Use the free Inverse Square Light Falloff Calculator below. Everything runs in your browser, with no sign-up and no upload.
Move a flash one step closer and the picture gets much brighter. Move it one step back and it falls off a cliff. That is not your flash misbehaving. It is the inverse square law, and once you can calculate it, you can predict it.
What is Inverse Square Light Falloff Calculator?
The inverse square law says light intensity falls with the square of the distance. Double the distance and the light drops to one quarter, which is two stops less. Halve the distance and the light quadruples, two stops more. Move from 2 metres to 3 metres and you keep only about 44 percent of the light, a loss of about 1.2 stops. The calculator gives both the stop change and the percentage, so you can think in whichever unit your camera uses.
How it works
The tool compares the old and new distances. The percentage is the square of the old distance divided by the new distance, times one hundred. The stop change is minus two times the base-two logarithm of the distance ratio, with a negative result meaning less light. The law assumes a point source in open space; large softboxes close to the subject fall off more gently, but the law is still the right first estimate for speedlights and studio heads.
How to use it
- Measure or estimate the current flash-to-subject distance.
- Enter the distance you plan to move to.
- Read the stop change and the percentage of light remaining.
- Adjust aperture, ISO or flash power by the same number of stops to compensate, if you want the same brightness.
Worked example
Your flash sits 2 metres from the subject and you move it to 4 metres to get it out of the frame. Doubling the distance quarters the light, so you lose two stops. To keep the same exposure you must open the aperture two stops, raise the ISO two stops, or quadruple the flash power. Move instead from 2 metres to 2.8 metres and you lose exactly one stop, because 2.8 is about 2 times the square root of two. Small moves matter most at short distances.
Tips and limits
Falloff is why backgrounds go dark in flash portraits: the subject is close to the flash and the wall is far away, so the wall gets a fraction of the light. If you want the background visible, move the flash further from the subject so the relative distances shrink, or add a second light for the background. Also remember that moving a light closer makes its falloff across a group of people more uneven, while moving it further back makes the light more even but weaker overall.
Please note: photography exposure tools are educational calculators. Sunny 16 and Zone System figures are starting-point rules of thumb, and a meter reading governs in real shooting. Time-lapse figures assume the camera can sustain the interval, and buffer or card limits may intervene.
Frequently asked questions
Does the inverse square law apply to sunlight?
In practice, no noticeable falloff, because the sun is so far away that moving a few metres changes the distance by almost nothing.
Why does a softbox fall off more slowly than a bare flash?
A large source close to the subject is not a point source, so the maths softens. The further the softbox is from the subject, the closer it behaves to the law.
How many stops do I lose by moving from 1 metre to 2 metres?
Two stops: the light drops to one quarter.
Try the calculator above with your own numbers, and use the result as a planned starting point for your next shoot. A few minutes of planning before you press the shutter usually saves a whole session of corrections afterwards.