Studio Lighting Calculator
Determines the fill light power needed to achieve a desired lighting ratio in a multi-light studio setup. Use it when balancing key and fill lights for portrait or product photography.
About this calculator
Lighting ratio describes the relative brightness of the key light versus the fill light on a subject. Because light intensity follows the inverse-square law, moving a light farther away reduces its power by the square of the distance ratio. The required fill light power is calculated as: fillPower = (keyLightPower × (fillLightDistance / keyLightDistance)²) / desiredRatio. A 2:1 ratio produces soft, even lighting, while a 4:1 ratio creates dramatic shadows. By adjusting either the physical distance or the power output, photographers can hit any target ratio precisely. This calculator solves for the fill light wattage given fixed distances and a desired ratio, letting you dial in settings before you shoot.
How to use
Suppose your key light is set to 400 W at 2 m from the subject, and the fill light is positioned 3 m away. You want a 3:1 lighting ratio. Plug in: keyLightPower = 400, keyLightDistance = 2, fillLightDistance = 3, desiredRatio = 3. fillPower = (400 × (3/2)²) / 3 = (400 × 2.25) / 3 = 900 / 3 = 300 W. Set your fill light to 300 W to achieve a 3:1 ratio at those distances.
Frequently asked questions
What does a 3:1 lighting ratio mean in studio photography?
A 3:1 lighting ratio means the key light side of the subject is three times brighter than the fill light side. It is measured by comparing the total light falling on the highlight side (key + fill) to the light on the shadow side (fill only). This ratio is popular for portrait work because it creates visible but flattering dimensionality. Ratios above 4:1 produce dramatic, high-contrast images, while a 1:1 ratio is completely flat.
How does the inverse-square law affect studio light placement?
The inverse-square law states that light intensity is inversely proportional to the square of the distance from the source. If you double the distance between a light and your subject, the intensity drops to one-quarter — not one-half. This means small changes in distance have a large impact on exposure. Photographers use this principle deliberately to shape lighting ratios without changing power settings on the flash heads.
When should I use a fill light instead of a reflector in a studio setup?
A dedicated fill light gives you precise, repeatable control over the fill intensity, which is critical when shooting tethered or matching light across a series of images. Reflectors are passive and their output depends on the angle and distance to the key light, making them harder to predict. For commercial and product photography where consistency matters, a powered fill light is preferred. Reflectors work well for quick portrait setups where exact ratios are less critical.