This section is from the book "The Fundamentals of Photography", by C. E. K. Mees,. Also available from Amazon: The Fundamentals Of Photography.
Photography is the art of making representations of natural objects by mechanical and chemical processes. These representations deal with differences of brightness, color being ignored, except in color photography, and the object of the photographic process is to translate, as accurately as possible, the degrees of brightness which occur in natural objects into corresponding degrees of brightness in a photographic print.
It is not possible to convey any impression in a photograph of the brightness of an object of even brightness; a piece of black velvet seen in bright sunlight is brighter than a piece of white paper in a dark room, so that it is impossible to speak of the brightness of paper or the blackness of velvet unless there is some standard of comparison by which it can be measured. If black marks are made on the white paper and then photographed, the resulting print will reproduce the relative intensity of the black marks and of the white paper.
When a representation of a natural object is made on a flat surface, the form can be represented only by differences of brightness or color. Shape is only possible in sculpture. The painter uses differences of brightness and of color, while the black and white draftsman uses only the differences of brightness. Except in the special branch of color photography, photographs deal only with the reproduction of objects in their degrees of brightness.
Fig. 60. Two Tones.
Fig. 61. Three Tones.
The different degrees of brightness are spoken of by artists as "tones." If a piece of white paper on which black marks have been made is photographed the result will be a picture in two tones (Fig. 60). Between these extremes are other tones spoken of as halftones. Figs. 61, 62, and 63 show the effects of additional tones. In Fig. 64 the six tones complete the representation of a solid object, from which it will be seen that form and substance are shown by degrees of brightness. In the mind the forms of natural objects are comprehended by the degrees of brightness that occur in them. It is the business of photography to reproduce these different degrees of brightness,which may vary from white to black.
Differences in brightness which occur in nature may be produced by differences in the illumination of the object. If a plaster cast is lighted directly from the front the outlines will be visible but there will be no variation in tone. It will have a flat, even appearance (Fig. 65). If the cast is lighted from one side shadows will be formed, there will be variations in illumination, and in this way tones will be produced by shadow (Fig. 66).
Fig. 62. Four Tones.
Fig. 63. Five Tones.
Fig. 64. Six Tones.
Fig. 65. Front Lighting (Flatness).
Fig. 66. Side and Top Lighting (Tone Graduations).
The brightness of an object depends not only upon the illumination falling upon it, but also upon the reflecting power of the object itself. Things differ very much in reflecting power. If a piece of white paper represents a reflecting power of 80%, a piece of gray paper may reflect only 44% of the light falling upon it, and so on down the scale, a piece of black paper reflecting only about 5%. The brightest thing known is white chalk, which reflects 90% of the light falling upon it; that is, of all the light falling on the white chalk 90% is reflected back. Snow does not reflect quite as much light as chalk. The ordinary red brick wall reflects only about 20%. Good black printers' ink reflects about 10%, and the blackest thing, black velvet, will reflect about 1% or 2% of the light falling upon it.
Since in natural scenes both the reflecting power and the illumination vary, some parts of a landscape consisting of clouds in sunlight, and others of dark rocks in the shade, the range of contrast is often very considerable. For photographic purposes a scale, or contrast of 1 to 4, in which the brightest thing is only four times as bright as the darkest, is very low, and such a subject would be called flat; a contrast of 1 to 10 is a medium soft contrast; 1 to 20 a strong contrast; 1 to 40 very strong and 1 to 100 an extreme degree of contrast. All these degrees of contrast occur in subjects such as landscapes, street and seashore scenes.
Since the more nearly we can reproduce in our picture the range of brightnesses which were present when the picture was taken, the better the picture will represent the original scene, our object in photography must be to get an accurate reproduction of the various tones or brightnesses which occur, keeping each tone in its same relative position in the scale as it occupied in the subject which was photographed. This is, of course, easier to do if the range of brightnesses is small than if it is very great.
When we make a photograph we do the operation in two separate steps. We first make a negative upon a highly sensitive material and obtain a result in which all the tones of the original are inverted, the brightest part of the subject being represented by a deposit of silver in the negative which lets through the least amount of light, while the darker parts of the subject are represented by transparent areas in the negative which let through the most light. This negative is then printed upon a sensitive paper, in which operation the scale of tones is again reversed so that the bright parts of the subject which were represented by heavy deposits in the negative now appear as the light areas of the print and the dark portions of the subject which were transparent in the negative are represented by dark deposits in the print.