According to the classic rules of landscape layout, the sky occupies 3/5 of the space above the line of horizon. In photography this is interpreted by major differences in luminosity between the sky & the rest of the image. Cokin P120 Gradual Neutral Grey ND1.7 P-Series graduated filter reduces the total amount of light reaching the film, but without affecting the colour balance. This filter will also re-balance the luminosity between the upper & lower part of a scene & the foreground/background.