International photographer (Jan-Dec 1941)

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INEQATIVE EXPOSURE By CAPTAIN DON NORWOOD, LI. S. A., Ret'd. One hundred and fifty years ago, in Southern California, the old Mission Padres associated distance to be traveled in a day with the rate of speed at which a mule traveled. The missions located roughly 25 to 30 miles apart stand as evidence of this. In a later period when there were roads of a sort, and horses and coaches, standards of the distance to be traveled in a day were changed and extended. Today, an automobile will cover five or six hundred miles easily in a day, while an airplane will cross the continent in the same length of time. Again the standards have changed. Time moves on, and as it does, men's standards in various fields of activity change and progress. This is true in the photographic field of the standards set up for negative exposure. Thirty years ago if a negative carried an image at all it was considered passable. The image might be very dense from overexposure, or very thin from underexposure. The laboratory people would try to doctor it up. It could be further juggled around when it came time to make a print. Anyway it got by somehow. About seven years ago the advent of photoelectric brightness meters occurred. Brightness meters being those which measure the light reflected from a scene. These meters were a big factor in changing the standards of negative exposure. By the use of these meters it became possible to so expose negatives that the entire image density range of all normal scenes would lie on the straight line portion of a characteristic H. & D. curve. It was still necessary, of course, to adjust printing exposures to compensate for variations in negative image densities. Now the time has come when it is possible to move on to still higher standards of negative exposure. This is made possible through the development of a new photoelectric meter known as a "Prevailing-Illumination" meter. Negatives exposed under the control of this meter are so precise that all may be printed within a very narrow range of printing exposures. Assuming, of course, that processing is maintained at a high level of constancy. The principle on which this new meter operates will be described. Let us first i onsider a photographic scene. To a photographer, a photographic scene may be defined as follows: "A complex array of assorted brightness, emanating from various sized areas, located at varying distances from the camera; further complicated by the clfccls of color." Consideration of this definition will lead one to realize what a tough proposition a brightness meter is up against. Of all ihose brightnesses in a scene, which should be measured? How much weight should be given to each measurement when balancing them off to arrive at a significant figure for the exposure? What about contrast as it affects exposure? What about corrections for color? What about corrections for distance? Haze? Backlighting? The problem is a serious one indeed. A careful and extended study was made in order to discover if it could not be simplified in some way. This study brought out the fact that all the brightnesses in any given scene have one factor in common. This common factor is the prevailing-illumination. The prevailing-illumination can be measured by a suitably designed instrument, and the value so obtained can be used for exposure control. The reason for the above is as follows: Any photo subject brightness is a product of two factors, namely, illumination, and its own reflectance. Reflectances remain substantially constant. Prevailing-illuminations show wide variation. The range of diffuse reflectances encountered in photographic subjects may extend from that exhibited by black velvet at two per cent, up to that of white velvet at eighty per cent. It will be noted that these values of two per cent and eighty per cent cover a range of 1-40. This range of 1-40 fits very easily into the latitude of negative emulsions which is usually about 1-125. Since the range of reflectance can be taken care of by the film latitude, it then remains only to measure the variable, the prevailing-illumination. With this done the lens diaphragm and shutter time may be properly set to compensate for the variable. In this manner the group of reflectances to be found in a scene will always come through onto the film with the same range of values. Consider some given scene. In one studio it may be lighted up to a level of 350foot candles. In another studio it may be lighted up to a level of only 50 foot candles. We know that the release prints carrying this scene may be practically indistinguishable one from the other. In addition, the two negatives carrying the scene may be practically identical as regards densities. The range of reflectances remained the same of course for both takes.. On one case we had a high level of illumination, which was pulled down by the camera exposure controls. In the other case a low level of illumination, of which a much larger percentage was passed by the camera exposure controls. The point which it is desired to emphasize, however, is that in order to get perfect negatives for both takes, the factor which logically should be measured is the only one which shows variation, that is, the prevailing-illumination. The range of reflectance constants will be taken care of by the emulsion latitude. When prevailingillumination is measured, and then compensated for by the camera exposure controls, it will be found that any given subject reflectance will always show up with the same density in the negative. Consider a face in close-up for example. Flesh tones have a reflectance of between 30 and 40 per cent. A girl's face may show a reflectance of 40 per cent. In a print this should always show up at about the same given density. When the method of negative exposure control described herein is used the face will always show up with a constant density in negatives. Piecing these two facts together will show why it is possible to print all negatives with a fixed printing exposure, or within a very narrow range of printing exposures. It is interesting to examine prevailingillumination as such. All prevailing-illuminations may be classified into three types. Examples of each type may be visualized if we consider a white stucco garden wall with sunlight shining on it through the branches of a tree. Type 1 Prevailing-Illumination. See Figure 1 . In this type the subject and scene is for the most part in direct illumination from the primary light source. The wall has only a few leaf and branch shadows on it. In this case the prevailingillumination is the clear sunlight, and that is what should be measured for exposure determination. Type 2 Prevailing-Illuminating. See Figure 2. In this type shadow area fills most of the scene. Only a few shafts of direct sunlight strike through onto the well. Or there might be none. In this type the prevailing-illumination is that existing in the shade. Its value should be measured at the position of the principal subject. Type 3 Prevailing-Illumination. See Figure 3. In this type the sunlight and shadow portions of the wall are about equal in area and importance. The principal subject is illuminated by patches of both sunlight and shadow. In this case the prevailing-illumination is a mean between the illumination value existing in the shadows and that existing in the direct light. For exposure determination both should be measured, and the mean value determined. It might be further noted here that this Type 3 Prevailing-Illumination is not con a