A photograph’s brightness is set by three dials — how wide the lens opens, how long the shutter stays, how sensitive the sensor is — and they trade against each other exactly. Open one stop, halve the time, same picture (but a different look). Slide the aperture (or tap) and watch the shutter compensate.
The exposure triangle in stops. Exposure value EV = log₂(N²/t) where N is the f-number and t the shutter time; a fixed scene needs a fixed EV, so the three controls trade in whole STOPS (factors of 2). Open the aperture one stop (N ÷ √2, so N² halves) and the shutter must double to hold EV — reciprocity, exact. The f-stops here are the real √2 series (f/1.4 … f/16); the shutter is solved to keep EV constant while ISO stays fixed, and a wider aperture shrinks the depth of field. A fail-loud self-check throws unless one stop of aperture equals one stop of shutter at constant EV.
Ideal reciprocity (real film has reciprocity failure at long exposures; digital adds noise at high ISO); depth of field is shown qualitatively. The EV = log₂(N²/t) arithmetic and the stop trades are exact.