Inverse-square law

Definition

For a compact source in free space, illuminance scales as 1/d². Move a bare speedlight from 1 m to 2 m and you lose two stops; 1 m to 3 m is a bit over three. Guide numbers assume this falloff on-axis with no bounce. Softboxes, walls and the sun (effectively at infinity) break the simple square. Use it to pre-visualize flash distance, then meter or chimping a test shot before the client walks in.

Calculators that use this

• Studio, storage & utilities

  Inverse Square Law Calculator

  See how light intensity falls when you move a point source: ratio of (d1/d2)² and the stop change of 2 log2(d2/d1).

• Exposure & light

  Flash Guide Number Calculator

  Turn a flash guide number, f-number and ISO into maximum subject distance, with GN scaled from the ISO 100 rating.