A Neutral density filter is a filter that is placed on the front of the lens, similar to any other filter, but it reduces the intensity of light at all wavelengths. What this allows the photographer to do is to set an F-stop, ISO, or shutter speed that would normally over explose the image given the existing light conditions.

One of the most frequent uses for them is when a photographer wants to achieve that ubiquitous ‘mist’ effect when photographing water in an image, such as sea and rivers. The best neutral density filter to use is the Big Lee Filter, otherwise known as the ‘Big Stopper’. Other uses of such filters are:

• Reducing the depth of field on a really bright day or brightly lit subject
• When using a flash on a camera in certain photographic conditions such as limiting exposure time
• Using a wider aperture to stay below the diffraction limit
• Reduce the visibilty of moving objects
• Add motion blur to objects, eg Race cars, horses etc
• Extend exposure times

There are various different types of Neutral density filters. For example:

• Gradulated Neutral density filter. The density of the filter changes across the surface of the filter, useful for applications where one particular part of the image is far brighter than the other. Moon shots at night for example
• Specialist neutral density filers

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In this video below Mike Browne discusses the aformentioned filter and explains how to use it