Understanding iCloud Photos
If you're new to iCloud or to Apple's Photos app, then you may want to refer to the the links at the end of this article.
First, what iCloud Photos is not: iCloud Photos does not add extra storage to your phone or computer. iCloud Photos is not a place to archive old pictures. iCloud is not like having an extra drive. iCloud Photos is not a backup service.
iCloud Photos offers two interconnected services--Synchronization and Optimized Storage.
Foremost, iCloud is a synchronization service. When you engage iCloud Photos, then the Library on that device is kept exactly the same as the iCloud Photos Library.
Photos on my Mac and Photos on my iPad— the Sidebars show the same folders and albums
So, for instance, if you take a picture with your iPhone, it is added to the iPhone's Photos Library, copied to iCloud Photos Library at iCloud.com, and then copied to the Photos Library on each of the other devices that you have connected. If you delete a picture on your Mac, then that picture is deleted at iCloud.com and on all the other devices. If you accidentally misspell your nephew’s name in face recognition, then it will be misspelled on every device and at iCloud. So you can't think of iCloud Photos as a full backup service, since it backs up mistakes as efficiently as it synchronizes pictures.
Secondly, with iCloud Photos you can use Optimize Storage on any of your connected devices. If you set a device to "Optimize Storage,"on your iPhone, for instance,
Choose Optimize or Download Originals
then Photos may store only smaller images on the phone and rely on iCloud to keep the full sized Original images. This is great, since with the lower resolution images you can scan through pictures very quickly, and they look great on the phone’s small screen. And if you want to edit a picture on an optimized device, Photos will reach out to iCloud to get a full sized image for you to work on. Your optimized Library may take up less than 10% of the space of a fully downloaded Library. But an optimized Library may be kept larger than that if the extra storage space is not needed.
Synchronizing with Mobile Devices
Keep in mind that iCloud is designed primarily for mobile devices, and since syncing uses the battery on an iPhone or iPad, and since lots of people are very anxious about battery drain, the mobile devices will avoid syncing when not plugged in. And since syncing uses processing power, and people hate slow-downs, the iPhone or iPad will avoid syncing while you are using it. And since syncing requires bandwidth, the iPhone and iPad will avoid syncing when the internet connection is weak. So, if you want faster syncing, then find good wifi, plug the device in, and leave it alone.
For a Mac, it’s much the same— syncing is faster when the Mac is plugged in, turned on, and running without having Photos in the foreground--you don't want it to think that you are using the Photos app. You may have to tell it not to shutdown when it’s not being used. Oddly, this setting switch is at SystemSettings>Battery>Options.
Macs are different from Mobile Devices
An iPhone has only one Photos Library. A Mac can have many Photos Libraries. The Library Chooser is accessed by closing Photos and option-clicking the Photos icon:
I have several Photos Libraries on my Mac
The Library that other apps, like Safari, Messages, Mail, etc, can access is called the System Library. The System Library is the only one that can synchronize with iCloud Photos. So on a Mac, we can have a Favorites Library with just the pictures we want to share with our phones or iPads, and we can keep archives of pictures that don't get synchronized with iCloud. We can even keep these extra Libraries on an external drive if we want.
Is iCloud Photos Synchronization for You?
iCloud Photos is a miracle for those of us who have multiple devices. There are no more wires to connect the devices. There’s no need for waiting to get enough pictures to make it worthwhile to sit down with the Mac and the iPhone to transfer pictures. There’s no wondering if all the pictures transferred. It’s all automatic!
I edit my pictures on my Mac, mostly, but I love having pictures and edits immediately available on my iPhone and on my iPad. The iPad is the perfect tool for showing pictures to friends and relatives — the screen is large enough to see the details, and the colors are accurate and gorgeous.
However, if you have only one device, you have only one iPhone or one iPad or just a Mac, then you are probably not interested in synchronizing your pictures with other devices.Then iCloud Photos may not be a good choice. Remember iCloud is not a backup service, it's a synchronization service. If you make mistakes in editing your pictures, those mistakes will be immediately copied to iCloud.
Is Optimized Storage for You?
