TELEPHOTOGRAPHY APPLIED TO ARCHITECTURE.
by Ernest Marriage.
From The Photogram, 1898, pp 10 – 15
Telephotographic lenses for detail are proving of great service to architects, and there is no doubt that such a lens is a very valuable addition to the outfit of any photographer who has to undertake much architectural work. The accompanying illustrations will show at a glance, and in a way more convincing than a lengthy description, the capabilities of a telephotographic lens. All three examples were taken without altering the position of the tripod, and the photograms have been reduced in exactly the same proportions. No. 1 was photographed by an anastigmatic lens of 8¼ inches focus; No. 2 by a combination of the same lens and a negative lens of four inches focus, the distance between the negative lens and the plate being eight inches; No. 3 under similar conditions to No. 2 but with this distance increased to sixteen inches. Similar results with an ordinary (positive) lens could be obtained, in the case of No. 2, by the use of a lens of about twenty-six inches, and No. 3 of 41 inches focal length.
For all architectural work the camera should have parallel bellows. For telephotography the camera and tripod should he as firm as possible, as the lens is, of course, heavier than an ordinary lens, and longer exposures have to be given. Cameras with parallel bellows are nearly always deficient in the amount of swing that can he given to the back, as a range of about thirty degrees is very necessary. This statement may seem an exaggeration, but it is not the case. I have had occasion to swing the camera back from ten to twenty-five degrees, and in one or two instances did not find this amount adequate, but as the camera then in use would not give more, I had to rest content. Figs. 1 and 2 show how this amount of swing can be arranged for in such a way that there are no projecting stays when the camera is closed.
A. B. C and D are binding screws. The slotted stays are filled in black. The strut (f) is only dotted in for convenience, it has to be bent in the middle (as shown at the side), in order to clear the clamp (C). The clamp (A) in fig. 1 is shown screwed into a socket out of the way, when in use it is screwed in at the hole (A1). Fig. 2 shows the position of the camera back and stays when the back is swung thirty degrees. For a slight amount of swing it is not necessary to alter the position of the screws, the stay (H) should be pushed forward as far as it will go, but otherwise the arrangement in fig. 1 can be adhered to.
A telephotographic lens requires rather different treatment from an ordinary positive lens, on account of the narrow angle of view it embraces. It will always be found best to keep the telephotographic lens opposite the centre of the plate. To photograph bits of architectural detail above the camera it is therefore necessary to tip the camera (preferably by means of a tilting table) and swing the back to the perpendicular, instead of resorting to the usual expedient, and simply raising the lens. A tele-photographic lens used with a moderate camera extension does not cover a large plate; it is, therefore, not practicable to raise the lens much, but the camera back may be swung over twenty degrees without seriously affecting the definition, if the positive lens is stopped down to f16 or f22. In examples 2 and 3 the camera was tilted and the back swung about sixteen degrees; the stops used were f16 and f11 respectively.
In practice it pretty soon becomes obvious that some means of tilting the camera other than altering the legs of the tripod is required. A ball and socket joint between the camera and stand is useless for this class of work; a tilting table is much more reliable. Fig 3 is a diagram of the most convenient form, showing the table in two positions. The stay (C) is slotted, so that the points A1 and B1 can be brought as near as may be required, the clamps at A and B slide freely along their respective slots in the sides of the table, so that an angle of ninety degrees is finally obtained. This tilting table is a great improvement on commercial patterns that have hitherto been obtainable, and the plan was suggested to me by Alexander Mackie, F.R.P.S.
For work with a half-plate camera, a telephotographic lens composed of a six inch positive lens, fitted to a three inch negative lens, is perhaps the most useful size. The six inch lens should be capable of covering the half plate at a moderate aperture; it will be useful in cases where a lens of rather wider angle is required, though for general use a lens of longer focus should be used. The positive lens should have an initial intensity of f8 at least.
The next point to be considered is the estimation of exposures. Some people may prefer to judge the amount required by looking at the image on the screen but as the intensity of the lens varies with the focal length employed, the plan is even less practicable than when an ordinary lens is used. It is better to arrive at the exposure by calculation, and the first step is to find the intensity of the lens at the particular extension in use. The simplest and most accurate method is as follows : —.
1st— Find the linear magnification of the image compared with that given by the positive lens above. Divide the distance from the negative lens to the screen by the focal length of the negative lens, and add one to the result, the total is the magnification.
2nd— Find the intensity of the lens. Multiply the f-number of the aperture of the positive lens by the magnification, and the product is the intensity of the lens.
For example, suppose the distance between the negative lens and the screen is eight inches, and a four inch negative lens is used, and the positive lens is working at f16, then
The magnification =(8/4)+1=3
The intensity =3 x f16=f48
If the draw is increased to sixteen inches, then the magnification given is five, and the intensity is f8o.
