When using a matte box as clip-on, step down rings can be necessary, with or without thread. This ring is required when using a MB-215, 216, 255 & MB-256 matte box as a clip-on on a lens with a front lens diameter of 105 mm. This ring also allows the use of all 105 mm step down adapter rings with the MB-215, MB-216, MB-255 & MB-256.