van Monckhoven, Désiré van. A Popular Treatise on Photography. Translated By W.H. Thornthwaite. London, 1863.
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THE common camera-obscura is supposed to have been discovered by Baptiste Porta, about the year 1590. It may be simply described as a box, at one end of which is fixed a piece of ground glass, and at the other a convex or magnifying-glass, mounted in a sliding tube to regulate the focus.
All convex or concave glasses are called lenses; the focus of a convex glass or lens is the distance between the glass when exposed to the sun, and the point or spot of light where the rays unite (Fig. 43)6
If the distance between the magnifying lens and the ground glass of an ordinary camera be regulated to the focus as described, and the lens directed towards some distant objects, it will be seen, on so shading the ground glass with a black cloth placed over the head as to prevent any lateral rays of light falling on it, that the image of those distant objects are clearly represented reversed on the ground glass. If the camera be now directed to some near objects they will not appear distinct, and the lens will require to be drawn out further from the ground glass before they are shown with perfect sharpness. This adjustment of the lens is called focussing the image.
It will be found impossible, however, to focus the whole of the objects perfectly; there will always be certain parts of the picture which want distinctness. But if a piece of cardboard having a small bole in it, be placed a short distance in front of the lens, the image on the shaded ground glass will become much more distinct and sharp, or, in other words, the picture can be better focussed. Any similar arrangement to that described is called a stop or diaphragm.
Fig. 44. Ordinary form of Photographic Camera.
The foregoing remarks will explain the principles and construction of the photographic camera, which essentially consists of two parts-the lens or objective, and the box or camera. However, it will be easily understood that these two apparatuses, to be rendered suitable for the various purposes of photography, must of necessity require more special and complicated construction than has been described.
The ordinary form of a photographic camera, Fig. 44, consist:: of a box, B, in which slide another box, A, holding the frame 0 with the ground glass. For the purpose of fixing the sliding part A, a board, D, is fastened to the box Bin which is a groove, as shown in the drawing. On the lower side of the box A is fixed a plate with a thumb-screw gassing into the groove. Thus, within certain limits, the two boxes A and B can be adjusted to various lengths, and fixed at the required focus by the aid of the thumb-screw. The lens is placed in front of B.
This form of camera has generally a single sliding-box, A; but sometimes, however, for the sake of portability, they are constructed with several slides. The material of which these cameras are made is usually mahogany or walnut wood.
The tripod, or camera stand. for supporting the camera, is usually of two kinds-one to), travelling, the other for use it the operating room for portraits, &C.
Fig. 45. Tripod Stand.
The travelling stand, Fig. 45 consists of a strong wooden or metal triangle, upon which are made to slip three feet, which for greater strength and firmness are made double. These feet; for convenience in travelling, can easily be doubled up, and removed from the triangle by unscrewing the joints.
Fig. 46. Stand for Operating Room, and Camera for " Cartes de visite."
There are many different models of tripod stands for travelling, but they are all more or less on the principle: of the one just described, and are made lighter or stronger as may be required for the size of camera. A black cloth, so constructed as to cover over the extended tripod stand, forms a convenient dark chamber for changing sensitised glasses from or to the camera. back when dry collodion plates are employed.
The stand used in the operating room or studio, not requiring to be moved any distance from place to place, may be of a much stouter and heavier construction than the ordinary tripod stand. The form of camera stand represented Fig. 46, combines solidity with facility of adjustment. The upper board supports the camera, and in order to allow of a tilting motion is connected with two half circles of wood, which can be fixed in position by a thumbscrew to an upright sliding frame, moving in a socket of the firm tripod stand. This sliding frame serves to adjust the height of the camera. Sometimes this form of stand has a winch, with rack or chain, to facilitate the raising and lowering of the camera. This addition is very convenient, especially when a heavy camera is employed.
Fig. 47. Camera without, Tail-piece.
A convenient form of camera for travelling, and suitable for plates 12 inches by 10 inches, and larger sizes, is represented at Fig. 47. B is a fixed box, pierced in front with a round hole, to which is attached the lens. Underneath, and oil the side (c) are two pieces of brass, in each of which can be fixed a thumb-screw, as seen at a. The sliding body A has also two similar pieces, one at d, the other underneath, each having a thumb-screw. The top of the tripod (which is seen in the figure) has a hollow, in which a long piece of wood b can be fixed; this has a groove in the middle, to admit the. two thumb-screws.
By this arrangement, oil turning the screw a, a very great degree of firmness is given to the part 13 of the camera; also to the tail-board b, and the top of the tripod. The sliding body A can be drawn out or in, and fixed as the operator wishes. The instrument can also be used on its side (the two pieces of brass which are seen at c and d serve for this purpose), enabling upright view.; to be taken with the same facility as long ones.
This form of camera is more convenient than the ordinary form, Fig. 44, especially for taking large views; besides which, the immoveable tail-board of the model, Fig. 44, prevents its being conveniently used on the side for upright pictures.
Fig. 48. Bellows-body Camera.
