Split Neutral Density Filter (c):kodak.comLandscape scenes often contain more dynamic range than our cameras can capture. Even though digital cameras have increased their resolution tremendously, the dynamic range they can capture has not changed much over the years. Fuji has introduced the Super CCD SR to extend the dynamic range, but the technology is expensive and has not been adapted throughout the industry.
Split Neutral Density filters are one solution to this problem. They regulate the amount of light hitting the image sensor. A Split Neutral Density filter works like graded sunglasses for your camera. The gradient will make sure that there is a smooth transition between the shaded and the clear area.
Usually one would use these filters with the darker area covering the sky, to reduce the brightness levels there and reduce the dynamic range of the scene to something the camera can manage.
I do not use these filters anymore for their many limitations. The transition is usually on a straight line, which works well for reasonably flat landscapes with a clear transition between sky and land. In other cases, this is not the case. I would not use a split neutral density filter to shoot an alley, a tunnel, or a city skyline (a fact that bit me in this shot).
Another big drawback is that you need too many of those filters, depending on your focal length. Wide-angle shots require a much smoother transition, since your lens covers a much wider area. Telephoto lenses require filters with a much harder edge.
These days I usually take multiple exposures or try to recover as much dynamic range by processing my RAW files through HDR software.
If a scene has no movement and there is plenty of time to set up a tripod, I always prefer to take multiple exposures. During post processing, I can create any kind of transition with any shape I want. Even easier is to run these images through the same HDR program. This is what I did for this image of False Kiva.