Same light, different settings
Change one setting and this solves the one that keeps the exposure identical — in real camera stops, not the rounded numbers printed on the dial.
Equivalent exposure
Now change what you want and clear exactly one field — that is the one solved for.
Nothing leaves this device
Why f/8 to f/11 is one stop and not 0.92
EV = log₂(N² ÷ t) − log₂(ISO ÷ 100)
The numbers engraved on your camera are rounded. f/11 is really f/11.314 — two to the power of three and a half — and 1/250 of a second is really 1/256. A calculator that takes the printed values literally will tell you that holding exposure from f/8 to f/11 needs 1/132, which is arithmetically correct and wrong to anyone who knows that is one stop.
So this page converts marked values to their exact ones before doing any stop arithmetic, then converts the answer back to the nearest setting your camera actually offers. Where that nearest setting is more than a third of a stop from the exact figure, it says so.
Aperture stops count double the logarithm because an f-number is a diameter ratio and light goes with area.
Equivalent is not identical
Three exposures can pass the same amount of light and produce three quite different photographs. That is the whole reason the triangle is worth understanding rather than automating.
Aperture changes depth of field. Going from f/2.8 to f/8 gains three stops and turns a blurred background into a legible one. See the depth of field calculator for how much.
Shutter changes motion. 1/1000 freezes; 1/30 blurs a walking figure; 1/4 turns moving water to smoke. Also the thing most likely to spoil a frame through camera shake.
ISO changes noise, and on most modern sensors far less than reputation suggests. ISO 6400 on a current full-frame body is entirely usable. A sharp frame at high ISO beats a blurred one at low ISO every time.
Which one to give up
Decide what the picture requires and let the rest follow. If depth of field is the point — a landscape, a group — fix the aperture and let shutter and ISO adjust. If motion is the point, fix the shutter. ISO is usually the variable to sacrifice, because noise is the most recoverable of the three faults: you can reduce noise in software, but you cannot un-blur motion or invent depth of field.
Related
ND and long exposure for when you want a slower shutter than the light allows, and astro exposure for the dark end.
Questions people actually ask
Why does the answer differ from a simple doubling?
It should not, and if it does the cause is usually that the marked numbers are rounded. f/11 is really f/11.314 and 1/250 is really 1/256, so working from the printed figures gives slightly wrong stop counts. This page converts to exact values first, which is why f/8 to f/11 comes out as exactly one stop.
What is EV?
Exposure value — a single number combining aperture, shutter and ISO, where each step is one stop. EV 15 is bright sunlight, EV 0 is roughly a dim room, and negative values are night. Two exposures with the same EV pass the same light, whatever combination produced it.
Is it better to raise ISO or use a slower shutter?
Usually raise the ISO. Modern sensors handle high ISO far better than their reputation, and noise can be reduced afterwards — motion blur and camera shake cannot be undone. A clean-but-blurred frame is a lost frame; a noisy sharp one is usually recoverable.
Why do half and third stops exist?
Because full stops are a coarse adjustment — doubling or halving the light. Modern cameras default to third-stop increments so you can fine-tune. This page accepts any value and snaps its answers to settings you can actually dial in.