The density of film refers to the amount of light a negative transmits. The denser or more blocked from light the sections of a negative are, the darker they will be in print. This is called negative film density. A system of measurement ranging from minimum to maximum called log density is used to assess the tonal range of negatives.
Log density is a simple ratio expressed as a logarithm. On a scale from minimum to maximum, "Dmax" is the maximum density or the most negative film density that a print can have. The higher the Dmax in portions of the negative, the better quality the black will be in those areas in the final print.
A negative with a short tonal range, meaning that there is less variation in the gradations of light, is considered to be of low negative film density. Conversely, a negative with a lot of tonal variation is described as having a long tonal range and a high negative film density.
Film Base and Fog
A 35 mm negative has a basic gray film base. The emulsion on the film base has a certain minimum density referred to as "fog." Fog is the result of a small proportion of unexposed emulsion being developed. The film base and the emulsion usually absorb half of the light that is it exposed to, while reflecting the other half.
Determining Negative Film Density
Film densities above film base and fog (fb + f) are the way that negative densities are expressed. Film base plus fog (fb + f) density is the equivalent of a 2x blocking factor, expressed as the ratio 2x, in stops (1 stop) or as a logarithm of the ratio. The logarithm to base 10 of 2 is rounded up to a 0.30 density.
Further Examples of Calculating Negative Film Density
If a certain section of the negative allows one-third of the light to burn through, it is described as having a blocking factor (blocking of light) of 3x. This is expressed as the ratio 3x, in stops (1.5) or using log density, the log to a base of 10 of 3, and rounded to the nearest hundredth decimal, to equal .48.
If a different section of the film allows no more than one-thousandth of the available light to burn, the ratio is described as 1,000x; in stops (10) or as a log density of 1,000 or 3.0.
Exposure, Development and Negative Film Density
Overexposed film is considered "blocked-up" or not transparent and has a high negative density. Underexposed film is considered "clear" and has a lower negative density. While exposure determines the detail of the shadows, development determines the highlight detail placement. Using log densities you can reduce your development time and produce a "thinner" negative, which is suggested for compatibility within the range of most scanners to print well. While a thin or underexposed and/or underdeveloped negative produces prints that scan well, higher paper grades and filters may be used to print the detail of the shadows.
Using Exposure to Control Density
You can manipulate the negative density of your film by overexposing it, but you sacrifice the tonal variations in the process.
Conversely, underexposing your negative in order to manipulate density may make the images appear as black and featureless.