How do you calculate a lighting ratio using two light sources?

• A. Measure each light's output with a meter; determine the ratio (e.g., key:fill 2:1 means the key is twice as bright); adjust power accordingly to achieve the desired contrast.
• B. Estimate the ratio by eye and adjust exposure after shooting.
• C. Use camera settings to balance brightness.
• D. Increase ISO until the scene looks balanced.

Answer: (A)

You determine the lighting ratio by quantifying how bright each light is on the subject and then comparing those values. Use a light meter to measure each light separately at the subject’s position: first isolate the key light and take a reading, then isolate the fill light and take another reading. The difference between those readings, expressed in f-stops, gives the ratio. A one-stop difference means a 2:1 ratio (the key is twice as bright as the fill); two stops means 4:1, and so on. Once you know the ratio, adjust the power of the lights to reach the desired contrast, keeping the overall exposure consistent by tweaking camera settings if needed. This approach provides objective, repeatable results rather than guesses. Estimating by eye or relying on camera settings or ISO alone won’t reliably establish the relative brightness of the two lights on the subject.