Adams and Archer’s original Zone System had 11 zones, numbered from zero through to ten, but with digital cameras we are mostly concerned with zones three to seven. Looking at the chart below left, start in the middle at Zone 5. This represents a mid-tone in the scene. Anything one stop darker than this would render as Zone 4, two stops darker Zone 3, and so on. Anything one stop lighter is Zone 6, two stops lighter Zone 7, and so on. Anything at Zone 2 — three stops below middle — is too dark to show detail, while Zone 3, although dark, has detail. Anything at Zone 8 — three stops above middle — is too light to show good detail, while Zone 7, although light, has detail.
For colour photographs you must consider colour, not just detail. A light colour will lose saturation beyond Zone 6. Although it will still have detail at Zone 7, the colour will be pale. And a dark colour can’t go below Zone 4 without becoming muddy.