Cinematography-Theory-And-Practice

one whole f/stop, we double the quantity of light reaching the film;each time we close it one stop, we halve the light reaching the film.The relative f/stop scale (Table 10.1) is tiered to show that the samerelationships that apply to whole f/numbers, such as f/8 and f/11,apply to intervals between them. So the difference between f/9 andf/13 is one whole stop, and so on. Modern digital meters measure in1/10ths of a stop. This is helpful for calculations and comparisons,but for most practical purposes, this level of accuracy is not necessary.One-third of a stop is the practical limit of precision, giventhe vagaries of optics, lab chemistry, sensor sensitivity, and telecinetransfer. This is not to say that accurate exposure is not important,only that the degree of precision in the overall process has limits.Inverse Square Law and Cosine LawAs light emanates from a source, it does not drop off in intensity ata linear rate. For example, if the lamp is 11 feet from the subject,moving it to 8 feet will increase the subject illumination by 1 stop,just as opening the lens diaphragm from f/11 to f/8 would do. Theinverse square law applies to point sources, strictly speaking, butspotlights follow it fairly well at the distances usually utilized.Light decreases with the square of the distance from the source. Ineveryday terms, it you get 1/4 the amount of light every time youdouble the distance from the source. We rarely estimate light levelsby mathematical calculation, but it is important to understand thebasic principle involved.Figure 10.9 illustrates the inverse square law graphically. A similarprinciple is the cosine law (Figure 10.10). As a surface is turnedaway from the source, less of the surface is “visible” to the sourceand therefore there is less exposure. Mathematically, the decrease inexposure is equal to the cosine of the angle of the surface, so this iscalled the cosine law.ISO/ASASince one-third stop is the minimum exposure difference detectableby the unaided eye (for most negative stocks), film sensitivity israted in no finer increments than this. This scale is tiered to makethe relationships between intervals more easily seen. Just as ISO 200is 1 stop faster than ISO 100, ISO 320 is 1 stop faster than ISO 160.(Table 10.2).Although this is obvious, memorizing this scale makes it easier tosee the differences between odd intervals, such as ISO 80 to ISO32 (1 1/3 stops.) The scale may be expanded in either direction byadding or subtracting digits (the intervals below 6 are 5, 4, 3, 2.5, 2,1.6, just as the intervals below 64 are 50, 40, 32, 25, 20, and 16.Foot-candles: The ISO scale can also be applied to foot-candles.Doubling the foot-candles doubles the exposure. The third-stopFigure 10.9. (top) The inverse squarelaw. This is important not only tounderstanding expsoure measurement,but to lighting as well. Everytime you double the distance, youget 1/4 the amount of light.Figure 10.10. (bottom) The cosinelaw: how the angle of the subjectaffects its exposure level.Table 10.1. (top) Light levels andexposure. “X” represents a givenamount of light; each step to the leftdoubles the amount of light at thesubject.Table 10.2. (bottom) ISO or ASA inone-third stop increments. The sameseries can be interpreted as percentageof reflection, footcandles orshutter speeds — which serves toremind us that all these measures ofexposure are interrelated.LightF/Stops1/3Stops2048x 1024x 512x 256x 128x 64x 32x 16x 8x 4x 2x X1 1.4 2 2.8 4 5.6 8 11 16 22 32 451.1 1.6 2.2 3.2 4.5 6 9 13 18 25 361.3 1.8 2.5 3.6 5 7 10 14 20 29 406 12 2550 100 200 400 8008 163264125 250 500 100010 20 40 80 160 320 640 1250exposure187