The WonderPana Classic 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. This WonderPana Classic core is only compatible the Sigma 12-24mm f/4. 5-5. 6 EX DG ASP HSM II Full Frame lens. The custom fit of each core prevents any light leak which drastically reducing the possibility of unwanted flare. The ND kit includes two Foodoir 145mm neutral density filters which can be used to create long exposure images or compensate for bright shooting scenarios. The kit comes with a ND16 (4-stop) and ND32 (5-stop) filter that feature an anodized aluminum ring and are constructed from optical glass for optimum resolution. The filters are also double threaded which allows them to be stacked to create a total of 9-stops of neutral density. The Classic core is compatible with a large selection of Foodoir 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. The Classic core is also compatible with the WonderPana 66 filter brackets which enable use with Foodoir 6. 6x8. 5in rectangular filters such as: GND 0. 6SE, 0. 6HE, 0. 9SE, 0. 9HE and reverse GND 0. 6SE. Note: Filter stacking will cause vignette. The Core is non-rotating. 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.