Photography Calculators

Studio, storage & utilities

Inverse Square Law Calculator

Double the distance to a small bare flash and you have a quarter of the light — two stops down. This is the point-source law. Softboxes, bounce and the sun (effectively infinite) do not follow it cleanly. Meter a test shot at both marks before you promise a rim light at 4 m.

Inputs

Units
m

First position of the light (or subject). Imperial shows feet.

m

Second position. Further is darker for the same power.

Results

Intensity at new distance / old
0.250

0.25 means a quarter of the light (two stops down).

Stop change
2.00stops

Positive = darker at the new distance.

Recommended gear

Calculation method

Intensity scales with the inverse square of distance. The ratio of new intensity to old is (old distance over new distance) squared. The stop change is two times log2 of (new over old): doubling distance is +2 stops (darker). Halving distance is −2 stops (brighter). One stop is a factor of two in light; a 2× distance is a 4× intensity change, which is two stops.

Exact formula

intensityRatio = (d1/d2)². stopDelta = 2·log2(d2/d1). Positive stops means darker at d2. Limits: Point source in free space; modifiers, bounce, falloff along a softbox and ambient omitted.

Key versus fill distance

If the fill is twice as far as the key and both are similar small flashes, fill is about two stops down before you touch power. That is a starting ratio, not a style.

Background falloff

Subject at 1 m, background at 3 m: the wall is about 3.2 stops darker than the subject for a light at the camera. Pull the subject forward to crush a messy room.

Frequently asked questions

Why doesn’t my octabox do this?

A large source at close range is more like a window. Falloff is gentler until you are several source-diameters away, then inverse square creeps back.

Sunlight?

The sun is so far that 1 m versus 2 m on Earth is the same. Inverse square is for studio lamps and flash, not for ‘moving the subject in the park’.

Guide number connection?

GN / aperture is a distance because flash follows inverse square. If you double distance you need two more stops of aperture or four times the power.

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