Work out inverse square law for light instantly with clear inputs, formula shown and shareable results.
Light from a point source spreads over a sphere, so illuminance falls with the square of distance. Moving from 1 m to 3 m cuts 800 lux to 89 lux - a ninefold drop, or about 3.17 photographic stops. The product E.d^2 is constant and equals the source intensity in candela.
Inverse square law
E2 = E1 x (d1 / d2)^2
Source intensity
I = E x d^2
Only beyond a few times its largest dimension. Close in, an extended source falls off much more gently, which is why big softboxes are forgiving.
Because the fall-off is quadratic. Moving from 0.5 m to 1 m loses three quarters of the light; from 5 m to 5.5 m loses under a fifth.