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Stops of Light Calculator

Enter two exposure values to see how many stops of light separate them.

How it is calculated

Enter two amounts of light in the same unit, such as lux readings, flash power, ISO values or exposure times. The difference in stops is stops = log₂(A ÷ B). A positive number means A is brighter (more light) than B, a negative number means A is dimmer. The ratio shown is 2|stops|, which is the same as the larger value divided by the smaller one and never carries a sign. Both values must be above zero. The values have to be linear light quantities: f-numbers are not, because the light gathered goes with the square of the aperture, so f/4 and f/2.8 should not be typed here as 4 and 2.8. One stop is a doubling, two stops are 4×, three stops are 8×.

Worked example

A light meter reads 500 lux in one spot (A) and 200 lux in another (B). Ratio: 500 ÷ 200 = 2.5. Stops: log₂(2.5) = ln 2.5 ÷ ln 2 = 0.916 ÷ 0.693 = 1.32 stops. A is positive, so spot A is brighter, by a factor of 2.50 ×. To even out the exposure, close down by about 1⅓ stops at spot A, or open up by that much at spot B.

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Common mistakes

  • Typing f-numbers directly (4 and 2.8 gives 0.51 stops with a ratio of 1.43 ×, while the real change from f/4 to f/2.8 is one full stop). Convert to light first, for example by squaring the reciprocal of the f-number.
  • Typing shutter speeds as denominators (250 and 125) and mistaking the sign: 1/125 s lets in more light than 1/250 s, but entering 125 as A and 250 as B gives −1 stop.
  • Mixing units between A and B, such as lux for one and foot-candles for the other, so the ratio is wrong before the logarithm.
  • Reading the ratio as a signed value; it is always 1 or more, and the sign sits only in the stops figure.
  • Expecting 0 or a negative reading to work; the logarithm needs both values above zero.

About this calculator

In photography, a stop of light represents a doubling or halving of exposure. This calculator finds the difference in stops between two exposure values as Δstops = log₂(E2/E1) — so if one setting lets in exactly twice the light of another, that's a 1-stop difference; four times the light is 2 stops, since each additional stop doubles again (2ⁿ brightness ratio for n stops).

Stops give photographers a single unit that works across aperture, shutter speed, and ISO simultaneously — opening the aperture one stop (say f/4 to f/2.8), halving the shutter speed (1/250s to 1/125s), or doubling the ISO (400 to 800) all add exactly one stop of exposure, and any combination can be swapped to keep total exposure constant. Photographers, cinematographers, and lighting technicians use stop differences to match exposure between two camera settings, calculate flash power adjustments, or convert a light meter reading into a usable aperture/shutter combination.

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Comments (1)

  • Editorial team

    For lenses, a simple workaround is to type the squared f-numbers, with the narrower aperture first. Entering 16 (f/4 squared) and 7.84 (f/2.8 squared) gives 1.03 stops, so the wider opening admits about twice the light; the extra 0.03 comes from rounding 2.8 for 2.83. Shutter speed and ISO need no squaring: 100 to 800 ISO is the default 3 stops. Which combination of settings are you trying to balance?

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