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.
South-West Corner, Second and Third Stories of Facade, San Michele, Lucca. Taken with 6-in. Ross Universal Symmetrical with 3-in. negative, i8Jj in. in draw from back of lens to plate. Stopped down to f/16, 56 seconds exposure. (Copyright of and kindly lent by J. W. Cruichshank, Esq.)
Although there is a considerable gain in using the Telephotographic lens, both as regards improved perspective and greater depth of focus, as compared with the lens of ordinary construction, there is a loss in rapidity when two lenses of the same focal length are compared. The comparative rapidities are now influenced by the "law of inverse squares" and their exposures are proportional to the squares of their respective distances from the subject. We have seen, however, that a Telephotographic lens of considerably shorter focal length than an ordinary lens can produce the same size of image at an equal distance, and hence the diameter of the positive element in the Telephotographic system may be smaller and yet give the same rapidity; this will be found both an advantage and an economy.
Reverting to the case of a "life-size" representation of a head, it is obvious that the distance of 80 inches is not by any means binding in the case of one and the same Telephotographic combination. Both theoretically and practically this lens may be placed at any greater distance we choose from the sitter. If a greater distance be chosen the scale of the image produced by its positive element becomes smaller and therefore requires greater magnification by its negative element in order to yield the same size of image finally.
It is probable that a distance of 10 or 12 ft. will be chosen lor the representation of a "life-size" head. At 10 ft, employing a Tele-photographic lens composed of a cabinet lens of say 12 inches focal length and 4 inches aperture as positive element, and a negative lens of 6 inches focal length as negative element (to vary the numerical examples), the primary image will be 1/9 of the original, and we must magnify it nine times for equal size. This requires a camera extension of 6 (9 - 1) = 48 inches, which shows from (18) that the lens has a focal length of 36 inches, and therefore an initial intensity of f/9.
If we wish to produce the image in the scale of half natural size, it is evident that all we have to do is to magnify the image 4 1/2 times. In this case we must take the camera extension 6 (4 1/2- 1) or 21 inches. The focal length of the system is now 26 inches, and the intensity
4/26 = F/65
It is hardly possible to give any hard and fast rules for the method of procedure in the various requirements of large portraiture, but it may safely be said that if a certain carte de viste or cabinet lens produces the size of image for which it was constructed at a given distance from the sitter, in really satisfactory perspective, these images may be taken as the basis of subsequent magnification by the negative elements with which they are combined to form Telephotographic systems. It is hoped that Table II. (p. no) may be of considerable assistance in this respect.
The magnification of the primary image produced by the positive element can be accomplished in either of two ways : (1) By using a negative element of very short focal length in comparison to the positive element, or, in other words, making f1/f2, or m, great, and using only a very small camera extension ; or (2) by using a negative lens of longer focal length, or making m small, and obtaining the same magnification by the necessarily greater camera extension. The latter method is far preferable in portraiture, and, in fact, in almost all cases, as the conditions favour greater covering power, more even illumination, and freedom from distortion of the lines near the margin. For portraiture the focal length of the positive element should not be more than 2 to 2 1/2 times as great as that of the negative lens, or m should not exceed 2 1/2.
Plates I. and II. show a nearly full-length portrait of a child ; Plate I. was taken at a distance of 10 ft. with an ordinary cabinet lens constructed for a short studio; Plate II. was taken by a c. de v. lens of 8 1/4 inches focal length, combined with a negative lens of about half its focal length, at a distance of 24 ft. The difference in the drawing is apparent; in Plate I. the hand and flower are too large, the background falls away too rapidly and appears too small, and the pedestal is out of drawing; in Plate II. the relative planes of the picture take their proper proportions. Plate III. illustrates a cabinet head, taken by the same Telephotographic combination at a distance of 15 ft. Again Plate IV. was also taken by the same Telephotographic combination at a distance of 10 ft. (N.B. It could have been as sharp as desired ; the method of producing the soft effect is referred to later), and Plate V. was taken by a skilful professional photographer, Mr. Habgood of Boscombe, who was asked to produce a head of the same size by his ordinary methods. He naturally selected the lens of the greatest focal length in his possession (a rapid rectilinear of 16 inches focal length) for the purpose. The distortion of the features in this presentment, as the lens had to be brought within 4 ft. of the sitter, is apparent ; the face appearing "bulged" and the size of the mouth distinctly exaggerated. Both negatives were purposely left untouched.
In addition to the precautions necessary for giving good perspective rendering in the image, the ill-effects of distortion in the lens itself must be carefully guarded against. For this reason the author gives preference to a non-distorting, well-corrected compound positive element, as against a single cemented lens of high intensity. Messrs. Zeiss have recently introduced a very interesting single cemented lens, illustrated in Fig. 57, which has the same high intensity as a portrait combination f/3. This is designed for combination with a single cemented negative lens illustrated in the same figure, in which m is advisedly kept small (as in such a combination, where the diaphragm is placed between the combinations, the distortion increases as m increases), and the photographer is warned not to utilise its covering power to the limits on account of the unavoidable distortion produced at and near the margins. Messrs. Zeiss describe the practical working of this lens, as they do that of all their Telephotographic lenses, in accordance with the method of treating the system under heading A., Chapter V (The Formation Of Enlarged Images). Dr. Rudolph, in his "Monograph " already referred to, furnishes a number of tables applied to particular constructions manufactured by the'firm.