International photographer (Jan-Dec 1941)

usiNq The exposure meter In any discussion of exposure meters emphasis should be placed on the erroneous belief that by simply owning an exposure meter you will get perfect pictures. An exposure meter, when used correctly, is a means whereby perfect exposures can be made. It should be remembered that the meter is an accurate light-measuring instrument, and that correct measurements must be taken to obtain the results which will permit exposing for perfect pictures. Photographic materials have come a long way since the development of the first sensitive emulsion, and figure 1 represents the curve as plotted of a typical film emulsion. Theoretically, a perfect film would have a line that runs diagonally, but because we do not have this film, we have to modify our exposure meters to take into account the characteristics of the films we now have to take pictures with. The film curve shown represents a typical scene as it is analyzed by the scientists in the Sensitometric Laboratory. The lower end of the curve represents the shadow portion of the film, the straight portion the contrast between the dark and light part of the picture, and the extreme end of the curve represents the highlights, or the greatest deposits of silver on the film. The curve representing the characteristics of a film is not a fixed, permanent thing, but something which is movable by means of exposure, development, and light conditions. Correct exposure combined with correct development will provide a perfect negative. One of the immediate problems that comes up in the development of exposure meters is the correlation of this curve to exposure meters. This is done in terms of film speed or film values. One of the earliest systems of rating film was the Scheiner system, which rated the film by measuring the threshold point of the curve. (Fig. I). There was a certain weakness to this system depending upon the position of the shape of the toe and chemical fog. This threshold point was measured by the film manufacturers, and in order to have the best possible product the most favorable value was used. The results were not too reliable as a film speed value. Later, an approach was made by the German Government, to more firmly establish the location of the measurement of the threshold point I Fig. 2) of the film. What is known as the DIN system resulted, wherein the film speed was dependent on the exposure required to produce a density of .1, but the methods of this system did not specify an exact developing technique. Since a change in developer can cause a modification in the curve of this line, this system is weak. In the first part of the twentieth century in England, Messrs. Hurter and Driffield did considerable research on photographic emulsion and sensitivity, and the findings made by them have given this curve a name: the H and D curve ( Fig. 1 ) . These gentlemen decided that a more reliable means of measuring film speeds involved the determination of the inertia point ( Fig. 2 ) . This particular system has merit, and for the films, then existing, was the best possible solution for determining speeds. The system is one of the most common in use in this country today. Film manufacturers, however, in the development of the new high speed film, created emulsion with long toe portions ( Fig. 3 ) rather than abrupt characteristics. These negative materials had H and D curves with a long curved toe portion, with a gradually curved shoulder, and with scarcely any straight line portion in some cases. We, therefore, have a condition where there is no straight line portion to measure. Another disadvantage is that occasionally the straight line portion of a fast film is exactly the same as that of a slower film, thus this system would give no indication of the actual film By Glen C. Anderson, General Eleetrie Company speed. Therefore, in the development of the General Electric film rating system we have made a compensation of the toe portion and the straight line portion of the curve and assigned proper film values. It is to be remembered that film values are assigned to films under laboratory conditions for average results. Individuals may and do vary film ratings, because of personal preferences for a specific density of negatives. It can therefore, be said that the exposure meter is an accurate light measuring instrument, and the film values that are assigned can be modified within the range allowable in order to produce a negative that most suits your needs. Also, an enlarger, that has condensing lenses in it can use a negative of less contrast than an enlarger of the different type of illumination system. Theoretically, the perfect exposure has the shadow portion at the lower end of the curve, and the high light at the upper end of the curve. Under certain specific conditions it is impossible to get such a range of light on the film because the intensity of light is beyond its range. Average photographic emulsions can record a brightness range of 128 to 1 . In some cases the brightness range is considerably higher than this, but for most exterior scenes it is in the neighborhood of 40 or 50 to 1. With this type of lighting being the general condition we do have some leeway in the placement of our exposure on our photographic emulsion. Many benefits will be derived from correct exposure and best results will result when the maximum film value is used. An over-exposure will cause: 1. Larger grain on the film. 2. Loss of detail in highlights. 3. Contrast loss. 1. A flat type of negative. 5. Need for long printing time. 6. Reddish hue in color films. 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 .8 .6 .4 THRESHOLD LOG EXPOSURE ;="<> I 2.8 2.62.4 >i 2.22.0 V) z 111 o 18 1.6 1.4 1.2 1.0 GENERAL rtECTR/C .8 .6 O/A/ D£f*s/rr = / A* .2 J LOG EXPOSURE 4 3 3 2 V ~l ° i I l i i 12 3 4 5 3CH£/N£R TH/feSHOLO 'H&D /A/ERTIA r,9n 18