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.
It is probably a matter of common observation that the finest portraits are produced by photographers having the advantage of a long studio.
Where the studio is sufficiently long, the photographer is always able to take up a standpoint from which a study of a head, head and shoulders, and so on to full length of the figure, is seen to the best advantage. In general, the distance must be increased as the amount and depth of subject which he intends to portray increases.
In order to do himself and his subject justice, these distances should never be departed from, whatever the scale of the image is to be. The lens should be placed at the correct viewing distance, and the focal length of the lens chosen to give the scale required, determining the size of the final image.
No one positive lens of ordinary construction and definite focal length can answer all purposes satisfactorily. If the focal length be great, suitable for a "life-size" head for example, the lens would be quite unsuitable for taking a "Cabinet" standing figure, because it would have to be removed so far from the subject that the image would appear flat. Conversely, if a lens of shorter focal length, suitable for the "Cabinet" standing figure, were employed to take a "life-size" head, it would have to be brought so close to the sitter that the drawing of the head would be altogether unsatisfactory.
The general tendency in ordinary photographic practice is to commit the latter error - viz., to aim at too large an image with a lens which is too short in focal length to allow an adequate distance to intervene between the subject and lens for good perspective rendering.
The Telephotographic lens affords the great advantage of always enabling the larger sizes of portraits to be taken under favourable conditions for proper perspective rendering. Again, its perspective is always better than that given by an ordinary positive system of the same focal length - due to the greater distance that may intervene between subject and lens.
Let us now illustrate some comparative examples of the employment of ordinary and Telephotographic constructions.
The production of a "life-size" head : We know that to accomplish this with an ordinary lens the sitter must be at a distance equal to twice the focal length of the lens from it. Suppose we take a lens of
5 inches diameter and 40 inches focal length (intensity f/8); the distance of the sitter from the lens must be 80 inches, and the camera extension also 80 inches. A distance of 80 inches is none too far for good perspective rendering, and yet it is seldom that even this distance intervenes between lens and sitter for a " life-size " head, because the focal lengths of most large portrait combinations are less than half this distance, or 40 inches.
Let us now form a Telephotographic lens by combining a rapid cabinet lens of 3 1/2 inches diameter and 10 inches focal length with a negative lens of 4 inches focal length, as illustrated in Fig. 56.
If we place the combination at the same distance (80 inches) from the sitter, the positive element alone gives an image \ the size of the original. In order that the final image may be of the same size as the original, we must magnify it seven times. To do so, the focusing screen must be placed at a distance E from the negative lens, where e=f1 (m-I)=4 (7 - 1) = 24 inches, as compared with 80 inches above, see (13). On adjusting the separation between the lenses by means of the rack and pinion mount, the image will now be found to be of the same size as the original. To compare our present conditions for rapidity with those above, we must determine the focal length of the combination when thus employed. Substituting in (18) p. 75 where e = 24, m=10/4= 2 1/2, ,f1= 10 and n (for equal size)=1; 4 Focal length From this result we can proceed to emphasise the valuable and important fact that a greater distance must intervene between a Telephotographic lens and the subject than is required for an ordinary lens of the same focal length. With the former we have found 80 inches is required, while with the latter it must be twice 20, or 40 inches. These results also exemplify in a striking manner the fallacy of the usual tenet that the correct viewing distance of the photograph should be determined by the focal length of the lens with which it was taken. The result by the 20-inch Telephotographic lens and the 40-inch lens of ordinary construction should of course be viewed from the same standpoint when the result is "life-size"; and, again, we see that if both lenses have the same focal length the result produced by a lens of ordinary construction should be viewed at a distance of 40 inches only as against 80 inches by the Telephotographic system. The latter is the more likely distance from which a "life-size" head would be viewed, and the result by the Telephotographic lens will appear in better drawing and be a more truthful representation.
6o + 10
= -.--------- = 20 inches only,
2 1/2 + 1 and hence this combination will work at the same initial intensity as the former with an aperture of only 2 1/2 inches.
Observe that as the lens produces the image of equal size with the object, the position of the screen must be 20 inches distant from its focal point, or focus for parallel rays. In other words, the combination focused for a very distant object would form an image at a distance of 24 - 20, or 4 inches from the negative lens. For parallel rays we see from (13) F = m e+f1 = 10 + 10=20 inches, and our conclusion is confirmed.