Graduated neutral density filter

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A GND filter held up to the horizon. Note the poor contrast in the overexposed part of the sky not covered by the filter.
A GND filter held up to the horizon. Note the poor contrast in the overexposed part of the sky not covered by the filter.
The same image with washed-out (white) pixels colored red. Note that clouds that are washed out without the GND filter show detail behind the filter.
The same image with washed-out (white) pixels colored red. Note that clouds that are washed out without the GND filter show detail behind the filter.
The same image with a GND-like effect applied to the right side of the image using a computer. While the overall effect is similar, regions of the image that were washed out are not recovered.
The same image with a GND-like effect applied to the right side of the image using a computer. While the overall effect is similar, regions of the image that were washed out are not recovered.
A Cokin 2-stop (ND2) graduated ND filter.
A Cokin 2-stop (ND2) graduated ND filter.

A graduated neutral density filter, also known as a graduated ND filter, split neutral density filter, or just a graduated filter, is an optical filter that has a variable light transmission. Typically half of the filter is of neutral density which transitions, either abruptly or gradually, into the other half which is clear. It is used to bring an overly-bright part of a scene into the dynamic range of film or sensor. For example, it can be used to darken a bright sky so that both the sky and subject can be properly exposed. ND filters can come in a variety of shapes and sizes and densities and can be used in all types of photographic applications from still photography, motion photography and scientific applications. Center filters are ND graduated filters that are slightly opaque in the center and are clear at edges. These are used to compensate for light falloff that is natural with large optics.

Although its importance may have lessened with the advent of the modern digital darkroom, GND filters are still an important tool for professionals because a digital sensor that is "blown out" or "washed out" (i.e., reading maximum values―white) captures no usable data in the washed-out area, an effect which cannot be corrected with later processing.

The filter comes in many types, basically can be separated in to two categories.

  • Hard Edge
  • Soft Edge

A hard edge is used when there is an abrupt change in brightness. For example a field with a horizon to a bright sky. A soft edge is a wider smoother change from light to dark. This is used when the light and dark portions are not distinctly separated. For example a mountain and sky. A soft edge filter is less noticeable than a hard edge. It also has the benefit of making the sky more intense, darkening the sky the closer on the top. Below is a picture of what a hard edge and soft edge filter would look like.

Soft Edge

Hard Edge

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