I have old iPhone 5 S & New iPhone X, & I'm afraid to Optimize the photos on the old iPhone to free some space; yet to have that to result in Optimizing the photos too on the new iPhone X. Even if I'm already selecting the option 'download & keep Original' upon my new iPhone X
iPhone X, iOS 12
The Optimize setting only affects the device where you turn that setting on. It does not affect iCloud or any other device.
There is also more information about iCloud Photos at Set up and use iCloud Photos - Apple Support
The Optimize setting only affects the device where you turn that setting on. It does not affect iCloud or any other device.
There is also more information about iCloud Photos at Set up and use iCloud Photos - Apple Support
Keeping Original Photos Size
