Studio Lighting Ratio Calculator - Key and Fill Power for Your Target Ratio

Use the free Studio Lighting Ratio Calculator below. Everything runs in your browser, with no sign-up and no upload.

Two lights, one portrait, and a question: how much brighter should the key be than the fill? The answer is the lighting ratio, and it decides whether the portrait looks soft and open or dramatic and sculpted.

What is Studio Lighting Ratio Calculator?

The lighting ratio compares key light power to fill light power. A 2:1 ratio means the key is twice as strong as the fill, a difference of one stop, and gives soft, flattering light. A 4:1 ratio is two stops apart and looks more dramatic, with deeper shadows. An 8:1 ratio is three stops apart and is frankly theatrical. The calculator converts power settings into a ratio and a stop difference, or works backwards from a target ratio to the fill power you need.

How it works

The ratio is key power divided by fill power, using the same units for both, such as watt-seconds, percent or flash stops. The stop difference is the base-two logarithm of the ratio. Working backwards, fill power equals key power divided by the target ratio. The tool assumes both lights are at similar distances; if one light is much closer, its effective power at the subject is higher, and the inverse square law, not the dial setting, decides the real ratio.

How to use it

Set the key light where you want it and note its power.
Enter the fill power, or leave it at zero and enter a target ratio instead.
Read the ratio and the stop difference.
Adjust the fill until the ratio matches the mood you want, then take a test frame.

Worked example

Your key light is at 200 watt-seconds and your fill at 100. That is a 2:1 ratio, one stop apart: gentle, commercial portrait light. For a moodier look you want 4:1, so the fill must drop to 50 watt-seconds. For a dramatic low-key portrait at 8:1, the fill drops to 25. The same key light, three different portraits, decided by one dial.

Tips and limits

Start with 2:1 for friendly portraits and corporate headshots, 3:1 or 4:1 for character portraits, and higher ratios only when you mean it. Keep the fill light near the camera axis so it fills shadows without casting a second set of shadows. And judge the ratio on the face, not on the dials: light distance, modifiers and reflectors all change the effective power, so the test frame is the final word. Note that ratios are about the lights only; ambient room light also acts as fill, so a bright room lowers your real ratio.

Please note: photography exposure tools are educational calculators. Sunny 16 and Zone System figures are starting-point rules of thumb, and a meter reading governs in real shooting. Time-lapse figures assume the camera can sustain the interval, and buffer or card limits may intervene.

Frequently asked questions

What ratio is best for a beginner portrait?

Start at 2:1. It is forgiving: shadows stay soft and open, and small positioning errors do not ruin the shot.

Do I measure the ratio with a light meter?

A meter is the accurate way: meter the key alone, then key plus fill, and compare. The calculator is for planning from power settings.

Does the ratio include ambient light?

No, the calculator only compares the two lights you enter. In a bright room, ambient light adds to the fill side and lowers the real ratio.

Try the calculator above with your own numbers, and use the result as a planned starting point for your next shoot. A few minutes of planning before you press the shutter usually saves a whole session of corrections afterwards.

Studio Lighting Ratio Calculator – Key and Fill Power for Your Target Ratio