Image: Apple
Optimize iPhone Storage can free space without requiring you to delete your photo library, but it changes which versions of your photos are kept locally on the device.
With iCloud Photos enabled, full-resolution photos and videos are stored in iCloud. When Optimize iPhone Storage is selected, the iPhone can replace some local originals with smaller, space-saving versions when storage is needed.
The full-resolution original remains available from iCloud and can be downloaded when you open, edit or share the photo, provided the iPhone has an internet connection.
Optimization is managed automatically. iOS decides when local space needs to be recovered rather than immediately replacing every original the moment the setting is enabled.
This also means Optimize iPhone Storage is different from deleting photos. Deleting an item from an iCloud Photos library removes it across devices, while optimization keeps the photo in the library and changes only how it is stored locally.
You can find the option in Settings under your Apple Account’s iCloud Photos settings. Users with plenty of free device storage can instead choose to keep full-resolution originals downloaded to the iPhone.
For people with large photo libraries and smaller-capacity iPhones, optimization can substantially reduce local storage use while preserving access to the complete library.