Light Distance Calculator
Light gets weaker fast as you move away from it. This shows how much light reaches a subject at a new distance, compared with the first distance, for lamps, flashes and studio lights.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
Light spreads out over a larger area as it travels, so the amount on a fixed-size subject falls with the square of the distance.
Divide the first distance by the new one, square it, and multiply by 100 to get the share of light left. Doubling the distance leaves a quarter of the light.
Worked example
Moving a light from 6 ft to 12 ft away leaves 25% of the light on the subject.
Questions
Does this apply to the sun?
Not in a useful way. The sun is so far away that moving a few feet changes almost nothing. The rule matters for close lights.
Does it work for daylight through a window?
Roughly, yes, but a window is a large, soft source, so the drop is gentler than this rule shows.
Why does a flash get dim so fast?
The light spreads over more area as it travels. Each step back spreads it wider, so the subject gets less of it.
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