Zone System
Ansel Adams's method of dividing the tonal range into eleven zones and placing chosen subject tones deliberately on that scale.
Formula / rule
Zone V = metered value
Measured in
zones (1 zone = 1 stop)
In depth
Zone 0 is pure black, Zone X pure white, and Zone V is the middle grey any meter reads. The photographer decides which zone a key tone should occupy and adjusts exposure accordingly, then controls development to place the highlights. Its discipline of previsualising the print before the shutter fires transfers directly to raw processing.
Examples in practice
Metering dark bark and placing it on Zone III means dialling −2 EV from the meter's suggestion, keeping it dark but textured.