This has probably been asked before but I'm at work and time is a little limited for my poor Boolean search skills, so I apologise if this is repetition. A friend of mine has a 16GB iPhone (I know, I told him before he got it, but what can you do?) and was running out of space. He messed about and deleted a load of photos (that were already on his Mac thanks to Photostream and Aperture) and cleared off a surprising 5GB of "other" but wanted a way to keep his free space under control. He activated iCloud Photo Library and selected "optimise device storage". Neither of us are too sure how this is supposed to work, so I'll run a couple of ideas past you and hopefully I'll understand it better. As I understand it, the iCloud Photo Library will store full size photos and videos until and unless your device gets low on storage then will shuffle things about, reducing the amount of space it takes up. How low is low? 1GB free, 500MB? I'm assuming it will not allow it to get TOO low otherwise there wouldn't be room for new photos/videos/apps? Anyway you get the idea, basically does anyone outside Apple know how this darn thing is supposed to work? As a sidenote, I've noticed with my own iCloud Photo Library that the space taken up by photos and videos is different on all my devices and on iCloud. And not in the direction you might expect. My library on my phone is 740MB, the library stored on iCloud is 666MB. Any ideas how, if the same same content is everywhere, it's taking up different space?