CHAPTER 3 FILM BASICS 41Slow-Speed Film: Maximum Detail
Slow-speed films of ISO 50 or less are mostly color. The reason is that grain has been
reduced so much in black-and-white films that few are still available at speeds under
ISO 100.
A slow-speed color film produces brighter colors and a crisper image than faster
color films. The original Kodachrome film was rated at ISO 25; Fuji’s hugely success-
ful Velvia film is rated at ISO 50.
One exception to color-dominating, slow-speed films is the black-and-white record-
ing film called Technical Pan 2415. Normally this ISO 25-rated film is used for
recording text and other high-contrast applications. Develop this film with a special
Technidol developer, however, and normal photographs are possible. The extremely
fine grain and high resolution of this film enable you to blow up 35mm negatives to
20 ×30 inches with no apparent grain.How Film Responds to Light
Recording an image on film involves a reaction between light and silver halide crys-
tals (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 crys-
tal 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 begin-
ning 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 3.7
Black-and-white
film construc-
tion illustrated. Thin protective coating
Emulsion (60% gelatin and 40% crystals)
Adhesive
Plastic base
Antihalation coating prevents light reflectionLight sensitive crystals