182 chaPter 8 The Film system 1 2 3 a b c Figure 8-33 When film is exposed to light (a), some of the silver halide crystals change into metallic silver according to the amount of light they received (2), while unaffected crystals remain in their silver halide form (1). During the processing of negative film, the unaffected crystals are removed (b), leaving behind various densities of metallic silver. Areas with heavy silver density block light (3) and areas with little or no silver particles allow light to pass through. of the actual light values. Imagine projecting this image, pushing light through this strip of film. The area corresponding to black, with no silver density, would be transparent, with only the clear base between the lamp and the screen, allowing the projector light to pass directly through. The area corresponding to a white detail in the scene would have built up a very dense and opaque mass of silver particles, allowing no light to pass through and would therefore appear black on the screen. Any density in between would be a shade of gray, with the inverse relationship to the actual brightness of the photographed detail (Figures 8-33). Identifying film stocks We have discussed the basics of all film stocks, but obviously there is more to it than this. All camera film stocks are further identified and ultimately chosen according to five major characteristics: 1. Color or black and white 2. Negative or reversal 3. Film speed (also called the exposure index, or EI) 4. Color temperature balance 5. Gauge/format Let's take a closer look at each category (Figures 8-34). color or black and White a b c d Obviously, this is a choice between film that renders images as tones of the grayscale (black and white) and films that duplicate the colors of the visible light spectrum as they appear in your scene (color film). As of the writing of this book, Kodak offers eight different color film stocks but only two different black-and-white stocks for 16mm production. It is clear that black-and-white film is receiving no research and development attention because very few people opt to make black-and-white movies these days--and for those who do, removing the color component is easily done in digital postproduction (Figure 8-35). The basic construction of color film is similar to that for black- and-white film stock--emulsion, subbing layer, base, and anti- halation backing--but there is one significant difference. Color film contains three layers of light-sensitive emulsion instead of one. Because every color in the visible spectrum is made up of some combination of the three primary colors of light--red, green, and blue--each of the three layers is sensitive to one of these primary hues. The first layer is sensitive to blue light, the second layer is sen- sitive to green light, and the final layer is sensitive to red light. This three-layered emulsion is called color tripak (Figure 8-36). Also, because all silver halide emulsions are sensitive to the blue Figure 8-34 A film can will always have the following information: whether it is color or black and white, negative or reversal (a), the Exposure Index (b), a color temperature rating (c), and the gauge (d).