Exposure Triangle
The teaching model that links aperture, shutter speed and ISO as three interdependent controls, where changing one requires compensating with another.
Formula / rule
Aperture ↔ Shutter ↔ ISO
Measured in
stops
In depth
The triangle is a mnemonic rather than physics: only aperture and shutter speed truly control exposure, while ISO sets amplification after capture. Its value is that it makes the side effects visible — aperture governs depth of field, shutter speed governs motion blur, and ISO governs noise — so a photographer chooses which cost to pay.
Examples in practice
Shooting indoor sport, you accept ISO 6400 noise so you can keep 1/500 s to freeze the players and f/2.8 to gather light.