Inverse Square Light Falloff Calculator
Light falloff by distance — where the same half-metre step costs two stops beside the subject and a sixth of a stop across the room.
The inverse square law describes a POINT source. A large softbox close to a subject is nothing of the kind, and falls off more slowly than this until you are a few times its width away — which is exactly the range portrait lighting works in. Treat the figures as the limiting case the physics tends toward rather than a reading off a meter. Nothing is uploaded.
The same half-metre costs two stops up close and a sixth of one across the room
"Falls off with the square of the distance" sounds like a steady decline. It is not — the curve is steepest where you are standing closest. Moving a light from 0.5 m to 1 m loses a full 2.00 stops; the identical half-metre from 8 m to 8.5 m loses 0.175. That is 11.4× more for the same physical step. And doubling the distance always costs exactly two stops, leaving a quarter of the light — not half, which is the guess.
| Move | Stops lost | Light left |
|---|---|---|
| 0.5 m → 1 m | 2.0000 | 25.0% |
| 1 m → 1.5 m | 1.1699 | 44.4% |
| 1.5 m → 2 m | 0.8301 | 56.3% |
| 2 m → 2.5 m | 0.6439 | 64.0% |
| 3 m → 3.5 m | 0.4448 | 73.5% |
| 4 m → 4.5 m | 0.3399 | 79.0% |
| 6 m → 6.5 m | 0.2310 | 85.2% |
| 8 m → 8.5 m | 0.1749 | 88.6% |
One stop is a factor of √2 in distance, which makes the useful distance series look exactly like the f-stop series — and for the same reason, since both are a square root of a light ratio. Doubling the distance is two stops wherever you start:
| Doubling | Stops lost |
|---|---|
| 0.5 m → 1 m | 2.0000 |
| 1 m → 2 m | 2.0000 |
| 2 m → 4 m | 2.0000 |
| 4 m → 8 m | 2.0000 |
| 8 m → 16 m | 2.0000 |
Which is why evenness across a group is bought with distance
Put a light 1 m from the front row of a group one metre deep and the back row is 2.00 stops darker. Move the same light to 8 m and that falls to 0.34 stops — nobody has to stand on a mark. A close light is a control; a distant one is forgiving.
| Light at | Front row | Back row | Difference |
|---|---|---|---|
| 1 m | 1 m | 2 m | 2.000 stops |
| 1.5 m | 1.5 m | 2.5 m | 1.474 stops |
| 2 m | 2 m | 3 m | 1.170 stops |
| 3 m | 3 m | 4 m | 0.830 stops |
| 5 m | 5 m | 6 m | 0.526 stops |
| 8 m | 8 m | 9 m | 0.340 stops |
Only the ratio of the two distances matters, though — not the absolute distance and not the depth on its own. 2 m to 3 m and 4 m to 6 m are both a factor of 1.5 and both cost exactly 1.170 stops, so moving the light back only helps because it shrinks the ratio a fixed depth represents. Double the group's depth as well and you are back where you started. Note also that this is evenness, not softness: those are different properties, and moving a light back makes it harder as well as more even.
How to use
- Enter where the light is now and where you are moving it.
- Enter how deep your subject or group is.
- Read the stops gained or lost, and the new aperture.
- Compare the falloff across the subject at each distance.
Frequently asked questions
What does the inverse square law actually cost me?
Doubling the distance costs exactly two stops and leaves a quarter of the light — not half, which is the usual guess. Halving the distance gains two stops the same way.
Why does moving a light a little matter so much?
Because the curve is steepest where you are closest. Moving from 0.5 m to 1 m loses a full two stops, while the identical half-metre from 8 m to 8.5 m loses 0.175 — more than eleven times less for the same step.
How do I light a group evenly?
Move the light back. With a light one metre from the front of a group one metre deep, the back row is two stops darker; at eight metres that falls to about a third of a stop and nobody has to stand on a mark.
Does the absolute distance matter, or the depth?
Neither on its own — only the RATIO of the two distances. Two metres to three and four metres to six are both a factor of 1.5 and both cost exactly 1.170 stops, so doubling the light distance and the group depth together changes nothing.
How many stops is one step of distance?
A factor of the square root of two in distance is one stop, which makes the useful distance series look exactly like the f-stop series. That is not a coincidence: both are a square root of a light ratio.
Does moving the light back make it softer?
No, harder — softness depends on the size of the source relative to the subject, so a light moved away is both more even and more contrasty. Evenness and softness are separate properties and this tool measures only the first.
Does a softbox follow the inverse square law?
Only once you are a few times its width away. Close in, a large source falls off noticeably more slowly than a point source would, which is exactly the range portrait lighting works in — so treat these figures as the limiting case.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.