PhotoHistory

PHOTOGRAPHY OF TO-DAY
THE CONTROL OF LIGHT

By H. CHAPMAN JONES, 1913

Man has controlled light, and so to a certain extent adapted it for his use, from the very earliest times. When the dweller in tents pushed open his tent door to let in the morning light, and when he shaded his eyes with his hand in order to see more clearly at a distance, he modified the light so that it should the better serve his purpose. If the tent door had remained closed there would have been some light inside, and if his eyes had not been shaded the distant object would still have been visible, but by letting in more light in the first case and excluding some in the second, he so controlled the light as to increase the visibility of the objects that he wished to see.

This method of controlling light leads us to the simplest kind of photography. Photographs are records made by light, and in every case where light has caused an obvious difference in the parts of an object exposed to its influence when these are compared with the parts shaded from it, a photograph has been produced. If a blade of grass is pulled up, root and all, it will be seen that the part that was above the ground is green and that the part that has been buried and in the dark is not green. This is a photographic record of how much was exposed to light and how much was underground. We bear upon our bodies a photographic record of the clothes we wear. Our hands and faces are darker than the covered parts, and in the summer when the light is the most active, the face gets sunburned while the forehead remains of a lighter color because it is shaded by the hat. The pattern of lace worn on a lady’s neck may be clearly visible when the lace is removed, if she has been unduly exposed to sunshine. White paint in the shade turns yellow, while in the light it remains of its original color. The darker patch behind a bolt knob is a photographic record of the existence of that knob. And many other examples might be given of photographic records produced, as we say, accidentally, and often to our annoyance and inconvenience.

There is a common idea that photography has to do with a comparatively small number of substances, if not entirely with a few compounds of silver. So far as the number is restricted, it is only a matter of adaptability to certain definite ends. A photograph can be made on any surface that is affected by light. A pattern may be cut out in a suitable material and placed, for example, on the sunward side of an apple that is ripening. In due time the apple will have a photograph on it, because the sunshine will be unable to produce its effect upon those parts that are shielded from its influence. Devices may be produced in a similar way by means of stencil plates or cut-out patterns upon innumerable other surfaces. In these cases there are only two tones or degrees of darkness, the one where the light has had free access to the surface and the other on those parts that have been covered over; but if instead of a pattern cut out of an opaque material we employ a glass plate with a deposit on it of various degrees of transparency, a shaded design may be produced. In this case the light is controlled as to its intensity, or the extent of its action, and not merely as to the place where it shall act.

The method just described is the simplest, crudest, and most obvious kind of photography. The impression is always of the same size as the original, and the original must be of such a character that it can be placed upon and in close contact with the sensitive surface. But we often want a copy to be larger or smaller than the object itself, even when this is flat and of a character suitable for giving a contact copy. And besides, we want pictures or records of all sorts of things, such for example as buildings, machines, persons, and landscapes, that are not flat and could not by any means be made to lie upon or against a sensitive surface, to say nothing of the impossibility of treating them so on account of their size.

In these cases it is necessary to produce an image of the object, getting the image of the required size, and to bring this, instead of the object itself, into contact with the sensitive surface. The image must be a real image, that is, one that can be received upon a screen after the manner in which an optical (or magic) lantern gives an image on the sheet. The image that we see of ourselves in a looking-glass is not a real image, it is only a virtual (or apparent) image; it cannot be brought into contact with a surface because it has no existence, except in the sense that it is an appearance.

All objects that we can see are visible because of the light that they give out and that travels from them to the eye that sees them. It is obvious that light comes from visible things if they are self-luminous, like the sun or a candle flame, and it is just as true of objects that shine by reflected light, such as the moon, the planets, and all the common things with which we have to do. The problem that we have now to consider is, how can this light be controlled so as to cause it to give the image that we want. The great bulk of light that emanates from an object of considerable size we cannot consider en masse, as the attempt to do so would lead to indescribable confusion. And this is not only impossible but unnecessary, because if we understand how the light from a very small part of the object can be manipulated so as to produce a correspondingly small part of the image, we shall have the information that we want, as it will only be necessary to apply the same considerations to every other small part of the object in order to understand the production of the complete image. We may imagine the object to be divided up in much the same manner as the design on a tessellated pavement with very small tiles; then in order to produce an image the light that comes from each tile must be made to produce a tiny spot of light on the surface that is to receive the image, each spot of light in its proper place in relationship to the others. We will consider two such small parts, and in order to facilitate the experiment?

