The Camera And Lenses
( Originally Published 1898 )
STEP by step with the progress of photographic chemistry went on the development of the camera. Something has already been said about the camera obscura, from which the camera of the photographer took its rise. In this, the principal apparatus used in drawing by the pencil of light, we have nothing more nor less than a dark box, to one end whereof a lens is adjusted, while to the other end is fitted a ground glass, upon which the image is cast, and by which it may be brought to a focus. When the object to be taken has thus been carefully focussed, the ground glass is removed, and what is called a dark slide or back " inserted in its place. Exposure to the light is effected by the withdrawal of a shutter and the removing of the cap from the lens, through which all the light must pass.
These are the essential features of a camera, and if the apparatus fulfils the conditions required it is a perfect camera, though of the simplest form.
The disposition of the various parts of the camera will be easily understood by reference to Figs. 10 and 11, in which A represents the body, B the lens, C the ground glass focussing plate or screen, and D the dark slide or back, which is made to fit into the groove in the body A when C is withdrawn. E is the shutter which is withdrawn to allow of exposure taking place.
Each of these four primary parts of the camera has undergone various changes and modifications of the form here represented with the progress of the art. The body of the instrument, for instance, which in the early days of photography was often made out of a cigar box and a spectacle lens, has developed into a somewhat complicated affair. In the camera in its first state there was no possibility of properly focussing an object. Then this end was attained by adopting a telescope action to the tube holding the lens, and working it by means of a rack and pinion. But this being found insufficient, the next step was to make the body of the apparatus consist of two boxes, one sliding within the other. Daguerre's camera was constructed on this plan (Fig. 12). This was a distinct advance on the single box, as it permitted of more than one lens being used.
The next improvement in the body of the camera was effected by the introduction of what is known as the bellows form in 1854. The credit of this contrivance is generally given to Captain Fowkes, R.E. Its advantages were at once so apparent that it was soon almost universally adopted; and, from that time to the present, an ordinary camera is rarely seen without the bellows or concertina body.
Several attempts to produce a lighter and more portable article than the ordinary box had already been made, one of the most noteworthy of which was Ottewill's folding camera, which was very convenient for outdoor work—always a great consideration for lovers of photography.
Numerous suggestions have, from time to time, been made for adding to the camera the convenience of a developing chamber ; and it is said that the instrument used by Scott Archer had some such contrivance added to it. In 1852 a camera was introduced by Mr. Newton, wherein were troughs of solution for developing the wet plates after exposure. Others seem to have turned their attention to appliances of a similar character, but with little success. Eventually the introduction of dry-plates, enabling the photographer to take a supply sufficient for the occasion when on an excursion, and developing them on his return, made the addition of a developing chamber to his camera a matter of less concern.
A convenient addition to the camera is the invention known as a double-back, which is a dark-slide holding two plates instead of one, there being one on each side. Changing boxes answer the same purpose. These are made to hold from one to two dozen plates, and are so contrived that each plate can be transferred to and from the dark-slide without exposure to the light. There are various forms of changing boxes, some very simple, others more complicated in construction. When all serve their purpose, it would be difficult to name a " best," especially as preferences enter so largely into selection.
Another arrangement of the modern camera, and one of very great importance, is the "swing- back." This is a contrivance by which that part of the camera holding the ground-glass screen can be "swung" through a small angle, either vertically or in a horizontal direction. By the vertical movement the ground-glass is swung backwards or forwards as required, the effect being that the top of it may be placed further from the lens than the bottom, or vice versa. By the horizontal or side-swing, one side of the ground-glass may be brought nearer to the lens than the other.
A similar adjustment adapted to the front of the instrument, called the rising front, enables the lens to be raised or lowered without moving the camera. In some cameras there is what is termed a swing-front as well as a swing-back, and it is occasionally a convenience that the front should be capable of slight lateral motion, although it is rarely used. It is simply an extension of the idea of the rising and falling front, and calls for no further description.
The use of the swing-back is perhaps best exemplified in the taking of a sitting figure, where, without the aid of this contrivance, it would be impossible with the lenses formerly in use to bring the knees into focus at the same time as the head. This is particularly noticeable in pictures taken with small single-lens hand cameras. A good example of the error fallen into by the operator who neglects this consideration is shown in the accompanying illustration. It is from a photograph of a lady reclining in a gondola, on her way to a ball. If she ever forgave the photographer who made of her "dainty feet" such barge-like appendages, she was surely more than mortal. The side-swing is much less used than the vertical movement, although there are occasions when it becomes extremely useful.
