Lighting geometry · ideal point source
Move the light. Understand the ratio.
Compare two source-to-subject distances using an explicit inverse-square model. Exact fractions and signed stop changes keep the direction clear.
Enter the two distances
Keep the model in view
Assume an ideal point source, the same emitted output and direction, a receiver facing the source in the same way, and no extra light or intervening losses.
Large nearby lights, bounce, reflections and ambient light can behave differently. This is not a camera or flash setting recommendation.
Read the assumptions and boundsIdeal comparison
New illuminance ÷ old illuminance Exact fraction
Ideal source-output multiplier Exact reciprocal
The model multiplier that would retain the old illuminance at the new distance. It is not a real flash dial or electrical-power setting.
- Illuminance change Approximate
- Compensating source-output change Approximate
Decimal ratios and stops use 10 significant digits. Tiny nonzero stop changes may use scientific notation. Display precision is not physical accuracy.
Farther means more than a linear change.
Doubling source-to-subject distance gives one quarter of the old illuminance in this model. The reciprocal source-output multiplier is four, equivalent to a positive two-stop model compensation. Moving closer reverses those directions.
The worked examples explain the signs. The guide separates geometric arithmetic from real lighting behaviour.