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This chapter is from the book

How Film Responds to Light

Recording an image on film involves a reaction between light and silver halide crystals (see Figure 3.7). The crystals, spread through the gelatin of the emulsion, are a compound of silver plus a halogen such as bromine, iodine, or chlorine. If a crystal were a perfect structure lacking any irregularities, it would not react to light. However, a number of electrically charged silver ions are also in the structure and move about when light strikes the emulsion, eventually forming an image. The crystal also contains impurities, such as silver sulfide, which play a role in the trapping of light energy.

An impurity (called a sensitivity speck) and the free-moving silver ions build a small collection of uncharged atoms of silver metal when the crystal is struck by light. This bit of metallic silver, too small to be visible even under a microscope, is the beginning of a latent image. The developing chemicals use the latent image specks to build up density, or the metallic silver required to create a visible image.

Figure 3.7Figure 3.7 Black-and-white film construction illustrated.

Chromogenic film is somewhat different from conventional silver halide film. A chromogenic emulsion contains dye couplers as well as silver halides. During development, the presence of silver that has been exposed to light leads to a proportional buildup of dyes. The original silver is then bleached out, leaving the dyes to form the visible image. Most color materials use chromogenic development to produce the final color image, as do several types of black-and-white film.

In Figure 3.8, notice the darker parts of the original scene: Receive less light, develop less silver density, and show less detail.


You probably are familiar with the C-41 process, common at all 1-hour photomats and pharmacies. The C in C-41 means chromogenic.

Figure 3.8Figure 3.8 Each of these images received one stop less light.

Eventually those parts of the negative become clear of silver and print as black. When a highlight area, like the sky, gets too much light, it blocks up with solid silver density, appears dark in the negative, and prints as white.

The Absolute Minimum

This chapter focused on film, which you might not have expected to see in a digital photography book. However, film can easily become part of your digital workstream. If you have any doubts about its lifespan, film will survive the digital onslaught, just as theater survived radio, cinema survived television, and bookstores survived Amazon.

Whether you choose digital or film, keep in mind these important points when taking pictures:

  • The speed of digital and film is measured using an ISO rating.

  • Double the ISO speed and you double the film's sensitivity.

  • ISO 800 film is recommended for indoor, low-light photography.

  • Slide film has less perceptible grain and is better for scanning.

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