Hold/Lock Reading button
Freeze the last reading so you can compose without re-metering.
Where to find it
Viewfinder Hold Reading button (bottom center, when a reading is active)
Summary
Tap or press the hold button to lock the most recent exposure reading. The app stops updating the meter so you can frame the shot without accidentally triggering a new reading.
Detail
How it works
What it does
After you take a meter reading, tap the Hold button to lock it. The exposure settings (aperture, shutter, EV) freeze and do not change even if you move the phone or tap the preview again. This is useful when you want to adjust composition without changing the meter reading.
When to use it
Typical workflow: point at the key shadow, tap to measure, tap Hold to lock, recompose, take the shot. Or in zone system work: measure zone III shadow, tap Hold, move to zone VIII highlight, measure (this adds a new spot to multi-spot mode), tap Hold again, and keep building your zone reading.
How to unlock
Tap the Hold button again (it now shows as active/highlighted) to unlock. Alternatively, switch metering modes or turn off Single Spot mode.
Multi-spot workflow
In multi-spot metering, hold works slightly differently: after the first spot is locked, each subsequent tap adds a new spot without changing the existing locked reading. So you can measure a shadow, lock it, measure a highlight (adding to the list), lock again, and so on.