Genustech Lens Adapter Ring with Nuns Knickers

A fixed donut adaptor ring only covers one barrel diameter, and this one is built for lenses that do not fit that mold: a 124mm plastic ring seats into the Production Matte Box, and an elasticated black-out fabric sleeve, the accessory the industry has always called nun's knickers, wraps around the lens barrel to seal the gap regardless of its exact diameter.

That makes it the right choice for a zoom that changes length during a shot, or any lens whose barrel does not line up with a standard adapter ring size.