Another form of camera, suitable both for the operating room and for travelling, is represented, Fig. 48. It will be found extremely light and portable. The construction of this form of camera will be easily understood from the cut. M is a square bellows body, connected with the wooden frames A, a The frame A, holding tile lens, is firmly fixed to a board n,-n, which can be lengthened or shortened at will. The frame a holds the ground-glass frame and dark slides.
Fig. 49. Camera, or Dark Frame.
Whatever may be the form of camera employed, the general principles to be observed in its use arc the carne. For the purpose of illustration suppose a camera mounted on its stand, as shown in Fig. 55, p. 49, and directed towards a person who stands for a model. Oil drawing out the sliding body more or less, a point is reached where the proper focus is obtained. As before mentioned, the head of the operator and the top of the camera must, be covered with a black cloth, so that the sharpness of the image on the ground glass can be properly examined. Fig. 55 represents an operator focussing a person who is placed before him. If the ground glass be replaced by a glass plate, covered with sensitised collodion, an imago will be obtained representing the model. The propel. placing of the sensitised plate is accomplished by means of a frame represented in Fig. 49, and which is always sold with and forms part of the camera. This frame, or camera back, is represented open, in order to make the figure better understood. The glass plate, coated with collodion, after having been taken out of the silver bath and well drained, is placed in the camera frame, with the layer or collodion towards the shutter a (which has been previously closed); then:he door b is shut and fastened with hooks, so that the sensitised layer
Is thus preserved from the light. The ground-glass frame is then removed from the camera, and tile frame (Fig. 49) put in its place. If the thin sliding board or shutter a be now raised, the sensitised surface of the glass plate, being exactly in the same position as occupied by the ground glass, receives the same image. When it is thought that the light has acted long enough, the shutter a is closed, the frame removed, and taken into the dark room, where, on opening the door b, the glass plate is removed, and submitted to an operation to be described under the title of developing the image.
Fig. 50. Glass Frame.
The inner portion of the frame that holds the sensitised plate has projecting corners of silver wire, so arranged that the plate only touches at these parts. Thus the woodwork of the frame is in a great measure protected from the corrosive action of the nitrate of silver; and if the precaution be taken of placing a piece of blotting-paper on the lower corners and back of the plate, previously to closing the door b, the risk of drops of nitrate of silver injuring the frame, and falling during its transport from the dark chamber to the operating room, will be avoided.
With a camera of a certain size it is very often required to take smaller pictures, as well as the fall size the lens will produce. This is accomplished by having some extra frames of wood (Fig. 50) fitting into the larger frame, and carefully adjusted, that when the plate is placed in one of these extra frames its surface shall be exactly coincident with the position of the ground glass upon which the focus was obtained; if otherwise, it would be impossible to produce a shag and distinct picture.
The glasses employed in photography are usually of patent plate, although good ordinary glass may be employed for very small sizes. The glass plates are kept in grooved wooden boxes (Fig. 51).
Fig. 51. Plate-Box,
There are two kinds of photographic lenses-those called single lenses, for views; and double, or combination lenses, for portraits.
The following are the diameters and focal lengths of portrait and view lenses suitable for the various sized glasses usually employed:
|Size of Pictures Inches||Diameter of Lenses. Inches.||Focal Length. Inches.|
|Ninth size||2½ by 2||1¼||3¼|
|Quarter plate||4¼ by 3¼||1¾||4½|
|Ditto for rapid action||4¼ by 3¼||1 3/8||5|
|Third size ditto||5 by 4||3¼||7|
|Half plate||6½ by 4¾. .||2 3/8||7|
|Whole plate.||8½ by 6½||3¼||9½|
FOR PORTRAITS AND GROUPS.
|Size of Pictures. Inches.||Diameter of Lenses. Inches.||Focal Length. Inches.|
|9 by 7||3¼ & 4½||11|
|10 by 8||4¼||13|
|12 by 10||4¾||15|
|15 by 12||5¼||19|
|Size of Pictures. Inches.||Diameter of Lenses. Inches.||Focal Length. Inches.|
|6 by 5||1¾||8|
|7 by 6||2||10|
|9 by 7||2 3/8||14|
|12 by 10.||3||16|
|16 by 12||4||24|
Fig. 52. Single, or View Lens.
The single, or view lens, is composed of an achromatic lens, mounted in a tube of brass (Fig. 52), as shown it, the figure. D, the brass cover of the lens. F, a small brass tube, holding diaphragms of various sizes. C, the brass tube, having the lens at A screwed within it, and also furnished with an outside screw, which allows the whole mounted lens to be fixed to the ring of metal A, which is attached to the front of the camera.
The view lens is used in the following manner:--It is fixed to the camera by being screwed into the brass ring; the cover D is removed, when the image may be focussed. All the diaphragms should be removed, except the one to be employed. The smaller the opening of the diaphragm, the sharper the image; but then a longer time will be required to produce an impression. Thus, a diaphragm of I inch requires four times as long exposure as one of z inch; but then the picture produced is much sharper. It is thus that the operator can, when required, sacrifice sharpness of detail for rapidity of action.