Take two lighted candles and set them up three or four inches apart, and support a piece of white cardboard, a foot square or rather larger, at a distance of about eighteen inches from them. On darkening the room, it will be seen that each flame shines all over the card and there is no suggestion on it of an image of the flames. Now take another card about the same size as the first, prick a hole in the middle of it with a large pin, and hold it half-way between the two flames and the first card. This shields the light from the card first set up, but each flame shines through the pinhole and produces a patch of light upon it. Here then we have at least the elements of an image. If we bring a third candle near to the others, it also will give its patch of light, and if we were able to go further and make a pattern with candle flames, as pyrotechnists make flaming and glowing devices with their fireworks, we should get an image of the pattern on the screen. As a matter of fact, a simple pinhole may be quite serviceable for some practical photographic purposes, giving a useful image of general objects.

Now examine carefully the spots of light obtained on the cardboard, bearing in mind that each should be of the same shape as the flame that produced it, though upside down. That it should really be upside down may be demonstrated by raising one of the candles and observing that as it is lifted up the spot of light that its flame has produced moves downwards. The light travels in a straight line, and if you like to represent its path by a knitting-needle, you might thrust it through the pinhole and see how that, when one end of the needle was raised the other end fell, like a see-saw with the pinhole for its support.

But what we specially want to notice is, that although the flames are bright the images are very dull, and that although the flames have decisive and sharp edges, the edges of the patches of light that stand for the flames on the cardboard are diffuse, perhaps so much so that it is difficult at first to recognize the shapes of the flames in the patches of light. We have an image, but it is dull and fuzzy instead of being bright and sharp as we want it to be. It is dull merely because the hole is so small that very little light can get through it. Make the hole larger and the image will be brighter, but it will be even further removed from sharpness. You may make another hole close to the first pinhole; this will add its own quota of light, but as the light travels in a straight line, this second hole will give its own image by the side of the image given by the first hole, partly overlapping it, and so the confusion will be increased. The pinhole is the first step in the getting of an image-forming apparatus, but we should be poorly off if it could not be improved upon.

Take one candle with the white card as before, but in the intermediate card make three pinholes in a row with a distance of about two inches between each and the next. The candle flame now gives three spots of light or images of the flame on the screen because the light travels in a straight line from the flame through each hole and onwards to the white card. If we could bring these three images together, so that they were all at exactly the same place on the card, we should have one image but of triple brightness. Now the only way by which this is possible is by bending at least two of the rays. This is the essence of the whole art of getting a bright and a sharp image, namely, bending the rays of light that otherwise would go astray, in such a manner that they shall all work together in the production of one image. In the present experiment this can be done with two small pieces of looking-glass held in the correct positions. You will find it rather difficult to hold both the mirrors still enough to get the three spots of light exactly superposed, but the principle of the concentration of the light will be clear.

Now imagine a fourth pinhole and a third mirror, and go on in imagination, for you cannot do it experimentally, making more holes and putting more mirrors to bend the rays that pass through them, and you can see in your mind’s eye the spot of light getting brighter and brighter. We saw before that a smaller hole gave a sharper image though a duller one, so fancy now that all these holes are very small, each with its mirror, and we have not only a bright image but a sharp image. If now these small holes were very close together the mirrors would have to be very small and each set at its proper angle; it is but another step to imagine no space at all between the holes, and the tiny mirrors all joined to form one continuous surface of proper curvature to gather all the light that a mirror or reflector of the given size and position could gather and concentrate to form the image.

