Cressi F-Dual MaskF Dual Mask The F Dual is a single lens mask with a unique assembly of lens and skirt without a frame. The mask design bonds a high grade silicone double feathered edge skirt directly to a single tempered glass lens. The silicone facial is assembled directly on the glass with no need for a frame. This system ensures good visibility in all directions. The low internal volume allows for easy clearing and the quick adjust strap buckles are molded