The proportionate exposures would be, for the positive lens alone, one; the telephotographic lens at eight inches extension, nine; and at sixteen inches extension, twenty-five. It will be seen that the exposure required increases directly with the square of the linear magnification. The absorption of light by the extra lenses is not a point of much consequence, and may be neglected.
For finding the proportionate exposures required for different f-values, there is a useful table given in most photographic almanacs (” Uniform System Numbers for Stops from f1 to f100,” calculated by S. A. Warbuton); paste this on a stiff card. The other side of the card should be covered with some good bold-printed matter, which will be found very handy for focussing the camera upon when the subject is badly lighted.
To estimate the exposure, a sensitometer is of great use; one in which the chief factor is the darkening of a sensitive paper should be chosen. There are several instruments of this type on the market, perhaps “Watkin’s Exposure Meter” is the most widely known; I have always used a “Stanley Sensitometer,” and to this meter the subsequent formulae and calculations apply. If the subject chosen is a carving in good relief (similar to illustrations 2 and 3), and the material light in colour, the exposure upon an ordinary plate should occupy about three or four per cent, of the time-reading of the sensitometer, when the lens used is working at f5.6. The following formula give the needful calculations : —
L = Time value of light as given by the sensitometer.
A = The uniform system number of the standard aperture.
C = The percentage chosen for that “ “
B = The uniform system number of the combination in use.
Then the exposure = (C x B x L) / A
Practical calculation for the exposure of photogram 2:-
Lens in use 8¼ – inch positive, four inch negative, positive lens stopped down to f16, distance from the negative lens to screen eight inches.
Magnification = (8/4) + 1 =3.
Intensity = 3 x f16=f48.
Uniform system number of combination at f48=144.
Uniform system number of standard aperture (f5.6) = 2.
Percentage chosen = 3.
L=6 minutes A = 2 B = 144 C=3.
Then the exposure should = (3 x 144 x 6) / (100 x 2) = 13 minutes
Similarly the formula for 3 is worked out_
L=6 A=2 B=400 C = 3
(3 x 400 x 6) / (100 x 2) =36 minutes.
In the exposure formulae it is assumed for the sake of simplicity that both examples were taken with the same aperture, and upon plates of the same rapidity. This array of figures may appear formidable, but the trouble involved is not great. Frequently it will be found that a series of subjects are wanted in similar positions; the camera will not need altering for the lot, and one calculation, except perhaps as regards variations in the sensitometer reading, will suffice.
It must not be taken for granted that the lens in question is only fitted for work at the two extensions quoted, any extension greater than about 6½ inches and within the capabilities of the camera would give as good results, but for the sake of clearness all the figures given apply to the illustrations 2 and 3.
A few observations on the method of using the lens may be of service. The subject should, in the first place, be focussed on the screen by means of the rack and pinion on the lens tube, the rack on the camera may be used subsequently as a finer adjustment if required. Focussing should be carefully done, as a very slight alteration in the racking of the lens makes a great difference in the sharpness of the image. Should the image be too large for the size of plate in use, there are two alternatives. Either reduce the extension of the camera, and consequently diminish the magnification of the image, or (if the lens will not cover the plate with a smaller extension) move the camera further away from the subject. It should be borne in mind that stopping down the telephotographic lens diminishes its covering power, if there is any danger that the plate will not be properly covered, the camera draw must he increased. There is seldom any use in stopping the positive lens down below f22.
Some subjects will not require the use of a tilting-table, but, as a rule, for architectural detail it will be found a necessity. Of course, if the subject is pretty much on a level with the camera it is not required, but such a circumstance is exceptional. The base board of the tilting-table should first be levelled, and any tilt that is required be obtained by then adjusting it; do not work with the legs of the tripod. The camera back should be placed parallel to the plane of the subject; generally that will mean that it must be kept perpendicular.
Negatives taken in diffused light are not so liable to be hard, and the shadow detail is better than when direct sunlight has been made use of. If, however, the subject is a more general view of a building, a stronger light may be of advantage to relieve monotony. Slow plates or films should be used, and plates ought in every case to be backed. Isochromatic plates are to be preferred, used with a screen they will give a clearer negative than an ordinary plate on a misty day. The state of the atmosphere is of more importance in telephotographic than in ordinary photographic practice. Wind is often a serious obstacle to work, especially if the magnification is great or the camera is not rigid enough.
The following pyro and soda developer will be found suitable:—
Pyrogallic acid 1 ounce.
Sodium sulphite 3 ounces
Citric acid ¼ ounce
Water to make 10 ounces
Washing soda 8 ounces
Sodium sulphite 10 ounces
Water to make 80 ounces
Bromide of potassium 1 ounce
Water to make 10 ounces
Carbonate of ammonia 1 ounce
Water to make 10 ounces
For use take 30 minims No. 1, 1 ounce No. 2, and 10 minims No. 3, make up to one ounce with water. If the plate is over exposed, add equal parts of 3 and 4 to the developer (say 15 minims of each per ounce of developer).