The Tiffen 6 x 4" Blue 4 Hard Edge Graduated Filter is a hard-edged graduated color filter providing a bluish color across the image, giving a cooling effect for your photographs. The graduated filter design keeps a portion of the image neutral, resulting in a more realistic look while allowing for the sky or other bright areas to take on the color of the density. The 4 density provides less of a blue cast than the 5 density filter, but more than the 3 density.