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The new Photos.app allows you to hold down option to open up other libraries, for example Aperture and iPhoto libraries.

How does iCloud Photo Library handle this? Does it work in a way like iTunes Match, where a 'central' library lives in the cloud, and various libraries add to it? Or will you have to overwrite the library if you open another.

This question stems from another question I had on how to import an Aperture library after setting up a new one, but is more tied to how iCloud Photo Library handles this more than the Photos app - and the workflow/system around this.

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Only one library can be the "system library", which in turn is used for iCloud Photo Library (if enabled), but you can have as many other libraries as you want.

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Yes - that's exactly it. The library stored in iCloud is the "truth" and whenever you opt in a device to the cloud, two things happen.

  1. It prepares to upload - trying to not have two exact duplicates get uploaded to the cloud.
  2. It downloads thumbnails for all cloud photos - trying to not have two exact duplicates in the final device version of the library.

Each device can opt to get full resolution images or to further optimize (resample/recompress) the original.

I say try since most people will find some photos they consider duplicates if they have long standing photo libraries, but it's actually pretty good in my experience. The wrinkle is that Macs can maintain as many Photos libraries as you wish, with only one tying into the cloud backed "truth".

You can work directly with that version of the "truth" Photos album through the Photos web app at https://www.icloud.com/#photos

The following information is synced through iCloud Photos:

  • Photo Metadata (keywords, favorite status, date, title, name, description geolocation and other EXIF metadata about exposure, ISO, digitizer, lens, flash, etc...)
  • Photo Albums

The following information is not synced through iCloud Photos:

  • Faces albums and databases

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