The rings have an inner diameter of 50mm, 60mm or 70mm to fit on 52mm/55mm, 62mm/67mm or 72mm/77mm lens diameters. The outer diameter is 120mm. They are made of soft black foam rubber, 3mm thick.

When you use your mattebox with rods support the rings are always a perfect fit. If you want to use for example the ARRI standard primes or other lenses without inner focusing with 80mm diameter, you should use the ring70, because the length of the lens is varying while focusing, but your matebox can´t move because it is fixed on the rods. And if you want to use still photography lenses like old manual focus lenses with smaller diameter, you should use the complete set for different diameters. You avoid light coming through the gap between the lens and the lensmount and reflections on dark filters show on your pictures dark parts.

If you clip-on the mattebox, you don´t need them, but even then they give you safety...