Rubber Bellows allow lens movement laterally and horizontally as well as forward and back to better accommodate telescopic type lenses. Rubber Bellows also allow use of a drop in 138mm or 4.5" round
filter (depending on bellows size) to effectively add another filter stage to the Matte Box. With the exception of wide angle lenses, this stage is often used for polarizing filters, as it will rotate within the backplate of the Matte Box. Available in several sizes ( see below selections), they allow use of additional "screw in" lens adapter rings to preclude light from bouncing around in the Matte Box by creating a snug fit to the lens barrel.