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How Equivalent Exposure Is Calculated

Aperture, shutter speed and ISO form the exposure triangle, and every one of them is measured in the same currency: the stop. One full stop always doubles or halves the amount of light that reaches the sensor, regardless of which of the three settings produces it. Full-stop apertures run f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16; full-stop shutter speeds double each time (1/1000, 1/500, 1/250, 1/125...); full-stop ISO values double each time (100, 200, 400, 800...). Because the three scales share the same stop unit, moving one setting by a given number of stops can always be cancelled out by moving another setting the same number of stops in the opposite direction — the total light reaching the sensor, and therefore the exposure, stays identical.

Worked example: a base exposure of f/8, 1/125 s, ISO 100. Opening to f/2.8 lets in roughly 3 stops more light (f/8 → f/5.6 → f/4 → f/2.8), so the shutter must close down by the same 3 stops to compensate: 1/125 → 1/250 → 1/500 → 1/1000. The result, f/2.8 at 1/1000 s at ISO 100, captures the same total light as the original setting — just distributed differently between aperture and time, giving a much shallower depth of field at the same brightness.

What You Should Know

Reciprocity, as this trade-off is traditionally called, is what lets you choose a creative priority — freezing motion with a fast shutter, blurring a waterfall with a slow one, or opening the aperture for background blur — without accidentally over- or under-exposing the shot. Whichever variable you change, shifting one of the other two by the same number of stops in the opposite direction restores the original exposure.

• Stops are not limited to whole numbers: modern cameras meter and adjust in 1/3-stop increments, so a change like f/8 to f/1.8 works out to a fractional 4⅓ stops rather than a round number, and the compensating shutter speed follows the same fractional math.
• The three variables are not truly interchangeable in practice: raising ISO adds visible noise, opening the aperture reduces depth of field, and slowing the shutter risks motion blur — equivalent exposure keeps the brightness constant, but each path there has a different visual side effect.
• This is the same math photographers use to translate a meter reading from one exposure mode to another, or to match exposure across two different cameras with different native ISO ranges.
• One exception: especially on film, very long exposures (typically beyond a few seconds) can suffer "reciprocity failure," where the stop-for-stop trade briefly breaks down and needs extra exposure time beyond the calculated value. Digital sensors are far less affected, but the effect is still worth remembering for night landscapes or star photography with very long shutter speeds.