This section is from the book "Telephotography: An Elementary Treatise On The Construction And Application Of The Telephotographic Lens", by Thomas Rudolphus Dallmeyer. Also available from Amazon: Telephotography and Telephotographic Lens.
Dr. Miethe, of Messrs. Voigtlander, Brunswick, adopts the method of employing a portrait combination of high intensity as positive element, added to a triple cemented negative, in his constructions destined for large portraiture in the studio.
There is a theoretical advantage as regards the brilliancy of the image, by keeping the number of reflecting surfaces in any optical combination as small as possible ; and the construction of Messrs. Zeiss fulfils this condition in reducing them to the minimum.
The degree of brilliancy due to a greater or less number of reflecting surfaces is at least difficult to trace in practical photographic work,, particularly in the studio.
Soft Effects in large Portraiture. - Since the early sixties a considerable number of photographers, aiming at artistic effects, have sought a softer degree of definition than is given by a perfectly corrected photographic lens. The latter defines too well for their purpose, and, in fact, shows up facial defects which are not even visible to the eye, some, indeed, being beneath the skin. In this case the retoucher's skill must be requisitioned, and upon this, if artistically applied, will the true likeness largely depend, even more perhaps than upon the drawing originally given by the lens.
In the portrait lens designed by the late J. H. Dallmeyer, a mechanical means of adjustment was given in order that the image might gradually depart from keen definition to an increasing softness in the image. This device introduces what is technically known as "Spherical Aberration." Fig. 58 roughly indicates how the effect is produced. (1) and (2) represent the posterior portion of the portrait combination. In (1) the lenses are very close together, and the rays leaving the combination converge accurately to a point, the focus. In camera and stand suitable for the purpose when large plates are to be covered. The ordinary studio camera, in which there is the usual amount of extension, will answer for the purpose of producing large heads such as those to which reference has been made.
(2) the lenses are separated from their normal position by unscrewing the hindermost lens, and the rays no longer converge accurately to one point; those refracted from the margin of the lens cross the axis nearer to the lens than those near the centre. The position where the image of any point in object is now best defined is no longer a point, but a small bundle of rays. Now points in the object situated on either side of the chief point or plane focused upon are represented in (1) by circles of indistinctness which contrast strongly with the finest definition in the chief plane focused for; in (2) by circles of indistinctness which have far less contrast with the definition in the chief plane focused for. The result by (1) may be compared to a drawing in which one plane is drawn by a finely pointed hard pencil and the rest by blunter points of the same pencil: the result by (2) may be compared to a drawing produced throughout by a blunt-pointed soft pencil. So that by sacrificing fine definition in one plane, we produce a softer but more uniform type of definition, which gives the appearance of greater "depth of focus." This softening, due to the introduction of spherical aberration, does not destroy the modelling of the subject, nor detract from the massing of light and shade, but greatly reduces and frequently eliminates the necessity for retouching.
If a portrait lens of this type forms the positive element of a Tele-photographic system we can produce a soft primary image and enlarge it by the negative element. The results will seldom require the retoucher's aid, and yet faithfully portray the leading characteristics in the features of the subject. Plate V. was produced in this manner but should not be viewed from too near a standpoint.
In 1893 Mr. J. S. Bergheim carried out some interesting and original experiments by combining single uncorrected positive and negative lenses with a view of suppressing unnecessary detail and aiming at a "quality" in definition he deemed better than that given by corrected lenses. His "Cinderella," for example, shown at the Royal Photographic Society's Exhibition in 1894, showed what his lens was capable of doing in the hands of an artist.
Mr. Bergheim placed the results of his investigations at the author's disposal for development. Fig. 59 illustrates the lens in its present form, showing the single uncorrected lenses of which it is composed. By placing the diaphragm in front of the front lens, distortion is eliminated. In this construction m is very low, and hence it has been possible to make the back single lens of rather larger diameter than the front, enabling a large amount of subject to be included upon the plate when the camera extension is considerable.
Rigidity and considerable variation in the camera extension are essential in a camera for studio work; Fig. 60 illustrates the type of
Hitherto the application of photography to the sciences of medicine and surgery has been somewhat limited on account of the necessity of bringing the lens very near to the subject in order to obtain images sufficiently large to show the necessary details. When a lens is brought in close proximity to the subject, it has been found that inconveniences, such as breathing upon the lens, nervousness in the subject, the obstruction of light caused by the instrument itself, the lack of depth of definition, etc, have interfered with an otherwise valuable practice. The increased distance between object and lens rendered possible by use of the Telephotographic combination enables the surgeon to obtain records of operations, skin diseases, and various phases of changed conditions in disease, etc, which were formerly practically impossible. The author has only indicated the application to this branch of photography in photographs of the human eye taken under quite ordinary conditions. Plate VI. shows two eyes of children, natural size, and an adult eye enlarged, but taken direct. In each case the lens was 4 ft. from the subject. For a reduced scale in the image it is obvious that a considerable distance may intervene between the lens and the subject to be photographed. High intensity in the positive element of the system is of course essential.
Studio Camera suitably mounted on heavy square table stand, for use in telephotographic portraiture.