I got an Olympus Varimagni angle finder (the old OM system one, from the 70ies) to take measurements. I designed an adapter to mount it on the E-M1 family cameras.
Here you see the Olympus Varimagni (left) and the Olympus VA-1 (right), both with E-M1 family adapters.
More about the adapters on my Angle Finder page.