Lighting Ratio
The relationship between the illumination on the lit side of a subject and that on the shadow side, expressed as a ratio or in stops.
Formula / rule
Ratio = (key + fill) : fill
Measured in
ratio / stops
In depth
A 2:1 ratio is one stop of difference and reads as gentle, flattering modelling; 4:1 is two stops and looks dramatic; 8:1 and beyond becomes theatrical. Ratios must be measured with an incident meter at the subject, reading each side separately, because judging them by eye is notoriously unreliable.
Examples in practice
Metering f/8 on the key side and f/5.6 on the shadow side gives a 2:1 ratio suitable for a corporate headshot.