By this means it would be possible to get a sharp and bright image, but such a mirror would be very troublesome to make, and could not be more than a mere ring, for otherwise it would come between the flame and the screen on which the image is desired and so cut off the light altogether. In the quite early days of practical photography mirrors were sometimes used for portraiture, but they were found inconvenient for this very reason, and also because much stray or uncontrolled light gained access to the apparatus, which of course had to open at the end directed towards the person being photographed. For astronomical purposes mirrors offer some special advantages. Dr. A. A. Common made some three feet in diameter and one five feet in diameter. All the light from a single star that falls upon such a mirror is concentrated upon a single point comparable in size to a pinhole, and the brightness of this point or image may be millions of times greater than the spot of light that a simple pinhole would give.

The large mirrors just referred to were not exactly spherical in curvature. The light reflected from its margins would not be sent towards the point where it is wanted to help to make the image bright. This fault or “aberration” is called “spherical aberration,” because it is the inevitable result of using a spherical surface for this purpose. It gradually increases from the center of the mirror towards its margins, but if the mirror curve is shallow, being only a small part of the whole sphere, the effect of this aberration is slight and an image of a useful degree of sharpness may be obtained.

We saw earlier that ordinary light consists of a mixture of many kinds of light, light of many different colors, as in the rainbow, if we regard the visual effects of its components, or of many different wave lengths if we regard it in the more fundamental way and irrespective of our eyes. One great advantage of mirrors as imageforming instruments is that they affect all these different components of light in exactly the same way. There is no separation of them, for the same law of reflection applies equally to all. It is the same law that comes into play when an india-rubber ball is thrown upon the pavement, or when a billiard ball rebounds from the cushion of the table (providing that no “side” or twist or spinning movement is given to the ball), namely, that the angle at which it falls or strikes is also the angle of the rebound, a law that probably needs no further elucidation.

We have already seen that in order to get a bright image it is necessary to bend some of the divergent rays of light, so that they also may be brought to contribute to the formation of the image. We have seen how these rays may be bent by reflection, but there is another and quite different way by which this bending may be accomplished. The path pursued by a ray or “pencil” of light may be bent by introducing into its path a different medium from that in which it is traveling. Of course all media used for this purpose must be transparent or the light would not pass through them. Of the more common we have, besides the air, water, glass of numerous varieties, and many mineral substances such as rock-crystal or quartz, Iceland spar, fluor spar, mica, the diamond, &c.

When a ray of light impinges perpendicularly upon the surface of a second medium, as when it passes from air into water or glass, the direction of its path is not changed, and this statement holds good whether the surface of the second medium is flat or curved. But if the direction of the ray is not perpendicular to the surface of the second medium, then the direction of the path of the ray is changed, the ray becomes bent, and the bending so produced is called, in optical language, “refraction,” to distinguish it from the bending of the ray by reflection. If the second medium is denser than the first, as in the case of the passage of the ray from air into, the path that the ray pursues is bent towards a line drawn perpendicularly to the surface. In passing from the denser to the rarer medium the ray is bent from the perpendicular, and, if the two surfaces of the glass are parallel, this second refraction is exactly equal to the first, but being in an opposite direction — the path of the emergent ray is parallel to the ray’s path before entering the glass; it is in the same direction, though shifted a little sideways.

Therefore the net result of passing the light obliquely through the slab of glass with parallel sides is no permanent bending or refraction of the ray, because what is done at one surface is undone at the other. But when the two surfaces of the glass are not parallel the result is different. Exactly the same laws apply, and the reader can easily work out the result for himself, but the effect is always that the direction of the path of the ray is changed. This gives us the second method of bending divergent rays and concentrating them upon any required spot. Consider the candle flame and the screen with its three pinholes, but this time the two outer pencils of light are bent by causing them to pass through wedge-shaped pieces of glass, and the three spots of light are superposed, just as when the mirrors were used. A wedge-shaped piece of glass or other similar transparent material is called a “prism.”

