The Optics Of Photography
( Originally Published 1898 )
PHOTOGRAPHY is the joint child of optics and chemistry. All that has been discovered about the influence of light upon the salts of silver and analogous substances might have been known, and yet, without the "dark chamber," the art of photography would have remained non-existent. It may even be said that the prior discovery of the camera obscura made photography possible. This simple instrument was the invention of Giambattista della Porta of Padua in 1569. The principle of it will be best understood by the simple experiment of darkening a room by closing the window-shutters, and admitting a ray of light through a small hole in them. If a sheet of white paper be fixed at a little distance from this aperture, the figures of external objects will be seen delineated upon it; and, by putting a small lens over the aperture, they are rendered much more evident from the condensation of the rays by the spherical glass. This will be readily understood by the following diagram (Fig. I). Let C D be a window-shutter having a small aperture at A, and E F a piece of paper placed in a dark chamber. If now an illuminated Object, R G B, be placed on the outside of the shutter, we shall observe an inverted image of this object depicted on the paper E F at B G R.
In order to understand how this inversion takes place, let us suppose the object R G B to have three distinct colours, red at R, green at G, and blue at B ; then it is plain that the red light from R will pass in straight lines through the opening A and fall upon the paper at R. In the same way, the green from G, and the blue from B, will respectively fall upon the paper at G and B, and an inverted image B G R of the object R G B will be depicted upon it.
If, instead of a darkened room, we substitute a darkened box (Fig. 2), the same effect will be perceived. Suppose, in the first place, the box to be without the lens, the rays would pass from the external arrow in nearly right lines through the aperture, refracted only in passing the solid edges of the opening, and form an image on the back of the dark box. The lens refracts the rays, and a smaller, but a much clearer and more perfectly defined image is the result. Such, briefly, is the principle of the camera obscura, the basis of the camera of the photographer.
It will be seen that, whether as regards the chemicals he uses or the instrument with which he works, light is the subtle agent, the grand motive power, wherewith the photographer produces his effects. Without going, therefore, into the nature of light, it will be well to enter a little into some of the laws governing its action which have a relation to photography.
A ray of light passing through a vacuum progresses in a perfectly straight line, and were it possible under such conditions to gaze at a brilliantly illuminated point, we should see it in its true position, that is, the rays coming directly to the eye. But all transparent matter, however attenuated it may be, has the property of bending or refracting the light ray. Hence we do not see the stars in their true positions, owing to the refracting power of the atmosphere.
The law of refraction may be demonstrated in the following simple manner. Take a bowl, and in the bottom of it place a shilling ; then move away until the eye loses sight of the coin, when it will appear as in Fig. 3, A representing the shilling and B the eye of the observer. The coin is of course invisible. Now let some one pour water into the bowl, taking care the while to keep the eye fixed on the same spot. The shilling gradually becomes more and more visible, until it comes entirely into view. The explanation of the phenomenon is that the ray of light-producing vision in the eye is refracted, or bent back, as in Fig. 4; C representing the water and A B the ray of light refracted.
The refractive power of water is also observable when we thrust a straight stick into it. The stick seems to be bent. Hence persons spearing fish have to aim at a point below the object of their aim, else the weapon will miss by striking too high. Another illustration of refraction is to allow a sunbeam S (Fig. 5) passing through an aperture in the window-shutter of a darkened room to fall upon the surface of a fluid contained in a glass vessel L L ; instead of proceeding to S', it will be found to alter its course on entering the fluid, and pass along the line to M. A ray of light, however, striking a refracting medium perpendicularly, as the ray N (Fig. 5), suffers no refraction.
The amount of bending back a ray undergoes depends on the comparative density or rarity of the substance through which it goes. If the medium which the rays enter he denser, they pass through it in a direction more closely approximating to the perpendicular. On the other hand, when a beam of light passes obliquely out of a denser into a rarer medium it proceeds in a direction further from the perpendicular. This refraction is greater or less according as the second medium through which the rays pass is more or less dense than the first.
In proof of this, take an upright vessel into a dark room, which admits but a single slanting ray of light through an aperture in the shutter. Let the vessel stand on the floor, a few feet from the window admitting the light, and let it be so placed that, as the pencil of light descends towards the floor, it just passes over the top of the side next the window, and strikes the bottom on the side farthest from the window. Let the spot where it falls be marked. Now, on filling the vessel with water, the ray, instead of striking the original spot, will fall considerably nearer the side towards the window. If salt be added to the water in such quantity as to form a dense solution, the point where the ray strikes the bottom will fall still nearer the window. So, if alcohol be substituted for water, the refraction will be greater; and greater still should oil be used.
These laws of refraction are of the utmost importance to the photographer, inasmuch as it is by their careful study and application that the optician is enabled to fashion lenses to meet his needs.
By a lens is meant what is commonly called a magnifying glass, which may be composed of any transparent substance, but in its application to photography it is generally made of glass—flint or crown—as colourless as can be obtained.
Lenses are of different shapes, and thence derive different names. The following figures represent sections of the variously shaped lenses used in photography.
A, B and C are convex or converging lenses, D, E and F concave or diverging lenses.
A is a double convex, and is used for magnifying objects, as in spectacles, telescopes, microscopes, and other similar instruments. B is a piano-convex, flat on one side and convex on the other. C is a meniscus or periscopic lens, convex on one side and concave on the other, both surfaces meeting, and of which we have an example in watch-glasses. D is a double concave lens.
E is a piano-concave, or plane on one side and concave on the other. F is an example of the convex-concave or negative meniscus lens, in which the surfaces disagree, or do not meet when continued.
In all these lenses there is an imaginary line, represented by M N, and passing through the centres of the surfaces, which is called the axis.
Thus, when the axis of a lens is spoken of, it means a line passing through it perpendicular to its surface. In converging lenses the rays of light passing through them unite at a point, called the focus, some distance beyond the surface. - Diverging lenses, on the other hand, cause the rays of light to go further apart, and are concave in form with broad edges. Of the varied qualities of these lenses it will be necessary to speak in a subsequent chapter; it suffices here thus briefly to distinguish them.
It was Porta's knowledge of the powers of lenses that suggested to him the idea of placing an objective in the orifice of the dark box, so as to bring the image produced as nearly as possible to a fixed point, and thus to reduce the camera to small and portable dimensions. The lens Porta used was a plano-convex, the rounded face being turned to the ground-glass. By thus hitting upon his ingenious contrivance he unconsciously took an important step in the evolution of the photographic art.