How do I find equivalent exposure settings when I change one variable?

Change one exposure variable by a given number of stops, and one or both of the remaining two must move by that same total number of stops in the opposite direction to keep the total exposure identical. A f/2.8, 1/60s, ISO 1600 exposure narrowed one stop to f/4 needs its shutter speed to lengthen to roughly 1/29s (call it 1/30s) to stay equivalent — or, keeping the shutter fixed at 1/60s instead, the ISO would need to rise to 6400 to compensate for the same one-stop aperture change.

Why There's Never Just One Right Answer

Because there are two other variables that can absorb a change in the third, any single settings change has infinitely many equivalent solutions, not one — the aperture example above could equally be balanced by splitting the compensation between shutter speed and ISO instead of loading it entirely onto one. Which specific solution is actually right for a shot depends on which of the three side effects (depth of field, motion blur, noise) you're least willing to sacrifice, not on the exposure math alone, which only guarantees matching brightness.

The Practical Shortcut

Count the stops on the variable you're changing, then move exactly that many stops on whichever other variable you've decided to sacrifice, in the opposite direction. A full-stop aperture sequence (f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16), a full-stop shutter sequence (doubling/halving each step), and a full-stop ISO sequence (doubling/halving each step) all move exactly one stop per step, so counting steps on one sequence tells you exactly how many steps to move on another.

Solve any two-variable exposure change automatically in the Exposure Triangle Solver, pre-filled with this exact worked example.