Density Against Log Exposure
Ferdinand Hurter and Vero Driffield published the sensitometry that still governs film in 1890. Their graph plots density against the logarithm of exposure.
Density is the base-ten logarithm of opacity. Transmitting one tenth of the light gives a density of 1.0. Transmitting a hundredth gives 2.0. One stop is 0.30 in log exposure.
The curve has four main regions:
| Region | What it is | What it renders |
|---|---|---|
| Base plus fog | Minimum film density | Clear film, the floor |
| Toe | Rising lower curve | Compressed deep shadows |
| Straight line | Constant-slope middle | Evenly separated mid-tones |
| Shoulder | Flattening top | Compressed highlights |
The slope of the straight-line section is the gamma. A steeper slope gives higher contrast. Gamma is not a fixed property of the film, as it rises with development time. Manufacturers often quote a contrast index, measuring the average usable gradient instead.
Film Speed and Grain
The ISO speed of black-and-white film is fixed on the toe. The speed point requires a density of 0.10 above base plus fog.
Underexposure destroys shadow detail first. Therefore, the classic rule is to expose for the shadows.
| Film speed | Crystal size | Grain | Resolving power |
|---|---|---|---|
| ISO 25 to 50 | Small | Very fine | Very high |
| ISO 100 to 400 | Medium | Moderate | Good |
| ISO 1600 to 3200 | Large | Coarse | Lower |
Larger crystals catch more photons, making them faster. They also leave larger grains of silver. Tabular-grain emulsions use flattened crystals to improve this trade-off.
Common pitfall: Treating the box speed as a property of the film alone. It only holds for film developed to a specified contrast.