Syncthing can do this and much more. Its the absolute best option to sync anything anywhere. It is very flexible, uses local connections, can only sync on wlan/certain wlans. The only thing that could be better is UX - for novel users its not so easy to understand. But people hanging out on HN should choose it as its just great!
I agree it's very good, and I use it for a lot of other folders eg notes, but I don't see how to use it as a backup tool for photos, without either using the option linked above or copying/hardlinking elsewhere on the NAS end. Unless there's a setting I'm missing that stops it from deleting on the NAS end?
EDIT: Don't get me wrong, it's my favourite file syncing tool, especially as one of the few open source syncing solutions. But I worry it always gets recommended as a backup tool and people don't realise it'll also sync deletions, so they'd suffer data loss.
