Linear vs Non Linear

Although the light meters built into digital and film cameras are essentially the same, the way these two processes record and respond to light is different in ways that are important to understand. One simple way to begin looking at this is to consider the graphic systems we use to record the relationship between exposures and print tonalities.

In 1890 the photographic researchers Ferdinand Hurter and Vero Driffield published a seminal paper demonstrating what happened when you plotted exposure and film density on a graph with logarithmic intervals.

If the relationship between light exposure and film density were consistently simple and proportional (or "linear") the resulting line on the graph would be straight indicating that, from the darkest value to the brightest, doubling the exposure by stops would produce twice the film and print density.

FIGURE 143

A linear ratio of exposure to negative density.

Instead, what they discovered was that the result was a line with a characteristic curve that was similar for all films and developers.

This curve indicates that equal amounts of exposure don't produce equal amounts of film density throughout the full range of tonal values. At the darkest and brightest ends of the scale the responses of film to light are compressed or "non-linear."

FIGURE 144 The characteristic curve of exposure to negative density.

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