Graduated neutral density filters, or graduated ND filters, are used by photographers to control very bright areas of a scene. A graduated filter is made of glass which is half dark and half clear with a soft or hard edge between the two parts. Graduated ND filters come in different density, typically from one to 10 f-stops.
Fix or hand-held the filter in front of your lens. Place the darker area over the part you wish to darken (i.e. bright sky). The clear part of the filter will keep its normal brightness. You’ll notice that once you have placed the filter in front of your lens, you’ll need to re-adjust the exposure. You can use a graduated filter to emphasize an area of your image by darkening a less important area, knowing that the viewer’s eyes are naturally attracted by lighter areas in a photograph.
Note: The transition between the dark and clear parts of a filter can create an unnatural line in your image so use the proper density filter.