Optimized storage is great for devices that have limited space! I use Optimize on my iPhone, and it’s especially good for me since I do most of my editing on the Mac. When you edit a picture and the large Original file is downloaded from iCloud Photos,
that file temporarily uses a bit more local space until you’re through, and that large file is purged. Since I don’t edit or print pictures from my phone, my Photos takes up less than 6% of the space that they occupy at iCloud. Of course, even if you do lots of editing on the phone or iPad, you can still save 85 or 90% of the space that pictures would otherwise take.
Perhaps the biggest downside of "Optimize" is that backups may not include all your pictures. I don't do "Optimize" on my Mac, because I want my backups to include all the pictures, all the metadata, and all the edits. If I were using Optimize Storage on the Mac, then the full sized image files would be stored at iCloud and not available for backup. When I bought my MacBook, I got it with a large enough hard drive that I could keep my Library on it.
Large hard drives are expensive on Macs, so not everyone can do that. Here is a plan for backing up Optimized Macs to an external hard drive when you have an Optimized Library:
Backup iCloud Photos from an Optimized Mac
A Photos Library can be on an External Drive
One way to deal with an ever expanding Photos Library is to keep the Library on an external drive. This works great on a Desktop Mac. The Drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. See this:
Move your Photos library to save space on your Mac
But on a Laptop? — don’t unplug that Photos Library!
Connecting an external drive to our MacBooks can be a handy way to have extra files available. But the thing is that an iCloud-connected Photos Library is constantly talking back and forth with iCloud over the internet, scanning catalogs and databases, making sure that the Libraries stay exactly the same. If we unplug the drive, interrupting that Library communication, then there’s a chance that Photos will get confused about what might be missing or different, and it will be forced to re-synchronize the Libraries’ databases. This can make the Library unavailable for a long time, like days for a large Library. For this reason it is best that we not use our Photos Libraries from an external drive on a laptop, where we’re likely to be plugging and unplugging it as we move around.
When you decide to turn on Optimize
If you decide to turn on Optimize to save storage space, Photos will start to remove the Original files. It might take a while. You may not notice.
When you decide to turn off Optimize
If you’ve used Optimize, and you want to change to DownLoad Originals, Photos will begin the download process. Awkwardly, it never tells you when it’s through! Editing, Printing, or Exporting forces Photos to download the original. So one way to force Photos to download the Original files on a Mac is that you can do a Print of your entire Library-- but you don't actually have to print! You select all, go through the print dialog, and Photos will download all the original files so that it can print them. Then you can cancel the print. You don't even have to be connected to a printer.
When you decide to turn off iCloud Photos
If you decide to turn off iCloud Photos, you will get this scary warning, here from an iPhone:
It warns: “Photos and videos that have been optimized to save space will be removed from this iPhone. The original full resolutions will still be available on other devices using iCloud Photos.”
Pictures that are optimized don’t have their full sized originals on hand, and they are pretty useless by themselves, so they will get erased when they are separated from their connections to iCloud. But that won’t affect the pictures at iCloud.com or on any other connected device.
We can choose “Download Photos & Videos,” but we need to be sure that there is room on the device to hold them. Presumably we’re using Optimize because the space is limited, so this could be a bad choice. If we're moving the Library to an external drive, for instance, this would be a good change.
What happens when you run out of Storage?
That would be bad..
"Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light." "Right. That's bad."
When we run out of storage on our device, then Photos may not be able to manipulate and maneuver files enough to transfer them to iCloud. The Photos app may not even be able to delete pictures. If we use Optimize, then deleting pictures may not make much of a dent, anyway. These articles have some ideas:
Free up storage space on Mac - Apple Support
Manage storage on iPhone - Apple Support
When we run out of storage at iCloud.com, iCloud Photos may not be able to accept pictures, anymore. As Photos and iCloud Photos continue to try to synchronize, information may be lost. This can happen gradually, so we may not notice until it's too late.
Manage your iCloud storage on your Apple device - Apple Support
And "full" doesn't mean "FULL." We should not be operating our devices that are more than 90% full. They need to have at least 10% of their storage available for manipulating files.
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If you’re new to using Apple’s Photos app, you should see this: Basic Concepts and Terms About Photos
Also:
"iCloud" isn't one thing!
In addition to iCloud Photos, iCloud offers two services independent of iCloud Photos:
1. iCloud Drive, see
2. iCloud Backup for Mobile Devices
Back up your iPhone or iPad to iCloud and see What does iCloud back up?