with non reflex bolex you need the side viewfinder.
two knob : one for distance one for focal lens.
the camera have a tri lens turret.
up tele
center normal 25mm
down wide.
the critical focussing viewfinfer is up.
for tele , after focusing you must rotate the turret with the tele lens on the normal center position ,set the side viewfinder with the good distance (parallaxe) and focal, and use it for critcal composition, and after run
for the other ,it is the same,rotate to critical focussing the lens in use,et after come back to the taking place (camera window).
the upper viewfinder is only for critical focusing.
see a link with descrption of old bolex.
http://www.city-net.com/~fodder/bolex/history/