Why does halving flash power equal one stop?

A stop is a factor of two in light. Manual flash power is calibrated in fractions of full output, so 1/2 power emits exactly half the light of full power, which is one stop less. 1/4 is two stops down, 1/8 is three stops, and so on.

How do I convert a power change into an aperture change?

Open the aperture by the same number of stops the power dropped. Dropping from 1/1 to 1/4 loses two stops, so opening from f/8 to f/4 (two stops wider) restores the same exposure on the subject.

How does flash distance compensate for power?

Light falls off with the inverse-square law: illumination is proportional to 1/distance squared. To gain N stops by moving closer, multiply distance by 2 to the power of (-N/2). Halving the distance gains exactly two stops of light.

Does this account for modifiers or ambient light?

No. It computes the pure relationship between flash power, f-stops, aperture, and distance. Softboxes, bounce, gels, and ambient light add their own losses that you measure or meter separately.

Can I use non-standard fractions like 1/3 power?

Yes. Type any fraction. The stop difference is the base-2 logarithm of the power ratio, so it works for 1/3, 2/3, or any value, not just the clean 1/2, 1/4, 1/8 steps on a speedlight dial.

Flash Power Reduction in Stops Calculator

This tool converts a change in manual flash power into the language photographers actually use on set: stops, aperture, and distance. Set a starting power and a new power, and it tells you exactly how many stops you have gained or lost and how to compensate so the subject stays correctly exposed.

How it works

Manual flash power is expressed as a fraction of full output. Because a “stop” is defined as a doubling or halving of light, the relationship between a power ratio and stops is logarithmic:

stops = log2(power_new ÷ power_old)

So going from full (1/1) to 1/4 gives log2(0.25) = -2 stops — two stops darker. Each clean halving on a speedlight dial (1/1 → 1/2 → 1/4 → 1/8) is one stop, which is why the dial feels so predictable.

To keep the subject exposed the same after a power change you compensate with aperture or distance:

Aperture: open up by the same number of stops the power dropped. The f-number itself scales by a factor of sqrt(2) per stop, since aperture area (not the f-number) is what doubles.
Distance: light obeys the inverse-square law, illumination is proportional to 1 / distance². To recover N stops by moving the flash, multiply the flash-to-subject distance by 2^(-N/2). Halving the distance is a 2-stop gain.

Worked example

You are shooting at full power, f/8, with the flash 4 m from the subject. You drop to 1/8 power to recycle faster and freeze motion. That is three stops darker. To hold exposure you can:

Open the aperture three stops: f/8 → f/5.6 → f/4 → f/2.8.
Or move the flash closer by the distance factor 2^(-3/2) ≈ 0.354, so 4 m × 0.354 ≈ 1.4 m.

Notes

The stop math is exact for any fraction, including odd values like 1/3 power. The standard f-stop ladder (f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22) is shown so you can snap to whole stops. Everything runs locally in your browser.