35 Mm Film - How Film Works

How Film Works

Inside the photographic emulsion are millions of light-sensitive silver halide crystals. Each crystal is a compound of silver plus a halogen (such as bromine, iodine or chlorine) held together in a cubical arrangement by electrical attraction. When the crystal is struck with light, free-moving silver ions build up a small collection of uncharged atoms. These small bits of silver, too small to even be visible under a microscope, are the beginning of a latent image. Developing chemicals use the latent image specks to build up density, an accumulation of enough metallic silver to create a visible image.

The emulsion is attached to the film base with a transparent adhesive called the subbing layer. Below the base is an undercoat called the antihalation backing, which usually contains absorber dyes or a thin layer of silver or carbon (called rem-jet on color negative stocks). Without this coating, bright points of light would penetrate the emulsion, reflect off the inner surface of the base, and reexpose the emulsion, creating a halo around these bright areas. The antihalation backing can also serve to reduce static buildup, which was a significant problem with old black-and-white films. The film, which runs through the camera at 18 inches (460 mm) per second, could build up enough static electricity to cause a spark bright enough to expose the film; antihalation backing solved this problem. Color films have three layers of silver halide emulsions to separately record the red, green, and blue information (except for the Kodachrome process - see below). For every silver halide grain there is a matching color coupler grain. The top layer contains blue-sensitive emulsion, followed by a yellow filter to cancel out blue light; after this comes a green sensitive layer followed by a red sensitive layer.

Just as in black-and-white, the first step in color development converts exposed silver halide grains into metallic silver – except that an equal amount of color dye will be formed as well. The color couplers in the blue-sensitive layer will form yellow dye during processing, the green layer will form magenta dye and the red layer will form cyan dye. A bleach step will convert the metallic silver back into silver halide, which is then removed along with the unexposed silver halide in the fixer and wash steps, leaving only color dyes.

In the 1980s Eastman Kodak invented the T-Grain, a synthetically manufactured silver halide grain that had a larger, flat surface area and allowed for greater light sensitivity in a smaller, thinner grain. Thus Kodak could solve the problem of higher speed (greater light sensitivity—see film speed) which required larger grain and therefore more "grainy" images. With T-Grain technology, Kodak refined the grain structure of all their "EXR" line of motion picture film stocks (which was eventually incorporated into their "MAX" still stocks). Fuji films followed suit with their own grain innovation, the tabular grain in their SUFG (Super Unified Fine Grain) SuperF negative stocks, which are made up of thin hexagonal tabular grains.