Some photographers affect to prefer the old-fashioned, simple cameras, without so many latter-day appliances. They undoubtedly have their advantages—one being that they do not, perhaps, so easily get out of order. The various additions to the modern camera, however, have not Leen applied without reference to some distinct need, and they should not be slighted. A good instrument should be as light as possible, consistent with strength and durability, and should be as simple as possible in its-working. The back of the camera should be capable of being brought up to the front for use with wide-angle lenses, and the front of being drawn out to double-extension. It should have a rising and falling front, a reversing frame, and should swing both back and front. Such a camera is Messrs. Horn & Thornthwaite's " Whitehall," as shown in Figs. 14 and 15.
Having described the camera generally, it now becomes necessary to speak of the most important adjunct to it—the lens.
As we have seen, the lens which Porta used in his camera obscura was a plano-convex : the convex face of it being turned towards the ground-glass. Chevalier, a French optician, who made the cameras for Daguerre, turned the lens the other way about, placing its flat side next the ground-glass. The image thus gained in clearness and definition, but the extent of what is called the focal plane was diminished. This fault was remedied by the use of small discs of metal, each pierced with a different-sized hole, called diaphragms, but more commonly spoken of as " stops." Andrew Ross, an English optician who turned his attention to the construction of lenses in the early days of photography, made a further advance by changing the plane surface of the lens into a concave. Another step towards the lens of today was taken when, in place of a single lens, combinations of lenses were adopted.
In order to make the reason for these changes clear, it will be necessary, without going into details too abstruse, to enter a little into the nature and property of lenses.
It has already been shown that a ray of light, in passing through a prism, is not only bent out of its course, but broken up or dispersed —that is to say, we get a band of colours, or a spectrum, which, no matter what the substance of the prism, always occurs in the same order, namely, red, orange, yellow, green, blue, violet. In a properly-formed spectrum, we should see the band of colours crossed by dark lines, which always fall in the same place, and so enable us to fix definitely the particular coloured ray of which we desire to speak.
It will be seen from the accompanying diagram (Fig. 16) that the prism bends the pencil of light towards its base. The same thing happens if we reverse the prism, the rays being still refracted towards the base.
If we now bring the prisms together, so that the base of the one rests on the base of the other (Fig. 17), and let R be a ray of light proceeding from an object and impinging upon the prism at C and D, we shall see that they are refracted till they cross each other at E. If, on the other hand, we place the prisms the other way about, that is, apex to apex, it will be seen that the rays—still bent towards the base—can never meet (Fig. 17).
In these two arrangements of prisms we have in reality the principle of the convex or converging, and the concave or divergent lenses respectively.
As one or both of the surfaces of the lens are portions of spheres, the curved surface or surfaces must be struck from a centre. Thus, in Fig. 18 it will be seen that the surfaces of the lens are drawn from M and N, the line connecting which is its principal axis, as it is called. The rays of light that coincide with the principal axis of a lens do not undergo refraction, but pass through it in a perfectly straight line; all others are refracted.
Besides its principal axis, every lens possesses what is called an optical centre, which is the point through which pass the axis of oblique rays of light. The rule for finding this centre is to draw two parallel lines from the centre of curvature of each surface, both being oblique to the principal axis, and to connect the two points at which they cut the surface by a straight line, which must be continued till it cuts the axis; this point is the optical centre. The accompanying diagram will make this clear. Fig. 19 is a double-convex lens, in which A B are the radii of curvatures of both surfaces, the lines from which to the further surfaces are parallel to each other. From their points of impact on their respective surfaces, at C E, a connecting line is drawn; and at the point D, where this line crosses the axis, is situated the optical centre.
'The centre of a piano-convex lens is situated in the convex surface of the lens, as will be seen by carrying out the above rule. When one side of a lens is flatter than the other, the optical centre moves towards the convex surface.
In the case of a deep meniscus or periscopic glass, the optical centre is outside the lens, as will be perceived from the annexed diagram (Fig. 20), in which A B are the parallel lines from the centres of curvature of the two surfaces, oblique to the axis ; and D C a line joining the two points, continued till it cuts the axis at E, which is therefore the optical centre.
The importance of the optical centre arises from the fact that the focus of a lens is measured from this point.