Jean-Louis Seguin

JK Camera used to sell a rack-over reflex front for the Bolex that did just what you're talking about. Of course, you could only see through the viewfinder when the camera was stopped and the mirror racked-over. Jean-Louis

You have to rotate the thread C-mount ring until the alignment is just right. Berthiot made a special tool that fits into the two little notches in the ring but if you don't have the tool, I'd suggest using a block of hard rubber to turn the ring. I've seen people use pliers but it's too easy to slip and scratch up the rear of the lens. When adjusted right, the zoom stick should be pointed straight down. Jean-Louis