Optimized Storage won't free storage on iPhone

Optimization should kick in when your storage starts getting low. With 20GB of free space, you have not yet reached that point.

Before you ask, it's not publicly know exactly what algorithm Apple uses to determine what's considered "low."

Yes that will work initially, you will start out with all of your photos in an optimised condition and with the library using up around 1/10th the space it uses at full resolution, however over time as you take new photos and open existing ones they will be stored at full resolution because of the dynamic nature of optimisation.

In my experience, as long as there is sufficient space, full-resolution pictures will always be downloaded to the device. Optimization is a dynamic process. As storage space is decreasing, more and more images will be replaced by lower-resolution files, not the other way around.

In other words, as you take more and more new pictures, more of the older pictures will be replaced by small thumbnails. Unless something goes wrong, you should not run out of empty storage space.

You can also turn iCloud Photo Library off and go back to using My Photo Stream to automatically sync new photos to your computer. Just make sure all your images have been imported to your computer. You can leave iCPL turned on in Photos on your Mac.

Rysz wrote:

In my experience, as long as there is sufficient space, full-resolution pictures will always be downloaded to the device.

That's not my experience and I've done it many times (I keep two phones just for testing this kind of stuff). When you first enable iCloud photo library on a device, assuming you also enable optimisation the photos are downloaded in low-resolution images. This is not simply apparent because my 50 GB library takes up just 5 GB but because each photo shows it's being downloaded when you open it for the first time.

Yes, the thumbnails are downloaded first, but over time, in the background, full-resolution images are then moved to the device (assuming there's enough free storage space and left powered overnight).

For example, since my new iPhone and iPad have much more storage than my previous models, now I never see the images needing to be downloaded in full-resolution when I click to view or edit them on those devices. They're already stored in full-resolution, as I can confirm by the size of the local library. The old devices, with identical settings, but limited storage, store a portion of the library in reduced-resolution and only the most-recently taken and viewed images at full-resolution.

Don't know why my experience is different than yours...

Your first statement said...

In my experience, as long as there is sufficient space, full-resolution pictures will always be downloaded to the device

Then you said...

Yes, the thumbnails are downloaded first, but over time, in the background, full-resolution images are then moved to the device

To me, they don't appear to be the same thing.

Additionally, I don't feel they revert to full resolution images in the background, but rather as and when they are opened. Again this is born out from my testing, the phone I use daily started out using 5.0 GB but has since expanded to 16.8 GB and I have 16.3 GB of free space on my phone. My test phone is still only using 6.3 GB of space for the same library and has 16.2 GB of free space available on it, my second test phone also uses 6.3 GB for the library and has 13.9 GB of free space.

IMO the reason the library on the phone I use has expanded so much is because I've opened photos I keep on it over time which I don't tend to do very much on my test phones.

FWIW, Apple description of how Optimized works seems to be clear that you start with full-resolution images, which then are reduced in size as needed.

But with respect, that's the reverse of what we are talking about, we are talking about optimised photos automatically being replaced by full resolution when there is plenty of room, the wording on your screen shot is talking about replacing full resolution images with low resolution images when there is a shortage of room.

[To me the two quotes you cite describe a sequence of events, not an either/or situaltion.]

Do you think the difference in behavior between what you're seeing and what I'm experiencing could be do to how Apple sets the threshold for min. free space? Meaning, +/- 16 GB on your devices vs. +/- 150 GB free on my brand new iPad Pro (after the full resolution library downloaded and Photos was barely used)?

Just curious to understand this...

I'm not sure but I can't see the purpose in it downloading full resolution versions if you aren't going to look at them, and what criteria would it use to choose which it did and it which it didn't if it's not guided by the ones you look at.

I would think that the purpose would be in having off-line and/or immediate access to photos not viewed recently and without anticipating the need in advance?

I guess without understanding how Apple's algorithm works, we'll never know.

Appreciate the discussion!

FWIW, Apple description of how Optimized works seems to be clear that you start with full-resolution images, which then are reduced in size as needed.

That probably won't work. As posted by Rysz, it will keep originals until space is needed.