The Film and its Processing

The film used was Kodak Safety Film 5035, 35 mm, commonly known as Kodacolor II. Its ASA rating is 100. The photograph in question was located at frame 11. The so-called "characteristic curve i.e., exposure vs. optical density), modulation transfer function, and spectral sensitivity for this film are presented in Figure 2(a), (b), and (c), respectively (Eastman Kodak Co. Manual, 1980).

The concept of "graininess" and "granularity" of photographic film is important here. Graininess is defined as the subjective sensation one gets when viewing an enlargement of a photograph of a random pattern of variations in texture, color, or both in regions of homogeneous luminance and color exposure. Granularity is the result of an objective measurement of the film using an instrument known as a densitometer which measures the local density variations that give rise to the sensation of graininess (Kodak Publications F-20, 1973). Most silver halide crystals that make up photographic film are dispersed in a gelatin and coated in thin layers on a supporting (paper, etc.) base. Importantly, these crystals vary in size, shape, and sensitivity to light energy. In general, they are also randomly distributed. As the Kodak manual states, "Within an area of uniform exposure, some of the crystals will be made developable by exposure; others will not. The location of the developable crystals is random" (Kodak Publication F-20, 1973, p. 3; italics mine).

One result of this random distribution of light-sensitive crystals (grains) is that patterns can be produced which have nothing to do with the object that was originally photographed. If such a pattern is perceived as having a recognized shape, it is possible to conclude that the shape represents an object somehow related to the primary object when, in fact, there is no functional correlation with the object.

The diameter of the film's crystals is also important. The Kodak film manual indicates that a typical crystal diameter ranges from 0.2 to 2.0 micrometers. At a normal viewing distance of 25 to 35 cm the human eye can just discriminate a crystal (grain) on the order of 0.05 mm diameter. Normally the eye does not perceive the granular structure at low magnifications. The finer the mean diameter of the crystals the higher the magnification can be before graininess is noticed. It is the random nature of the exposed crystals, each