Often called the H & D curve after its progenitors Ferdinand Hurter (1844-98) and Vero Charles Driffield (1848-1915), this shows the relationship between exposure and photographic density. As the latter is a logarithmic quantity, it is plotted against the logarithm of the exposure. Much of the curve is approximately linear, and it determines the exposure needed to produce an image with a satisfactory range of tones. A typical characteristic curve is shown in Fig. 1. A great deal of information is available from this curve:
• Emulsion speed. The exposure required to produce the minimum useful density is at the point 0.1 density units above the zero-exposure density level, called the base-plus-fog level. This is used to establish the ISO speed index.
• Inherent contrast. This is the ratio of subject contrast to negative contrast, and is the slope of the line joining two standard points on the curve representing an average exposure range. The steepness of the curve increases with increasing development time.
• Spectral sensitivity. By exposing the emulsion to different wavelengths a set of characteristic curves will show the variation of sensitivity and inherent contrast with wavelength. In practice the emulsion is positioned under a density wedge and exposed to a calibrated spectrum.
• Latitude. This shows the extent to which exposure error is tolerable. The lower limit is the toe of the curve; below the 0.1 density point the shadow detail disappears. The upper point is determined by graininess, which increases with density.
• Processing variations. A comparison of characteristic curves shows the effect of different developers on emulsion speed and contrast. More important, families of curves made for different development times and temperatures are used to produce time/temperature charts for developing to required contrasts.
A colour negative has three characteristic curves, for red, green, and blue respectively. These are measured with a densitometer fitted with colour filters (Fig. 2). Colour transparencies are measured in the same way. In this case the characteristic curve slopes the opposite way (Fig. 3).
Characteristic curves for print materials have a slope that varies very little with development time. The slope needs to be matched to the density range of the negative; hence the need for various grades of paper (or a variable-grade paper). As development proceeds the curve moves to the left, indicating the latitude in exposure (Fig. 4).
There are more immediately practical conclusions to be drawn from the characteristic curve. For example, if it has a very steep slope, the emulsion is high contrast, suitable chiefly for copies of black-and-white drawings. A long ‘toe’ and a more gentle slope indicate an emulsion with good exposure latitude that will also allow push processing. In transparency material the density range between toe and shoulder indicates the maximum recordable subject contrast.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
— Graham Saxby
See also densitometry; sensitometry and film speed.Bibliography
- Saxby, G., The Science of Imaging: An Introduction (2002)
