Point Light Distance Exposure Compensation Calculator

Method, formula and assumptions

Ideal illuminance ratio = (old distance/new distance)^2. Exposure change is log2 of that ratio. Equivalent f-number = old f-number x old/new distance; equivalent power divides original power by the ratio.

Input definitions and supported units

• Original light-to-subject distance (m): Measured or assumed numerical value, from 0.01 to 1000; must be positive.
• New light-to-subject distance (m): Measured or assumed numerical value, from 0.01 to 1000; must be positive.
• Original f-number: Measured or assumed numerical value, from 0.1 to 128; must be positive.
• Original relative light power (%): Measured or assumed numerical value, from 0.001 to 100; must be positive.

Worked example with answer

Moving from 1 to 2 m leaves 0.25x illuminance, a -2-stop exposure change. f/8 becomes f/4 for aperture compensation; an original 25% relative power needs 100% at unchanged aperture.

How to use the results

Power above 100% means the entered setup cannot compensate by power alone. Aperture compensation changes depth of field and should be considered separately.

Results retain full precision internally and display up to four decimal places. Very small nonzero results use scientific notation rather than appearing as zero. Check the labels before comparing values across units.

Limitations and references

Assumes a point source and linear relative power. Large modifiers, near-field geometry, room bounce, TTL metering and flash power calibration differ in practice.

The method is a self-contained arithmetic, geometric or explicitly stated mathematical model. There are no imported tariffs, empirical presets or equipment specifications; measured values and allowances must come from your own process.

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