The international photographer (Jan-Dec 1935)

Fox The INTERNATIONAL PHOTOGRAPHER June, 1935 MAKING THE MOST OF EXPOSURE By Herbert C. McKay Reprinted from the American Annual of Photography, 1935, bv special permission of American Photographic Publishing Co. PART I gjl I is difficult to overestimate the importance of exIffi'li 'if^A posure. Not only is correct exposure a vital factor Sb«l in making a good negative, it is practically the only phase of technique over which the photographer has any control. Lighting and posing lie well within the field of esthetics, while emulsion characteristics are fixed by the manufacturer and the process of development has, fortunately, been highly mechanized. Sensitive materials are available in abundant variety, yet so fully standardized that compensation for variability of sensitivity is rarely, if ever, necessary except when changing from one emulsion type to another. Formulas are available which enable us to secure the best possible results by developing according to a time and temperature chart. By virtue of this standardization and mechanization, a great amount of drudgery has been eliminated, leaving the mind of the photographer free to cope with the problems of individuality in the use of his medium. The mechanization of exposure is to be desired, but not to a degree which removes it entirely from the control of the photographer, because the correct exposure is not necessarily the desirable one. Certainly, however a knowledge of the correct exposure is vital to success in making any negative. What Exposure Is and What It Does. — Exposure is the act of permitting light to fall upon the sensitive surface of a plate or film in such a manner that the image of the scene before the lens may be faithfully recorded upon the sensitive surface. This much is common knowledge, but until we have a clear conception of the elementary mechanics of the light action it will be difficult to appreciate the importance of knowing the correct exposure to give. While the reaction is fundamentally a photo-chemical one, we will assume a diagrammatic mechanical interpretation for the purpose of explaining the fundamental phenomena of photographic exposure. It must be thoroughly understood that, for the purpose of this elementary explanation, facts will be dealt with broadly, and that the explanation given does not, from a strictly technical point of view, exactly coincide in every particular with existing facts! Technical readers will appreciate the departures from fact, while non-technical readers need not give any consideration to this, because the statements as made will serve to give a mental image of what takes place and will suggest how the knowledge may be utilized in securing better results when making photographs of any kind. The sensitive material used in photography is known as the emulsion. Whether it is coated upon celluloid to make films, upon glass to make plates, or upon paper for use when enlarging or printing, the fundamental characteristics are not radically changed. The emulsion is essentially a mixture of silver bromide and gelatine. The gelatine used is the same substance used for making candy, desserts and other foods, but in a more highly refined form. It is curious that certain chemicals naturally found in the gelatine have a profound effect in making the emulsion more sensitive to light than is the silver bromide alone. The emulsion is extremely thin, but the grains of silver bromide are so infinitesimally small that the emulsion is actually a stratum of material thickness in which there are suspended many layers of silver bromide grains. Of course, these are not arranged in distinct layers: they are scattered throughout the emulsion in a naturally accidental pattern. For the purpose of explanation, however, yve shall make use of a purely imaginary emulsion in which there are ten layers of sensitive grains, all uniform in size and arranged in perfect symmetry (Fig. 1). We shall assume, moreover, that, when light falls upon any grain, it will immediately turn black (without the aid of development). We shall assume that no grain will be in any way affected until the grain immediately above it has become fully blackened, so that the light effect passes in distinct steps from one layer to another. Finally we shall ignore the decreasing proportionate effect of exposures of high and low intensities. We are concerned with building a skele ton of essential facts. When it is complete we can clothe it with the flesh of actual conditions, but we must always remember how the hidden mechanism works. These explanatory examples serve only to illustrate the essential facts of exposure and are not necessarily true to fact in details. In order to expose the emulsion, we must assume a light unit. This unit is of such value that it will pass into and completely blacken just one of the sensitive grains in the top or first layer of the emulsion. Two units of light will blacken one grain, pass into and just blacken the second grain and so forth. A fractional unit will not have any effect at all (in our demonstration emulsion). If our entire emulsion is exposed to light of one unit intensity, all of the grains in the top layer will turn black. We must assume that total opacity in the emulsion requires all ten layers to be blackened. Then, the blackening of this single layer will give a faint greyish tinge to the whole emulsion. If we use five units of light, five grain layers will be blackened and the emulsion will become medium grey. If we use ten units of light, the entire emulsion will be blackened and it will then be totally opaque (in actual practice it is difficult to produce a true opacity by exposing the emulsion and then developing it, but it is easy to produce a depth of tone which is opaque as far as practical printing conditions are concerned.) Having established an experimental emulsion, a definite, experimental light unit, and their basic reactions, we are ready to learn what happens during exposure. Normal Exposure. — Let us assume that the experimental emulsion is divided into ten equal and adjacent areas. These areas are exposed to uniformly increasing light. The first section receives one unit, the next two units, and so on until the tenth section receives ten units of light, which in this example is maximum intensity. In each of the ten areas the grain layers will be blackened to a depth corresponding to the light intensity which fell upon that area, one layer being black ro oi <k • u» Figure 9 Please mention The International Photographer when corresponding with advertisers.