Cokin Z121M Colour graduated camera filter

Unlike human eyes, CCD or film have no emotions or imagination. They only see light as levels of
brightness. A large difference in brightness between sky and foreground is beyond their ability to see and record. This is where graduated neutral-density (ND) filters come in. By using one of these filters, you will be able to bring to CCD or film what you see with your eyes. Grad ND filters can’t create light but they can help you capture it properly.
Pro landscape photographers often use Grad NDs to darken a bright sky so that both the sky and subject can be properly exposed thus capturing greater details.
Use these filters when you need to control the contrast range in your landscape images to avoid blowing out highlights, like snow-covered mountains or dramatic skies. How do you know when you need one? With some experience, you get a pretty good idea of when the light range in a scene is beyond the ability of the CCD or film to record, but one sure way is to use your in-camera spot meter and evaluate the scene. Check the difference between the exposure needed for your foreground and the exposure needed for the background. If there’s no difference between the two, then you don’t need a filter. But if there is, say, a 2-stop difference, then you need a 2-stop filter (ND4 = ND0.6).