Defining a lighting ratio
Figure 11.1 The relationship between the brightness measured in stops and the lighting ratio
A lighting ratio is the figure we give to a measured relative difference in brightness between two parts of a scene. As opening the aperture by one stop doubles the amount of light reaching the film, two surfaces where one is brighter by one stop of exposure than the other will therefore have a lighting ratio of 2:1.
Where there are several surfaces, each a stop brighter than the next, each time you open the range of your comparative readings by one stop the amount of light reaching the film will double, therefore ...