photos in iphone and computer

Are your iPhone and your Mac's Photos Library connected to the same iCloud Photo Library? If so, then any photo you delete from one will be deleted from both (and any other devices connected to the same library, such as an iPad). That is the purpose and function of iCloud Photo Library. If you need to save space on the iPhone, check out the Optimize iPhone Storage option. If you are trying to achieve some other effect, there may be other approaches, such as multiple libraries on the Mac (only one synced with the phone).

Sorry. I’m working from my phone and did not see your follow up questions at first.

You can move some of the photos from the library (on your Mac) that is connected to the iCloud Photo Library to another library. Since only one library per Mac user can be connected to iCloud Photo Library at a time, photos you remove from the synced library would be removed from the phone and be safe in the second library on your Mac without syncing back. Several of our veteran members recommend this strategy, even going so far as to keep most of their pictures in the non-synced library.

The duplicate merge does propagate across iCloud when I have used it. The feature is also in the current versions of iOS and iPadOS, and the result is visible even to older systems sharing the same iCloud Photo Library if you have any. I take that to mean it is a real change to the library rather than an illusion, but I have not used it enough to measure the space savings.

The best method depends on the end goal.

One possible end goal (the goal usually suggested here) would be one large offline library intended as an archive/master library (often on a separate volume) and one smaller active library with current projects and older favorites. One path to that would be to clone the current library to form the archive, then pare down the current library to form the smaller active library. The challenge for the future is making a schedule for yourself to update the archive and pare back the active library to keep it lean.

Some users find it better to spin off one library per major work project, major trip, or year. The clone-and-pare strategy works there only if your photo life is less continuous than mine.

Sorry. This topic needs a longer slice of my attention than I can give it this afternoon. I was hoping than one of our more senior members with more extensive experience on split libraries and pre-written reports might drop in and lend a hand.

I will try to get back to you in a few hours when I am at a computer keyboard and have fewer conflicting calls on my attention. My apologies.

You can disconnect your phone from iCloud Photo Library entirely, but then no photos at all will go from the phone to iCloud. The switch is all or nothing. Sorry.