Technology of grain in film photography

Historically, film technology has been locked into a rigid relationship among film speed (sensitivity), sharpness, and grain structure. Alter one and you surely change the other two in the opposite direction. Demands for ever faster films led to the introduction of high-speed films, especially of the color print variety.

Indeed, we do get to use higher speed film. Emulsion speeds of ISO 200, 400, 1000, and even 3200 are available. There is a price to be paid for all this speed. Initially, high-speed film was very grainy, with huge, rough-edged crystals. These films were not particularly sharp. We had speed, but it wasn't useful for anything except small snapshot-size prints. If you produced anything over a 5 x 7-inch enlargement, the quality just wasn't there. Critical photographers were not content to settle for mushy, ill-defined, low-light images and they voiced their disapproval with their wallets. High-speed-film sales didn't go anywhere.

Then, several major producers of photographic film around the world launched a major research effort to change the traditional film-quality triangle relationships. They asked one question: Could one design a film that was faster (more sensitive) while at the same time limit or eliminate the adverse effects that such speed increases had on image sharpness and the appearance of grain? The first positive answer to this question came in 1983 when Eastman Kodak announced a new method of making silver halide crystals. Until this time, crystals were formed in a more or less pebble shape and were randomly distributed within the film emulsion layer. To change this, Kodak researchers did two things. First, they computer-designed the now-famous pioneering T-Grain (tabular), a crystal whose thinner, flatter shape presented more surface area and thus was able to capture more light for the same amount of silver than conventional grains. To increase their efficiency, T-Grains were also designed to align their surface parallel to the emulsion surface automatically.

What Kodak accomplished in one decisive stroke was to make a grain structure that improved on previous crystals in a number of highly significant ways. First, the new structure was more regularly shaped with fewer large crystals that had heretofore been the cause of visible and objectionable grain patterns, specifically in high-speed film. Also, the new design was able to limit the number of very small grains that had downgraded sharpness. An additional benefit of T-Grain research is that it allows for a measurably thinner emulsion layer, which in turn reduces light scatter Film Resolution.

By definition, film resolution is a measure of a film's ability to record distinctly very fine and closely spaced details in subjects recorded on the emulsion. Thus, resolution is a quality of film that generates detail. Usually, the more detail that is observable, the higher the film resolution. Conversely, low-resolution film generates relatively fewer image details. In just about all instances, pictures shot in high-contrast light will generate substantially more lines of resolution than under low-contrast light. Resolution figures for film are quoted for both high- and low-contrast lighting. The figures for high-contrast are as much as 60 percent greater than those for low-contrast targets. As with measures of granularity, resolution is described numerically and verbally.

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