Cinematography-Theory-And-Practice

Figure 10.15. A theoretical ideal film— one that exactly reproduces theexposure changes of the subject ina one-to-ratio with negative density.contrast film. Figure 10.16 shows the difference between a high-contrastemulsion and a low-contrast one. In the high-contrast example,for each additional unit of exposure, it changes 2 units of negativedensity. Looking at the brightness range of the exposure against thebrightness range of the negative density, we see that it will showmore contrast in the negative than actually exists in the scene. Theslope of this line is called the gamma of the film: it is a measure ofits contrastiness.Contrast refers to the separation of lightness and darkness (calledtones) in a film or video image and is broadly represented by the slopeof the characteristic curve. Adjectives such as flat or soft and contrastyor hard are often used to describe contrast. In general, the steeper theslope of the curve, the higher the contrast. The term gamma refers toa numerical way to describe the contrast of the photographic image:gamma is the slope of the middle, straight part of the curve.Gamma is measured in several different ways as defined by scientificorganizations or manufacturers. They are all basically a wayof calculating the slope of the straight-line portion of the curveby more or less ignoring the shoulder and the toe portions of thecurve. Gamma does not describe contrast characteristics of the toeor the shoulder, only the straight line portion. But there is anotherwrinkle. In the lowest range of exposure, as well as in the highestrange, the emulsion’s response changes. In the lowest range, the filmdoes not respond at all as it “sees” the first few units of light. Thereis no change in photochemistry at all until it reaches the inertia pointwhere the amount of light first begins to create a photochemicalchange in film or an electrical change on a video tube. After reachingthe inertial point, then it begins to respond sluggishly: negativedensity changes only slightly for each additional unit of light. Thisregion is the toe of the curve. In this area, the changes in light valueare compressed.At the upper end of the film’s sensitivity range is the shoulder in filmterminology, and the knee in video. Here also, the reproduction iscompressed. The emulsion is becoming overloaded; its response toeach additional unit of light is less and less. The end result is that filmdoes not record changes in light value in the scene in a linear andproportional way. Both the shadows and the highlights are some-cinematography192