Figure 2.2. Characteristic curve of a color negative.

The figure shows how a negative records the relative scene luminance as film densities. Marked are the locations of the 2% black, 18% gray, and 90% white point. The scene luminance is expressed as log exposure, where 0.3 units equal one stop. Only the density of the green layer is shown and it is measured above base meaning the density of the unexposed film is subtracted.

The slope of the straight part of the curve, known as “gamma”, is around 0.6. Therefore, an exposure increment of 0.3 log E increases the density by 0.3 * 0.6 = 0.18 log D.