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The Silver Image—Image Quality Attributes - Granularity and Graininess, Edge (Adjacency) Effects

resolution characteristics film emulsion

When considering the silver image, there are a number of important attributes that characterize the recording medium. In addition to the macroscopic characteristics covered above under the section Sensitometry, there are micro-scale issues because the image is made up from individual grains of silver, coated as a thin layer that has unusual light-scattering and absorption properties, and is processed in a developer.

Granularity and Graininess

A property that becomes immediately obvious when looking at a large enlargement, or a negative under high magnification, is made up of individual silver grains. On a microscopic scale this means that even an area that measures as uniform optical density on a standard densitometer does in fact exhibit statistical density fluctuations when measured on a microscopic scale. The objective measure of these statistical fluctuations is made with a device known as a microdensitometer and the measure is known as granularity.

This granularity gives the image the visual property known as graininess. Graininess can be defined as the subjective consequence of granularity. Granularity and graininess are a function of both the emulsion type and the processing conditions employed. In general, fine-grain emulsions produce images with low graininess and granularity. Different developers will also produce different levels of graininess and granularity at the same densities from the same emulsion product.

A further level of information on the emulsion’s properties is provided by the noise power or Wiener spectrum of the granularity. This splits the photographic noise signal into individual frequency components in a fashion analogous to audio noise analysis. This type of analysis is useful in separating lower-frequency components that give the visual impression of mottle from other noise signals.

Edge (Adjacency) Effects

The characteristic curve covered earlier described the macroscopic recording characteristics of an emulsion. However, it is found that this does not adequately describe the recording characteristics of fine detail and around the edges in an image. In such cases the recorded density is also a function of the density of adjacent areas. As a result such phenomena are known generically as adjacency effects.

Adjacency effects are primarily driven by development conditions and, in particular, by changes in development conditions across edges and fine detail. This can be explained with reference to Figure 4, which illustrates the situation in a stylized fashion for clarity.

Adjacency effects are a function of developer formulation and the level and type of agitation employed. As is illustrated in Figure 4, there is no reason for the enhancement to be symmetrical around the edge.

Resolution and Sharpness

One of the oldest metrics of image quality of optical systems is resolution, which can be defined as the ability to record fine detail. A chart such as that illustrated in Figure 5 is used and the resolving power is the maximum number of lines and spaces per millimeter that can be individually distinguished.

In practice, resolution measurement of photographic films was found to be a fairly good measure of their image recording characteristics when the image was meant to be read directly from the film. An example of such an application is aerial film where the interpretation would often be done from the negative. However, in most cases negatives involve an additional imaging step such as printing before viewing. These processes have their own imaging characteristics so resolution measurements are less applicable. As a result these types of measurements have fallen from favor. With the widespread availability of film scanners, this situation may change as film resolution appears to be a reliable measure of the image recording characteristics of camera films for scanning.

The resolution of a photographic emulsion is much lower than would be expected from the grain size, by one or two orders of magnitude. This difference is due to light scattering and reflections within the emulsion layer. However, fine-grain emulsions, in general, do yield a higher resolution than larger grain varieties.

It should be noted that there is a poor correlation between perceived sharpness of a film and resolution. Measurements of Modulation Transfer Function (MTF) are much more applicable in such cases.

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