The WonderPana FreeArc provides the freedom to use and rotate 66 and 145 filters on your 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 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. The WonderPana 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; once attached the photographer can easily slide in our 6.6" square filters (among the largest and most durable in the business). 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. Fotodiox Pro ND Filters are true neutral density filters designed to darken the image. They feature an even, neutral gray color for creative photography applications. The ND16 filter is a 4 stop value which allows you to slow down the shutter speed or use a larger aperture in bright light conditions. The Grad ND darkens the sky while leaving the foreground unaffected, perfect for shooting sunset or sunrises where the sky is blown out and the ground is dark. These filters combined are a perfect combination for times when you need to darken the overall scene yet darken the sky even more so, such as beach sunsets for example. Kit Includes: FreeArc Rotating Core Unit 145mm Neutral Density 16 (4-Stop) Filter WonderPana 66 - 6.6" Wide Upgrade Bracket 6.6x8.5" Grad ND 0.6 Soft Edge Filter 145mm Heavy Duty Inner Pinch Cap