Lumen has no cloud of its own. Devices never talk to each other — they meet at a WebDAV folder, and whoever holds that folder is your cloud. That can be a NAS or a rented server, but the simplest version is a computer you already own.

Turning on Host mode makes the spare machine do both halves at once. It serves the library over your network, and it becomes the preview worker: for every photograph that lands on the server it does the slow work once — the full RAW demosaic — and publishes a smart preview and a thumbnail everything else can use.

That is why phones stay fast. Your phone never demosaics a 60-megapixel raw file; it opens a 2560-pixel preview the mini already made, and asks for a full-resolution render only when you export.

Nothing here is demanding. The machine's one hard requirement is that it is on when you want your photographs.

A Mac mini is the classic choice — silent, small, and it idles at a few watts. An old laptop with the lid shut works, as does a desktop PC. What matters more than speed is the disk: everything you shoot ends up there.

Lumen runs the whole imaging pipeline in the app itself, so there is nothing else to install — no database, no web server, no Docker.

A server that sleeps is a library that intermittently does not exist. Lumen has to be running, not merely installed — the previews, the full-resolution renders and the folder tidying all happen inside the app.

Two settings and you can forget the machine exists. Close the lid, put it on a shelf, and let it get on with it.

Install Lumen on the phone and open Settings → Your photo server. The fields are the same four: Server URL, Username, Password, and Library folder on the server — /photos.

That is the whole setup. The grid fills with thumbnails the host has already published; opening a photograph pulls its smart preview, and every slider works on that preview at full speed.

Two settings are worth a moment. Keep recent photos offline keeps the last 25, 50 or 100 photos on the phone — previews and, within the cache limit, their full-resolution renders — so a plane or a basement doesn't end your editing. Full-resolution cache limit decides how much room the big files may take; past it, the least recently used are dropped.

Everything so far works on your own network. To use it from a café, you need a way in — and there is a right answer and a wrong one.

Use Tailscale. Install it on the spare computer and on your phone, sign both into the same account, and they behave as though they were on the same network wherever they are. Nothing is exposed to the internet; the traffic is encrypted between your own devices.

Lumen helps: the Host panel spots Tailscale addresses and labels them. Use that URL in your phone's settings and it works at home and away, with no switching.

Open the library folder and you will find ordinary files in ordinary directories. No database, no opaque bundle. If Lumen vanished tomorrow your photographs would still be there, in dated folders, readable by anything.

Back up the whole folder — Time Machine, a second drive, rsync to a friend's house, whatever you already trust. The originals are the irreplaceable part; previews, thumbnails and proxies rebuild themselves from those.

Updates matter most on the host. It renders every preview and every full-resolution export the rest of your devices depend on, so an out-of-date host holds all of them back. Lumen does not update itself — install new builds by hand, and do the host at the same time as your phones, not months later.

Beyond that there is little to do. Downloaded originals are capped (20 GB by default, and never within 5 GB of filling the disk), proxies expire after a week, and the host tidies the folder structure as photographs arrive.