Genustech GARD Donut Adaptor Ring

The GARD slots into the back clamp ring of a matte box and steps down to whatever lens adaptor ring is on the lens, closing the light gap that would otherwise sit between the two. Without it, a wide lens with a small adaptor ring leaves an opening straight into the back of the box, and that opening is where flare gets in.

It is machined from high grade Delrin, so it is rigid enough to hold shape in the clamp ring but soft enough not to mark the lens ring it sits against. An angled inner lip does the stepping, which is what lets one part cover the whole run of lens adaptor rings from 52 mm upward rather than needing a donut per size.