Contrast Index
A measure of the average slope of a film's characteristic curve over the range actually used in practice.
Measured in
CI
In depth
Contrast index replaced the older gamma figure because gamma measured only the straight-line portion, which most real exposures never fully occupy. A value around 0.56 to 0.58 suits normal scenes and diffusion enlargers; lower values suit contrasty subjects, higher ones flat light. Adjusting development time is how the index is controlled.
Examples in practice
Cutting development by twenty per cent lowers the contrast index enough to make a harshly lit scene printable.