This 52mm Wide Angle Adapter Ring for 4 x 4" Filter Holder from Formatt Hitech is used to fit the filter holder and screw onto the desired lens, therefore getting the effect of the filter that was slid into the holder.
The adapter ring places the holder millimeters closer to the lens not to cause any vignetting on the wide-angle lens. Hitech's rings are milled from aluminum in the United Kingdom.
Milled aluminum ring
Places filter holder millimeters closer to the lens not to cause vignetting
Provides a secure attachment