Flash inverse square falloff calculator

It returns the distance ratio, remaining light as a ratio and percentage, the signed exposure change in stops, and the opposite compensation needed to preserve the original exposure. Use metres, feet, inches, or another unit, provided both entries use the same unit. The calculation is deterministic and intended for direct flash or continuous point-like sources where inverse-square behavior is a useful working model.

Why flash exposure changes so quickly with distance

Light from a compact source spreads across a larger area as it travels. Under the inverse-square model, illumination is proportional to one divided by distance squared. Move a flash from two metres to four metres and the distance doubles, but the subject does not receive half as much light; it receives one quarter. One quarter corresponds to a loss of two stops. Move the same flash from four metres to two metres and illumination becomes four times stronger, a gain of two stops. Enter the original light-to-subject distance as initial_distance and the new placement as final_distance. The unit can be metres, feet, centimetres, or any other consistent length because the calculation uses only their ratio. Measure from the emitting flash head to the subject, not from the camera, stand, or photographer. The returned direction says whether the light moved farther, closer, or stayed unchanged. A negative exposure_change_stops value means less light reaches the subject; a positive value means more light. compensation_stops uses the opposite sign and tells you the exposure adjustment required to restore the starting brightness. This distinction prevents the common ambiguity in phrases such as “a two-stop change,” which can describe either the light loss or the correction.

How to read the ratio, percentage, and stop results

The calculator first divides final distance by initial distance to produce distance_ratio. It then squares the inverse of that ratio to obtain light_ratio. A light_ratio of 0.25 means the new position supplies one quarter of the original illumination, while light_percent expresses the same result as twenty-five percent. The stop conversion is log base two of light_ratio, because each exposure stop represents a doubling or halving of light. If exposure_change_stops is minus two, compensation_stops is plus two: open the aperture by two stops, raise ISO by two stops, lengthen the shutter by two stops for a fully ambient or continuous-light calculation, or increase flash output by two stops when sufficient power is available. For flash photography at or below normal sync speed, shutter speed primarily changes ambient exposure rather than the brief flash contribution, so aperture, ISO, flash power, or distance are usually the relevant controls. Fractional results are normal. Moving a light from three metres to four metres, for example, does not land on an exact whole-stop boundary. Set precision when you need fewer or more decimal places for a worksheet or software fixture. Rounding affects displayed values only; all derived quantities come from the unrounded inputs before they are formatted.

Where the inverse-square model works and where reality differs

Inverse-square falloff is most useful when the flash behaves approximately like a point source, the subject distances are measured consistently, and no automatic system silently changes output. It is excellent for planning manual speedlight placement, comparing test setups, teaching lighting ratios, and estimating how much power or aperture must change after moving a lamp. Real photographs can differ because a softbox or umbrella has physical size, especially when it is very close to the subject; the source may not behave like a mathematical point. Bounce surfaces, reflectors, grids, zoom settings, atmospheric loss, room reflections, and uneven beam patterns also alter the result. TTL flash can mask falloff by automatically increasing or decreasing power, and maximum output may prevent it from delivering the requested compensation. Treat the answer as a controlled baseline, then confirm important setups with a flash meter, histogram, or test frame. The calculator compares illumination at one subject position; it does not calculate guide number, aperture from guide number, depth of field, background brightness, or the difference in exposure between several people at different depths. The browser calculation is free, while an automated API request costs $0.002. No image, camera metadata, location, or account-specific lighting profile is needed, and the computation uses no network service or stored measurements.