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 and the rest of the image. Cokin P121L Gradual Neutral Grey ND2 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 and lower part of a scene and the foreground/background. With progressive graduation from the bottom to the top of the filter the P121L is the same colour but with a lighter tone than the P121.