This photo shows the 138mm to 100mm adapter ring (Arri part # 333060 and K2.47222.0) mounted on the rear of an Arri MB-20 Matte Box. This adapter has a three-fold purpose. On the inside of the adapter is a retaining ring which holds in any 138mm round filter. The rubber bellows ring fits around your 100mm lens barrel and keeps light from entering and refracting onto the lens. It also accepts step down rings for smaller outside diameter lenses including 95mm, 92mm, 90mm, 86mm, 85mm, and 80mm.
There are several adapter rings to choose from, but it is important that you know the O.D. (outside dimension) of your lens barrel. Contact the manufacturer if you have any doubt, and we'll also try to assist you.
CAT 333060 K2.47222.0