These filters come in rectangle sheets with gray color on the top portion to clear on the bottom portion of the filter, with an area of less dense gray in the middle. Their purpose is to hold back the brighter areas of an image like the sky, reducing the dynamic range of the image. Since the eye can see a considerably greater range of light than can be recorded by a digital sensor, or film, particularly slide film, these filters help reduce the range of light bringing detail to the shadow areas.