Several factors, including graininess, resolving power (resolution), and acutance, affect the definition or capability of an emulsion to produce a clear, sharp image.

GRAININESS. The negative image consists of nearly an infinite number of density deposits of metallic silver. To the naked eye, these grains of silver appear as a smooth, continuous deposit; however, when the image is magnified significantly, a speckled, granular, or mottledeffect becomes apparent. This appearance of the enlarged image is called graininess.

The graininess of a negative depends upon the size of the silver halides in the emulsion, the exposure the emulsion received, and the clumping action of the metallic silver grains during development. Although each emulsion has an inherent grain size, the graininess effect in a negative can be minimized by giving the film correct exposure and proper development. As a general rule, slower speed emulsions have a finer grain (a smaller grain size) than faster speed emulsions.

The Eastman Kodak Company uses modem tabular, or T-grain, technology in some of their emulsions. In doing so, flat, tabular crystals are used which are very efficient in absorbing light. In T-grain emulsions, fewer silver halide crystals are needed, because the crystals have a relatively large surface area for light to strike, but are thin and contain a small amount of silver halides. These properties provide higher resolution and lower graininess compared to other films with the same relative film speed.

Not all black-and-white films form the final image in black metallic silver; for example, Ilford's XP-1 black-and-white film produces a dye image. The advantage of this technology is the film can be processed in color-processing chemicals along with color film and then printed on black-and-white paper.

RESOLVING POWER. This is a term used to define the ability of an emulsion to record fine detail. The resolving power, or resolution, of an emulsion is expressed as line pairs per millimeter. Resolving power is determined by photographing a lined test chart at a greatly reduced size then counting the lines present in one millimeter of film.

ACUTANCE. This is the ability of an emulsion to produce sharp edges between image density differences. Do not confuse this with the ability of an emulsion to produce sharp images controlled by lens focus. Although an image is in focus, the line between a

Figure 2-2. Cross section of black-and-white film.

highlight area and a shadow area is not perfectly sharp. This is because the high-density area tends to bleed onto the low-density area due to the scattering or diffusing of light in the emulsion during exposure.

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