The WonderPana FreeArc provides the freedom to use and rotate 66 and 145 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). 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 features an even, neutral gray color for creative photography applications. The ND16 filter offers a +4 stop EV value which allows you to slow down the shutter speed or use a larger aperture in bright light conditions. The Graduated Neutral Density darkens the sky while leaving the foreground unaffected. These two filters combined are a perfect combination for times when you want to darken the overall scene and darken the sky even more so, such as beach or snow sunsets & sunrises for example. Fotodiox ND & Grad ND filters are a true neutral grey and do not color cast the images.