Zone System Calculator — Ansel Adams Exposure Zones

The Zone System maps the 11 tonal zones (0–X) to exposure values, with each zone representing one stop of light.

A light meter always reads toward Zone V (middle grey). If you meter a subject and want it to appear at a different zone, you adjust the exposure setting accordingly. Because a higher EV setting means less light reaches the sensor (a darker result), placing a tone on a zone darker than V needs a higher exposure EV, and a zone lighter than V needs a lower exposure EV.

Exposure-setting EV = Metered EV + (V − Target Zone)

For example, if you meter dark foliage at EV 10 and want to place it on Zone III (dark with detail), the exposure should be EV 10 + (5 − 3) = EV 12 — that is 2 stops less light (underexpose 2 stops) than the meter's mid-grey recommendation, which renders the tone two zones below middle grey.