The international photographer (Jan-Dec 1935)

Eighteen The INTERNATIONAL PHOTOGRAPHER June, 1933 MAKING MOST OF EXPOSURE (Continued from Page 5) within the sensitivity range of the emulsion. But what would have been the case had the original series of light intensities been on the order of one to ten? In that case a single unit of overexposure would have merged the two heaviest deposits, two units of overexposure would have merged the three heaviest deposits, and so on. In a like manner, any degree of underexposure would have merged two or more of the lightest areas, so that any degree of incorrect exposure would have resulted in an incorrect reproduction. When the intensity range of the original equals the sensitivity range of the emulsion there is only one correct exposure. Any departure from this cannot result in faithful reproduction of the original. In such a case although the latitude of the emulsion has not changed, there is absolutely no permissible latitude of exposure. What, then is the case when the experimental emulsion of ten-degree latitude is exposed to a light series ranging in intensity from one to twenty units? Ten units, any ten consecutive units, will exhaust the latitude of the emulsion. If the most intense light is allowed barely to reach the bottom layer, we have the latitude exhausted by light intensities from eleven to twenty, while the intensities from one to ten have not been able to affect the emulsion. The entire strip shows a full range of intensities and is apparently normal, but the lower intensities have not been registered. In practice, a print would be brilliant, with a full range of tone, but the shadows (representing weaker lights) would be inky black and without detail. This is a type of underexposure which cannot readily be recognized by the beginner. No matter how the exposure is manipulated, some error creeps in. Suppose we expose so that light intensity six just blackens the top layer (Fig. 6). Then the ten areas of the emulsion will reproduce light intensities six to fifteen. The first five intensities will not appreciably affect the emulsion, and the last five wll use up the full range and affect the emulsion in exactly the same manner as the fifteen-unit intensity. This means that weak lights will not record and intense lights will show no differentiation. In practice, the print will show inky black shadows without detail, and burned out highlights, also without detail. This is commonly seen in photographs of a brightly lighted landscape, where shadow detail and extreme highlights are sacrificed to retain the desired fidelity in the middle of tones. As we shall see later, the best exposure in actual practice is that which makes use of the known limits of latitude and the knoivn limits of light intensities, balancing the two to the best average advantage. Should the experimental emulsion be given an exposure which would permit the weakest light to affect the first layer, we should have the ten greatest intensities all recorded just like the tenth one, sacrificing all detail in all light intensities above the middle average. Tonal Compression. — There are times when the important portions of a subject have such a great range of intensities that they exceed the normal emulsion latitude, yet they must be reproduced. To illustrate such a condition, we shall again consider the range of twenty light intensities and our original emulsion with ten degrees of latitude. We have seen that when the weakest intensity is allowed to act, the final eleven areas of the emulsion are totally blackened. Obviously we cannot accept such a condition. Imagine, then, each layer cut in half (Fig. 6), so that each unit of light intensity blackens only one-half of a grain. In such a case the full range of twenty light units would be just sufficient to give us the full tontal range of which the emulsion is capable, the tones of adjacent areas differing by a half tone instead of a full tone. In such a case we have a proportionate reproduction. Although we have not extended the actual range of tones, we have succeeded in representing twenty full degrees of light intensity of the original by twenty half tones. This is a satisfactory solution of the problem. In practice this effect can be obtained by controlled development. Fortunately, the actual blackening of the negative does not occur until the emulsion is developed. This blackening does not take place instantly, but is a gradual process. At first practically all of the light-affected areas become faintly grey. As development proceeds, the areas which received the most light grow in intensity at a rate corresponding to the intensity of the original light. Thus we have all the tones represented almost alike at first, but during development the difference, that is the amount of difference, between any two areas increases. Fig. 7 shows diagrammatically the growth of contrast during development; la represents the low contrast of underdevelopment; 7b represents normal development; 7c represents the excessive contrast of overdevelopment. Fig. 8 shows the difference between ideal and actual emulsion development reactions. In actual practice the difference in density between any two underexposed or overexposed areas is less than the difference between any two areas in the middle range — assuming a uniform series of exposures throughout. If we give sufficient exposure to allow the faintest light to act on the emulsion, that effect will be apparent early in the process of development. The development is watched, and as soon as the area which received the most intense light acquires the density which we regard as maximum, the action of development is stopped. In this way we can, in actual practice, achieve the same result as we did in the experimental emulsion by dividing the grains into independent halves. In the finished negative, although the proportionate difference between tones will be only one-half that of the original, the full range will be reproduced in steps which are proportional to the original. This compression of tone scale by means of controlled development depends for its success on giving that exact exposure which will just serve to record the deepest shadow of importance. It cannot be performed with fullest success unless the photographer knows the relation which exists between the full range of original light intensities and the latitude of the emulsion. In compression control we give the exposure necessary to register the shadows, and we stop development just before the stage at which the highlights start to become too opaque. Hence it is easy to understand the reason for the old golden rule of photography: Expose for the Shadows — Develop for the Highlights. The latitude of most modern emulsions is such that for most average subjects an exposure can be given which will produce satisfactory results when followed by standardized development. This cannot be done, however, unless the exposure given has been the one correct exposure for the circumstances! To secure the best technical results under any circumstances it is essential that the correct exposure be given — and, with the rare exception of a short intensity range in the original, there is only one exposure which is correct for anv given subject and light! There are, of course, times when the best technical result is not the best pictorial result. There are many times when the photographer will deliberately give the wrong exposure in order to secure some definite effect; but even these cases demand a knowledge of the right exposure, so that the error may be made that will give just the effect desired. This deliberate distortion of exposure, as well as the methods used in making a scientific determination of the exposure factor, will now be discussed. It is well known that many special effects are secured by Figure 10 Please mention The International Photographer when corresponding with advertisers.