this aluminium wheel adapter screws into the aperture ring thread around the lens so you can set the aperture more easily-- its only for the 1960s 50mm F4 chrome schneider componon (which takes 29.5mm filters) --it could be machined out to fit some 30.5mm filter front lenses in the small 25mm thread fixing barrels--but currently will only fit the stated lens--but any questions please ask