Key to Fill Ratio Calculator — Portrait Lighting Ratios

A lighting ratio compares the brightness of the lit side (key + fill) to the shadow side (fill only) of a face — the additive convention used by the ASC and studio meters. Because the bright side is lit by the key AND the fill while the shadow side has fill only, the stops between the two lights is log2(ratio − 1): a 2:1 ratio means the key equals the fill (0 stops, flat), a 3:1 ratio means the key is 1 stop brighter than the fill, 5:1 is 2 stops and 9:1 is 3 stops. Higher ratios create more dramatic, contrasty portraits.

2:1 (flat, key = fill) for beauty, fashion and high-key headshots. 3:1 is the classic portrait ratio — the key 1 stop over the fill gives enough contrast to show form without losing shadow detail. 5:1 to 9:1 (2–3 stops) for dramatic, moody or character portraits. Higher ratios require careful shadow control and suit male or editorial subjects.

Set your key light first for correct exposure, then add fill light at reduced power. For a 3:1 ratio the fill should be exactly 1 stop dimmer than the key (half the key power); for 5:1 set the fill 2 stops down. You can also control ratios by moving the fill light further away (inverse square law) instead of dialling its power.

In a studio with no ambient contribution, the ratio is determined entirely by key and fill lights. On location, ambient light acts as fill. To achieve higher ratios outdoors, you may need to overpower the ambient with your key light or use flags to block ambient from the shadow side.