Graduated ND filters are used by landscape photographers and have been for many years. They let the photographer capture the equivalent of an HDR image in a single photo. So what are they? They’re a piece of glass that starts out very dark at the top to cut down light gathering abilities while the bottom clear. The light transmission becomes greater and greater as you get closer to the bottom. When placed in front of your lens, you can place the top part of the filter over the sky to cut down the light coming in and therefore clearly expose for the shadows.
The most obvious examples of their use are for horizon photos and images shot during the golden hour. When combined with a low ISO and a narrow aperture, they can also allow the photographer to capture long and dreamy exposures. Way before the age of digital photography, film photographers would use graduated ND filters before doing any sort of dodging and burning in the darkroom.
PhotoPlus Mag did a video a while back about how to use a graduated ND filter that does an excellent job of explaining how to use them (sans the slightly boring host). They even show the typical high contrast example where the scene has deep shadows and very bright highlights. In modern day, the best digital camera sensors may probably be able to capture these scenes without needing a filter. However, you’ll need to have a very flat color profile and do a lot of post-production work.
The video is after the jump.