The WonderPana FreeArc is a line of all metal filter holders for super-wide angle (SWA) and ultra-wide angle (UWA) lenses that make it impossible to use standard filters. With the FreeArc, the filter stage is rotatable 360� when paired with the WonderPana 66 filter holder and Fotodiox 6.6×8.5in rectangular filters: GND 0.6SE, 0.6HE, 0.9SE, 0.9HE, and reverse GND 0.6SE (66 bracket and rectangular filters sold separately). This action allows you to control the horizon line of Fotodiox rectangular GND filters without having to adjust your camera. The custom fit of each core prevents any light leak which drastically reducing the possibility of unwanted flare. This WonderPana FreeArc core is only compatible the Sigma 20mm f/1.4 DG HSM Art lens and can be used with Fotodiox’s large selection of 145mm filters. These include double threaded slim filters such as: ND4, ND8, ND16, ND32, ND100, MC-UV, CPL, MC-CPL. Other filters include single threaded ultra slim ND4, ND8, ND16, ND32, and ND100. Note: No filters are included with this kit. The rubber ‘nubs’ on the core do not serve a purpose other than in the manufacturing process, they may be discarded. All pictures are for illustration purposes only. Actual product may vary slightly due to ongoing improvements in design.