This is a unique extension tube with a helicon extension that can be continuously varied from 22mm to 68mm. This extension requires the lenses to sit inside the opening and therefore does not work with larger barreled lenses except at or near the full extension. Most of the lenses with the Bay VI filter mount will fit without problems. The 90mm APO Macro will work from about 45mm out to full travel.
It can be mounted onto other extension rings or the extension bellows.