Graduated neutral density 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-spot filters are ND graduated filters that are slightly opaque in the center and are clear at edges. These are used for special effects or to compensate for light falloff that is natural with large optics.
Graduated filters were used in the early twentieth century, for example for darkening skies in landscape photos. One advertised brand was called the "Lifa graduated filters for sky, cloud, and landscape photography. These "sky filters" were not necessarily neutral, since they were used for black-and-white photography, and sometimes used a yellow top half to darken blue skies more. In a discussion at the Royal Photographic Society in 1910, some "debatants" held the use of such "graduated color screens" to be quite limited.
Although its importance may have lessened with the advent of the modern digital darkroom, graduated ND filters are still an important tool for professionals because a digital sensor that is clipping ("blown out" or "washed out") captures no usable data in the clipped area, an effect which cannot be corrected with later processing because data has been lost.
The filter comes in many types, basically can be separated into 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.
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- L. R. Gwyer (1908). "The Prismatic Filter". The Photographic Times 40 (Photographic Times Publishing Assn.). p. 113.
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- Katrin Eismann, Sean Duggan, and Tim Grey (2003). Real World Digital Photography. Peachpit Press. ISBN 978-0-321-22372-2.
- Bryan F. Peterson (2004). Understanding Exposure: How to Shoot Great Photographs with a Film Or Digital Camera. Amphoto Books. ISBN 978-0-8174-6300-7.
- Outdoor Photographer How To: Graduated Neutral-Density Filters
- Jesse Speer Photography >> Making the Image : Digital Graduated ND
- An effective demonstration picture on flickr.com, photo by www.saraheinrichs.com
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