Calibration

A phone camera is not a calibrated photometer, and its readings will sit some distance from the handheld meter you already trust — not always by the same amount at every brightness. Calibration closes that gap: you give the app readings from a reference meter, and it fits a correction across the Zone System, zone by zone.

Once calibration is active, every corrected EV label carries a dot (•).

Reference meters

Calibration data is grouped per meter, so a Sekonic and a Gossen keep separate profiles and you can switch between them. In Settings:

• Reference Meters — add, select or delete a meter.
• Use Calibration — turn correction on or off. Available once the selected meter has enough data to fit.
• Export Calibration JSON — write a meter’s data points out, to back up or to share.

Calibrating

1. Take a snapshot and enter offline mode.
2. Optionally add custom spots by tapping the image — each one is another sample, and custom spots are how you reach zones the automatic three do not land on.
3. Tap Calibrate to open the calibration panel.
4. For each spot, enter the EV your reference meter reads for that same area, using the stepper.
5. The Highlight, Midtone and Shadow spots are fixed to Zones VII, V and III. Custom spots can be assigned to any zone from 1 to 10.
6. Tap Save to record the points.

Repeat across different scenes. Each session adds points rather than replacing them.

How the correction is fitted

Each zone gets its own linear regression, computed once that zone has three or more data points. Zones that do not yet have three interpolate a correction from their nearest calibrated neighbours, so the curve stays continuous rather than stepping at the gaps.

Two things follow from that, and they are the whole practical advice:

• Calibrate across a range of lighting. Three points all taken in the same bright daylight describe one operating condition, not a line.
• Use custom spots to reach the outer zones. The automatic three cluster around VII, V and III. Zones 1, 2, 9 and 10 only get data if you place spots there yourself and tell the app which zone they are.