This is a handy way to quickly set the infinity rail distance for various lenses. I set infinity at the '5' mm mark on the actus rear standard - gives me a lot of leeway instead of trying to use '0'. Each lens has a matching dowel which is marked for that lens. I've left the 120mm dowel in to show how it fits. For this to work the rail stop attachment must be removed. This will also work for all of the Arca bellows cameras.