Exposure Triangle
The exposure triangle describes the shared relationship between aperture, shutter speed and ISO — all three measured in the same unit, the stop, meaning a change in any one can be exactly offset by an opposite change in either of the other two while keeping total exposure identical.
The metaphor is useful but incomplete: matching total exposure only guarantees matching brightness, not an equivalent photograph. Aperture also controls depth of field, shutter speed also controls motion rendering, and ISO also controls visible noise — three separate creative effects layered on top of the shared exposure math, which is why choosing between equivalent settings combinations is a real creative decision, not just an arithmetic one.
Frequently Asked Questions
Is ISO really an equal member of the exposure triangle?
It's measured in the same stop-based unit and trades directly against the other two for total exposure, but unlike aperture and shutter speed, it doesn't change how much actual light reaches the sensor — it only amplifies the signal already captured.
Which variable should I set first when shooting manually?
There's no universal order — a common approach is to fix whichever variable the scene constrains most (aperture for depth of field, shutter speed for motion) and let the remaining variable absorb whatever exposure the first two leave unaccounted for.