The values in a tone reproduction curve generally represent the darkest tone that has some detail and a diffused white highlight. This doesn’t mean there aren’t additional luminances beyond these points. “deep crevices in tree trunks or between folds in dark clothing, shadows deep in bushes, or other small dark recesses in which no detail is discernible…Whether they are reproduced properly or not in a photograph is often of little importance.” – Theory of the Photographic Process, p. 468. A similar statement can be made with luminances beyond the diffuse white highlight.
While the following example is a set of transparency reproduction curves, it’s the scales along the base which are important. One has the statistically average luminance range of 2.2. The other includes the cavity shadows and specular highlights. These aren’t forgotten in tone reproduction, just not usually included in the graphs. In my four quadrant normal example, the negative density range pretty much matches the paper LER. The LER is measured between a reflection density of 0.04 above paper base plus fog and 90% of the paper D-max. This doesn’t mean nothing should fall outside of that range. This is where the specular highlights and cavity shadows fall.
tone reproduction curve - range.jpg
And just to show how different materials and the way they are used influences the shape of the preferred reproduction curve. This example is of motion picture film, transparencies, and reflective print.
Preferred print, transparency and MP tone reproduction curve.jpg
My apologies to Michael for deviating from the main topic, but I believe this may help in the appreciation of the various factors involved in the perception of preferred print quality which is critical in determining an accurate film speed method. That a simple density measurement alone is insufficient to determine accurate exposures which produce consistent results with good a probability of quality.