What it does and doesn’t do
This is the setting people misread, so it is worth being blunt about the three things it is not:
- It is not a delete. Nothing leaves your library. The original of every photo is still yours, in iCloud, at full resolution.
- It does not reduce your iCloud usage. Every original still counts against your plan — if anything, this setting is the reason the plan matters.
- It is not instant. iOS offloads originals when the device starts running short on space, on its own schedule. Switching it on when the phone is half empty may free nothing at all.
What it costs you in storage
On the phone, it can genuinely help: what stays behind is a device-sized version rather than the full original, and on a big library that difference is real. What you pay for it is that opening an old photo, editing it, or sharing it at full size means waiting for a download — and with no connection, you get the smaller version or nothing.
What you can do about it in iOS
The choice lives in Settings › Photos, under iCloud Photos:
- Optimize iPhone Storage — originals in iCloud, device-sized versions on the phone.
- Download and Keep Originals — every original on the phone as well. The right choice if you travel without signal, or if the library is small enough to fit.
Both need iCloud Photos switched on; without it, the setting has nothing to optimise. And neither one changes what a delete does — with iCloud Photos on, a photo you delete on the phone is deleted from iCloud too, and lands in Recently Deleted for 30 days.
When it stops being enough
Optimising is a way of fitting a large library onto a small phone. It works right up until the library outgrows the iCloud plan, and then the only remaining moves are to pay for more or to actually make the library smaller. Kept is for the second one, on the device, with every delete recoverable for 30 days.