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Calibration
The readout says NO CAL. What does that mean?
No calibration offset is being applied. A phone differs from a reference meter by a fixed amount, usually between about two-thirds and two stops. Open Calibration, fill the frame with an evenly lit surface, dial in what a camera you trust reads, and save the point. The label then shows the offset in use, for example CAL +0.42.
My readings are consistently out by a stop or so.
That is what calibration corrects. No offsets ship for devices that have not been measured, because an invented number looks like a measurement.
The offset seems to change between sessions.
Check the lens identifier on the Calibration screen. Calibration is keyed to the device model and lens, so a lens change is the likely cause.
Can I move my calibration to another phone?
Calibration exports to JSON and imports back. Note that the offset belongs to the phone it was measured on, so a different handset needs its own.
Metering
Should I compare a spot reading against my camera's meter?
No — compare in AVG. A camera's built-in meter reads the whole frame, and so does AVG. SPOT and BOX read a part of it, which is the point of them.
Why does it only measure a 3:2 window?
That is the shape of a 35mm negative. Fixing it means aiming the phone and a reference camera the same way puts the same scene into both readings, so a calibration measured on one screen applies on another. A preview stretched to fill the screen would take in ceiling and floor that no 3:2 frame ever sees.
Which films have reciprocity corrections?
Only those whose manufacturer publishes the figure. Where none is published the app applies nothing rather than guessing, because a plausible wrong number gets trusted and costs a frame.
Permissions
Why does the app want the camera?
It measures light with it. Frames are read to work out brightness and are discarded at once; nothing is recorded, stored or sent. See the privacy policy.