Worked examples

Read the ratio and its reciprocal.

These original examples use only the declared ideal point-source model. Distances are from the modeled light source to the subject, expressed in the same unit. No example predicts a particular lamp, softbox, flash, camera or room.

1.2 m to 2.4 m: double the distance

The old/new distance ratio is 1.2 ÷ 2.4 = 1/2. Squaring it gives new illuminance of 1/4 of the old value. The illuminance change is −2 stops. The reciprocal source-output multiplier is 4/1, corresponding to +2 stops of ideal source-output compensation. That multiplier is not a promise that a real flash dial or electrical control will scale its output that way.

1.5 m to 2.25 m: a non-whole stop

The distance ratio reduces to 2/3, giving relative illuminance 4/9. Its reciprocal is 9/4, or approximately 2.25 times the old model source output. The compensating change is approximately +1.169925001 stops at ten significant digits. The negative of that value is the illuminance change. This is the fixed example shown at startup and reset.

2.5 m to 1.75 m: moving closer

The relative illuminance fraction is 100/49, which is greater than one. Moving closer increases modeled illuminance. To retain the previous modeled level, the ideal source-output multiplier is 49/100. The compensation is therefore negative, approximately −1.029146346 stops. A negative compensation value means less modeled output, not a negative quantity of light.

Same distance, same model

At 0.75 m before and after, both fractions are 1/1. Both stop changes are zero. The source, direction, receiver orientation and intervening conditions are still assumed unchanged. Equal distance alone would not keep real lighting unchanged if other conditions changed.

Very close decimal inputs

Two entered distances can differ by a tiny amount even when a ten-significant-digit decimal ratio displays one. The exact fraction preserves that difference, and the signed stop result may use scientific notation. This is a numerical distinction, not a claim that such a tiny physical distance or lighting change can be measured accurately. Read the source and scope in saved notes whenever sharing a result.