Zone System Calculator (Place Shadows, Check Highlights, N+ / N− Development)

How it works

camera EV = shadow EV + (5 − shadow zone) · highlight falls on zone 5 + (highlight EV − camera EV) · N = desired highlight zone − zone it falls on (+ expand, − contract) · dev time ≈ N × 1.3^N (expand) or ÷ 1.25^|N| (contract) · EV = log₂(N² ÷ t) at the metered ISO

A light meter renders whatever it reads as middle grey, Zone V. The Zone System uses that fact deliberately: meter the darkest shadow that must keep texture and expose two stops less to put it on Zone III; then meter the brightest textured highlight and see how many stops above the exposure it sits. If it lands on Zone VII the scene is normal; higher means a contrasty scene that needs less development (N−) so the highlights do not block; lower means a flat scene that benefits from more (N+). The calculator does the placement arithmetic and turns the result into a development time and camera settings.

Worked example

A spot meter reads EV 8 on a shaded rock face and EV 14 on sunlit clouds, ISO 400. Placing the rock on Zone III means exposing at EV 10 — f/2.8 at 1/125 s — two stops under the shadow reading. The clouds then fall on Zone IX (6 stops above the exposure) when Zone VII is wanted: a 6-stop subject range calling for N−2, so a 9-minute normal development becomes about 5:45 to keep texture in the clouds.

Frequently asked questions