The rear bayonet mount is the depth of several filter have to build up the rings and glue them together so they become a solid mount, so that the front element of your 20mm fd will clear the rear element of the .7 front elements on wider lenses bulge outwards, only you can make the call for how high you have to go, relative to the lenses you would use this adapter with (buy enough filter rings to accomplish this) also need a rail system to take a lens support, as t