Optical Density and Density Range
Tonal values on film are measured in units of optical density. This normative scale ranges from zero density (0.0d) to 4.0d -- clear film to black. Density is measured on a base 10 log scale, the numbers represent the tens exponent of the relative density. A tonal value measured at 2d is 10 times more dense than one measuring 1d. A one photographic stop difference in density is about 0.3. For example, 2.5d is about one stop greater in density than 2.2d. In reality, the sensitivity of film ranges from .3d (fog and film base density) to about 3.7d. The shorthand for the minimum and maximum value that can register on a film or device is D-Min and D-Max. Of the two, D-Max is the more important because it is indicative of the ultimate sensitivity of the film or device.
Density range is simply the difference between the minimum and maximum tonal values that the film can register. The theoretical density range of film is then 3.7-.3 = 3.4d, or a bit over 11 stops. In practice, however, the typical photographer is able to use a fraction of this range.
Since film scanners are devices which translate the analog medium of film into digital representations, they too have a density range. Because their wider density range sensitivity pretty much encompasses that of film, density range is not now much of an issue for drum scanners. The first consumer desktop scanners, because of the limitations of CCD technology, started with D-Maxs of about 2.5. The best CCD scanners today attain a D-Max of 3.6. A scanner with a D-Max of 3.4 can image about one more stop of tonal value than a scanner with a D-Max of 3.1. When evaluating scanner performance, density range is a factor to be evaluated in tandem with resolution.
All this having been said, in the course of preparing this short treatise and reviewing various scans, I got the impression that there is one standard of measuring the density range for consumer CCD scanners and another for measuring density range for drum scanners. CCD scanner density range is generally consistent among CCD scanners in terms of comparability, but seems overly optimistic when compared to drum scanners.
For Gen-X'ers or younger: how was D-Max meaningful in the good (bad?) old days of photography. Aside from recording one's esthetic vision, one of the enduring challenges of photography has been to accommodate the limited tonal range of film to the scene seen by the eye. On a sunny day with a clear blue sky, the human eye experiences a tonal range exceeding 12 stops (4.0d). The final step, producing a print, required projecting a negative with a potential density range of 3.4d onto print paper with a density range of 2d. In today's terms, each step in producing a print confronted the print maker with a substantial loss of data. For mere mortals the final print was a visual tradeoff of effort and time for quality. Master craftsmen, like Ansel Adams and Edward Weston, became legendary partly for their skills at transcending the limitations of the print making process. The digital age is eclipsing the demand for these rare skills (and eliminating photographic suppliers). Film scanners exist in a transient period that will end with the advent of fully digital photography. Still to this New Yorker, who has had the opportunity to attend countless photography exhibitions, the thought that the days of the finely crafted black and white silver-based print with its smooth transition from velvet black to glaring-white are numbered is somewhat unsettling.