The lens requires from time to time to be cleansed with a soft wash-leather; but in remounting the same in its cell, it is of the greatest importance that the convex side of the lens should be towards the sensitive plate***.
Fig. 53. Double, or Compound Lens.
The compound, or portrait lens, is composed of a greater number of lenses. It has a compound lens in front, which is very thick, being formed of two ce4mented together, like the view lens, and two other lenses at the back. Whenever the lenses are removed to be cleaned, or for any other purpose, it is important that they be replaced in exactly the same order as when purchased otherwise the picture produced can never be good and sharp. Fig. 53 shows the form of the double; or compound lens. D is the cover, or cap; B G, a double tube, sliding one over the other, capable of being adjusted by turning the milled head F connected with the rackwork. Its use is, after the focus has been roughly obtained, by drawing out the sliding body of the camera, to finally adjust the same with the greatest ease and nicety. C is a portion of a larger tube, adapted to screw upon the tube B, and serves to protect the lens from lateral rays of light; B, A, the ends where the lenses are fixed. G, the tube, having a screw at the end for the purpose of attachment to the ring E. This ring of metal is fixed by screws upon the front of the camera. H, a diaphragm, or stop, fitting the interior of the tube C.
The above are the chief points of difference between the two varieties of lenses known as the single and double. The brass mountings of both forms are blacked inside, to prevent internal reflection and should this dull black coating be at any time rendered imperfect, it can be repaired by the application of a mixture of lamp-black and gum-water.****
With respect to the optical differences of these two forms, and their special uses, it is to be noted that the single lens, although of the same size as the double combination, does not give so brilliant an image upon the ground glass, because the diaphragms intercept an enormous quantity of light. The double lens, on the contrary, has two lenses, combined in such a way that a diaphragm is not necessary; consequently, the images produced are well illuminated. By stopping off a single lens, the sharpness of the image is rendered perfect, whereas with the double combination such a degree of sharpness cannot be obtained; therefore the first is employed for views and buildings, and the latter for portraits.
Another consideration also influences the selection It is easy to be understood that a sensitive coating is impressed more easily in proportion to the brilliancy of the light striking upon it. Thus, a lens of 3 inches in diameter, with ½-inch stop, will require one hundred times longer exposure than a double combination of lenses of the same diameter to produce the same picture. Now, is it not easy to expose for as long a time as may be required for a view, landscape, flowers, or inanimate objects, so as to obtain, what ought to be the chief aim, extremely fine detail in the picture? In most cases, therefore, although rapidity can be gained by enlarging the stop, results will show it had better be avoided. With the single lens just mentioned, a view, illuminated with the sun, can be easily obtained in twenty to thirty seconds, and if there be any persons in the view, they will without doubt be copied. 1f, with the same single lens, an endeavour be made to take a portrait of a person placed in the shade, it will require from three to five minutes. Is it possible to remain so long without moving? Certainly lot. Now the double lens will allow such a portrait to be taken in ten seconds; and although it gives a picture dot quite so sharp as the single lens, it nevertheless is greatly preferable and would in reality produce a sharper picture, for with the single lens no one could remain perfectly immoveable during a sufficient time.
In certain cases it is also necessary to use a stop with !he double combination lens; but instead of a stop of to 2 inch for a lens of 3 inches diameter, one of from ¼ to 1½ inch is all that is required. Fig. 54 shows such a stop at H; it is placed in the hood C of the lens, and is chiefly employed when a group of portraits is required to be taken. For the purpose of more clearly comprehending the reason for the use of a diaphragm when taking groups, &c. with a double or compound lens, it may be remarked that, the more extended the space occupied by an object, the more light will there be on the ground glass; but. at the same time, with, the double lens there will be less sharpness. An intermediate course is therefore adopted; that is to say, a diaphragm is employed, whereby a loss is incurred in the matter of exposure for the sake of wining sharpness and definition.
Generally speaking, a single lens gives so perfect a definition to the whole of the object, that the operation of obtaining a good focus is extremely easy. It is not so, however, with the double combination lens; therefore, as a rule, the Bead of the sitter should be carefully focussed, anti a little of the details sacrificed, if necessary, to obtain this point.
Compound lenses are manufactured and sold for taking pictures of a certain size; this must be understood to mean the utmost of their capabilities, and those photographers who may be desirous of obtaining first-class portraits should always employ a larger lens than is absolutely required. The following is a list of compound lenses particularly adapted to produce the finest portraits of the given dimensions:
A new form of lens has lately been constructed for views, consisting of two lenses, which allows a great degree of sharpness to be obtained with more flatness of field than the ordinary single lens. They are termed orthoscopic or caloscopic. In a special note (Note 8), will be given a description of some of the principles of optics, a proper understanding of which will render more clear many of the foregoing details.
For taking "carte de visite pictures" the best size of Compound lens that can be employed is 2,3- inches in diameter and 5-inch focus.