Middle-mouse button, the "\" key, or RS on controller will work. It will toggle between zoom levels. On Steam, you can rebind the key for zoom as you see fit!
In the next update, Zoom will respect the level of Zoom the user has specified more tightly, but it is not always going to be something you can count in certain scenarios like particular Quests or scripted events, and it might still have some oddities with local Co-op. We'll be looking to improve it best we can and will be eager to hear updated feedback afterwards.