The WonderPana FreeArc provides the freedom to use and rotate 6.6" and 145mm filters on your Wide and Ultra-Wide Angle Lenses. Because of the construction of these lenses, it"s normally impossible to install filters as there are no filter threads, in addition standard filter sizes are not large enough to adequately cover the lens"s wide angle view. The WonderPana FreeArc changes all that by providing a complete kit for mounting our collection of both round and square filters. The all-metal Core attaches directly onto the lens with a secure mounting system. With The Core in place, you can now mount any of Fotodiox"s new "slimline" 145mm round filters. The WonderPana FreeArc system is a perfect fit for panorama, nature, architectural, fashion and event photographers, as well as, analog and digital filmmakers. For even more filter flexibility, mount the WP66 square filter holders (these mount without tools on either side of The Core); once attached the photographer can easily slide in our 6.6” square filters (among the largest and most durable in the business - they are manufactured by a high-end optics manufacturer exclusively for Fotodiox). With the FreeArc, the square filter stage is rotatable 360 degrees. The Wonderpana FreeArc can accommodate either two square filters, or one round and one square simultaneously. Please Note: On the Zeiss 15mm lens, the WonderPana can hold either one round (145mm) filter -OR- one 6.6" rectangle or square filter. There is a step up ring included to allow 145mm round filters to mount properly with the correct clearance needed for the lens hood, It is recommended that this step up ring be removed before attaching a 6.6" filter to avoid scratching the filter when it is slide into place.