Introduction
The Lighting Distance Calculator applies the inverse square law to calculate light intensity at any source-to-subject distance. Enter the known lux or exposure value at a reference distance, then enter a new distance to see the resulting lux and the change in stops. The tool also works in reverse: enter the target lux at a new distance to find the source placement that achieves it. Use it for quick exposure planning when repositioning a light, or to translate a manufacturer's published lux specification at one distance to your actual working distance.
What This Tool Calculates
The inverse square law states that illuminance falls off with the square of the distance: E2 = E1 × (d1² / d2²), where E1 is the lux at reference distance d1 and E2 is the lux at the new distance d2. Stop change = log2(E2 / E1) = log2(d1² / d2²) = 2 × log2(d1 / d2). Worked example: 1,000 lux at 2m. Moving to 4m (doubling the distance): E2 = 1000 × (4 / 16) = 250 lux — exactly 2 stops less. Moving to 1.41m (halving the distance, sqrt(2)): E2 = 1000 × (4 / 1.99) = 2,005 lux — exactly 1 stop more. The inverse square law is exact for point sources and approximate for large-area softboxes and diffused sources.
The Formula and How It Works
A gaffer set a 600W LED at 3m from the subject, measured 1,800 lux. The DP asked for 1 stop less without gels. The calculator showed moving to 4.24m (3 × sqrt(2)) would halve the lux to 900 — exactly 1 stop reduction. For an exterior car commercial, a 6000W HMI was placed 8m from the hood at a manufacturer spec of 3,200 lux at 5m. The calculator derived: lux at 8m = 3200 × (25 / 64) = 1,250 lux — confirming f/2.8 at ISO 800 was achievable without supplemental fill. On a documentary, a gaffer needed to know what happens to a 200W LED fill (800 lux at 1.5m) when the camera moved back to 2.5m for a wider frame. The result: 288 lux — 1.47 stops less — triggering a request to open to T1.8 rather than reposition the light.
Real-World Examples
Distance-to-Intensity Reference (1000 Lux at 1m Reference)
At 1m: 1,000 lux (reference). At 1.41m (√2): 500 lux (−1 stop). At 2m: 250 lux (−2 stops). At 2.83m: 125 lux (−3 stops). At 4m: 62.5 lux (−4 stops). At 0.71m (1/√2): 2,000 lux (+1 stop). At 0.5m: 4,000 lux (+2 stops). At 0.35m: 8,000 lux (+3 stops). Use these ratios to mentally estimate stop changes on set — every factor of √2 in distance change equals 1 stop of exposure change.
Pro Tips and Common Mistakes
| Detail | Value |
|---|---|
| The inverse square law is exact only for true point sources. | |
| Large softboxes, sky panels, and fluorescent banks behave more like extended sources — they fall off more slowly than the law predicts at short distances and more closely follow it at distances greater than the largest dimension of the source. | |
| Always meter the result at the final position rather than relying on the calculation alone for critical work. | |
| The most common mistake is applying the formula to moving a soft source close to the subject: at 0.5m from a 4×4 softbox, the inverse square law significantly overpredicts the lux compared to a real reading.. |
Pro Tips and Common Mistakes
Pro Tips
- The inverse square law calculation takes 5 seconds with this tool and 30 seconds with mental math.
- On a fast-moving set where the director is waiting, running these numbers instantly — and showing the gaffer exactly where to place the light — keeps the pace without sacrificing precision..
Common Mistakes
Frequently Asked Questions
Does the inverse square law apply to LED lights?
Yes, for small LED chips and Fresnel LED lights at distances greater than the source diameter. For large LED panels and softboxes, the law approximates well at distances greater than the largest source dimension. At shorter distances, the output is more even than the law predicts.
How do I use the inverse square law on set without a calculator?
Memorize the doubling rule: doubling the distance = 2 stops less. Halving the distance = 2 stops more. For 1-stop changes, multiply or divide the distance by √2 (approximately 1.41). These two rules cover most on-set positioning decisions.
Does the inverse square law work for sunlight?
No. The sun is effectively at infinite distance, so the inverse square law does not apply for objects on Earth's surface. Sunlight intensity varies with atmospheric conditions and sun angle, not with the distance between the subject and the sun.
What is the difference between the inverse square law and the cosine law?
The inverse square law describes how intensity falls off with distance from a point source. Lambert's cosine law describes how illuminance changes when a surface is not perpendicular to the light source. On set, the inverse square law governs light-to-subject distance; the cosine law governs the angle of the light relative to the subject surface.
Start Calculating
Use the calculator above to run your numbers before your next production.
