Foodie Pro 6.6"x8.5" filters are the strongest in the industry at 4mm thick. Foodie 6.6x8.5. Reverse grad filters are primarily used for light reduction in skylines of landscape shots. The reverse grad is a vital tool if you are looking to take sunrise or sunset shots. This is because the ND part of the grad is upside down, with the darkest area is near the center, graduating to approximately one f-stop lighter on the top part of the filter and the lower part of the filter is clear to allow balanced exposure with the foreground. This ensures the maximum light reduction when the sun is at or near the horizon. This filter is made by Foodie Pro specifically for the WonderPana 66 & Free Arc Systems.