The photographic zone system is a technique for controlling the exposure and development of photographic film or prints, developed by Ansel Adams and Fred Archer in the 1930s. The zone system is based on the idea of dividing the entire range of tones in a photograph into 11 zones, ranging from pure black (zone 0) to pure white (zone X) with various shades of gray in between (zones 1-9).

The photographer uses a light meter to measure the brightness of the scene and then assigns different parts of the scene to specific zones based on their brightness levels. The photographer then adjusts the exposure and development of the film or print to achieve the desired tonality in each zone.

The zone system allows photographers to achieve a greater degree of control over the tonality of their images, resulting in more accurate and expressive representations of the scene being photographed. It is particularly useful for high-contrast scenes, where the range of brightness exceeds the dynamic range of the film or sensor.