.5). When using the single elements of the Series III lenses, sharp definition is obtained at F. 13.5 for the smaller lenses (up to and including the No. 3), and at F. 16 for the larger numbers. The single elements of the type B lenses give soft definition admirably suited for large portraits at an aperture of F. 15, and critically sharp definition when stopped down to F. 22 and less.