A lighting ratio compares two light levels, such as the separate contributions of a key and fill or the measured light on two sides of a subject. The numbers only have a clear meaning when the measurement method is stated.
Convert stops to a ratio
For comparable readings, one stop of difference is 2:1, two stops is 4:1 and three stops is 8:1. The relationship is ratio = 2 raised to the stop difference. A 3:1 ratio is about 1.585 stops, rather than exactly one and a half.
Key-to-fill versus bright-to-shadow
Suppose an isolated key contributes 200 lux and an isolated fill contributes 100 lux at the same measurement position. Their source-contribution ratio is 2:1. In an idealized arrangement where the fill reaches both sides equally and the key reaches only the lit side, the resulting levels would be 300 and 100 lux: a 3:1 bright-to-shadow ratio. Real faces and rooms rarely follow that model exactly.
Practical example
Record “key to fill, measured separately: 2:1” rather than just “2:1.” For a repeat setup, also record the receptor position and orientation, source positions and ambient conditions. A reflected comparison of two different surfaces includes their reflectance as well as the illumination. Recheck the final image after changes to wardrobe, blocking, bounce or negative fill.
Related terms
Key Light · Fill Light · Negative Fill · Stop · Incident Light Meter · Reflected Light Meter
References
Sekonic: source comparisons and lighting ratios · Sekonic: combined key-and-fill convention