In exactly the same way as in the case of the mirrors, we may imagine that the pinholes in the screen are increased greatly in number and that each has its appropriate prism to bend the ray of light as required, and we may suppose that the holes are made smaller to get a sharper image. But in this case we can go further than when using the mirrors, and imagine the small holes all over the screen, or all over a large circular patch in the middle of it, each with its own little prism. If, now, instead of all these tiny prisms a single piece of glass, with a surface that is continuously curved as indicated necessary by the prisms, is placed in position, the card with its multitude of pinholes that we have produced in our imagination may be removed, and all the light that falls upon the curved glass will go to form the image. The image will be bright if the curved glass, or “lens,” is large, because of the amount of light brought to its formation, and the image will be sharper than when only the pinhole was used, because we may regard the sharpness as the equivalent of a much smaller pinhole. Moreover the apparatus is convenient, because it is possible to enclose all the space between the lens and the screen that receives the image and so to shut out stray light. Thus we have realized another important step in the production of an apparatus that will give a good, useful image, for we have got the image both brighter and sharper than that produced by the simple pinhole, and we have obtained these advantages by means of an apparatus that does not surfer from the difficulty of having to put the screen that receives the image between the candle and the apparatus that produces the image.

If only the lens would do exactly what we have just represented it as doing, it would be perfect. Unfortunately this is not the case, for a single piece of glass, even though it were shaped without any error as we have imagined it to be, would surfer from several imperfections with regard to image formation; it could never bring all the light that emanates from each small portion of the object to form a similar small portion of the image. There are methods by which these imperfections can be reduced in amount, but none of them can ever be entirely got rid of, even though no limit were placed upon the cost or the complexity of the final product of the optician’s skill. It is not possible to make any lens, however complex, that will be at its best under all conditions. It is customary therefore to perfect lenses in different directions according to the uses for which they are intended. It is not fair to the optician to take a lens made for one specific purpose and apply it to another, any more than it would be fair to a tool-maker to complain that his tools were not efficient when they had failed in the doing of work for which they were never intended. In concluding this section we will endeavor to get some idea of the different kinds of work that lenses are required for in photography.

The three chief classes of instruments in which images are produced by lenses may be indicated by the telescope, the microscope, and the photographic camera of the usual kind. In the telescope a comparatively small image is produced of a distant object, in the microscope a comparatively large image is produced of a near and small object, and in both these cases the image is examined through another lens that magnifies it. The only part of the image that is really needed is the central part that the eye can see through the lens provided. Here, therefore, the extent of the utilized portion of the image is very small, generally less and often very much less than an inch in diameter, and the image within that area must be so well defined that it will bear magnification.

But in the case of the ordinary camera the extent of the plate that the image is required to cover is often very large. The most popular of the small sizes is the “quarter-plate,” and in order to cover this the image produced must be at least five and a half inches in diameter, while for a “half-plate” it must be eight inches in diameter, and practically the lens must give a larger extent of image than this to allow of adjustment. These are but small plates such as amateurs most often use, and no definite limit can be set to the size of plate that may be required for commercial purposes. This extent of required image is the essential characteristic required of lenses in general photographic practice. Here we take the image as it is. We do not magnify it as in the telescope and the microscope, and therefore the optician directs his attention to the getting of a large area of image good as a whole, rather than a very small area of image with such exquisite exactness in its details that it will stand much magnification before it shows its imperfections.

There is one other matter that we want in a lens, and that is size. It may be regarded as a window, and the larger it is the more light it will admit. With twice the amount of light, a given amount of work can be done by it in half the time, and this saving of time is often of the very first importance. It means that in instantaneous work the period of the exposure may be halved, and that the moving object may therefore be moving twice as fast without causing any more detriment to the picture by its movement. It is a little advantage in having one’s portrait taken to have to be still for only two seconds instead of four, but the rapidity of the lens is a matter of perhaps greater importance to the trade worker than to any one else. If the time required for the exposure is halved, it means that the operator can do more work in the day. Or if for any reason this should not be so, it certainly means that more work can be got from one camera in the day, and that therefore in a large establishment fewer cameras are necessary. And fewer cameras need less accommodation, less artificial light when that is used, and thus a general saving in workshop expenses. Large and quick working lenses, therefore, are not to be regarded as fads for the eccentric or luxuries for the rich, for they may prove very profitable even if they are very costly.