Characteristic Curve
A graph plotting a film's resulting density against the logarithm of the exposure it received.
Formula / rule
D vs log H
Measured in
density vs log exposure
In depth
Also known as the H&D curve after Hurter and Driffield, it has three regions: a toe where shadows compress, a straight-line portion where response is proportional, and a shoulder where highlights compress. The gentle shoulder is what gives film its celebrated highlight handling, and the slope of the straight section is its contrast.
Examples in practice
Comparing two films' curves shows immediately which will hold a bright sky and which will clip it.