ImageReasonLighting, Flash, and Studio
What fill power do I need for this key-to-fill ratio?
Enter your target key-to-fill ratio and the key and fill distances to see the relative fill output needed, and whether it's feasible with your key baseline.
Answer
Visualize
Calculate to see the key/fill diagram.
Compare
| Ratio | Fill power |
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Plan
Calculate a result to generate a copyable plan.
A ratio is a relative exposure decision
A key-to-fill ratio is a relative exposure decision, not a power-label setting. Equal distances make the arithmetic simple: a two-stop key advantage leaves the fill at one quarter of the key's relative output. Unequal distances change that relationship because direct illumination falls with distance squared. The planner keeps output and placement visible together.
With key and fill the same distance from the subject, a two-stop key-to-fill target needs a fill at one quarter of the key's relative output. If the fill is twice as far away, it needs four times as much output to maintain the same contribution. If that exceeds the available setting, lower the key baseline, move the fill closer, or accept a different ratio. Meter or test the real arrangement because modifiers and reflections complicate the point-source model.
This result models the values you entered. It cannot see the scene, verify focus or movement, predict camera-specific processing, or guarantee the final photograph. Use it as a stated starting point, then check the important subject area in a test frame or proof.
Make one controlled test with the important variable held constant. Inspect the subject detail, motion, highlights, and intended output, then adjust the setting that protects the priority you named.
FAQ
- Does a 4:1 ratio mean the fill is set to 25 percent?
- Only when the lights are comparable and the distances are equal under this simplified direct-light model. Distance and modifier behaviour change the practical setting.
- Why no universal best setting?
- There is no universal best setting because the relevant tradeoff changes with the subject, light, movement, output, and the part of the image that matters. This tool makes the stated tradeoff visible rather than hiding it behind a generic recommendation.