Neutral-density filter

The purpose of a standard photographic neutral-density filter is to reduce the amount of light entering the lens.

Doing so allows the photographer to select combinations of aperture, exposure time and sensor sensitivity that would otherwise produce overexposed pictures.

This is done to achieve effects such as a shallower depth of field or motion blur of a subject in a wider range of situations and atmospheric conditions.

ND filters can also be used to reduce the depth of field of an image (by allowing the use of a larger aperture) where otherwise not possible due to a maximal shutter speed limit.

Using a digital camera, the photographer can see the image right away and choose the best ND filter to use for the scene being captured by first knowing the best aperture to use for maximal sharpness desired.

Examples of this use include: Neutral-density filters are used to control exposure with photographic catadioptric lenses, since the use of a traditional iris diaphragm increases the ratio of the central obstruction found in those systems, leading to poor performance.

A neutral-density filter can increase the contrast and cut down the brightness, making these objects easier to view.

More significantly, most ND filters are only specified over the visible region of the spectrum and do not proportionally block all wavelengths of ultraviolet or infrared radiation.

The advantage of this approach is reduced bulk and expenses, but one drawback is a loss of image quality caused by both using two elements together and by combining two polarizing filters.

To create ethereal looking landscapes and seascapes with extremely blurred water or other motion, the use of multiple stacked ND filters might be required.

Typically these are rated at a 10-stop reduction, allowing very slow shutter speeds even in relatively bright conditions.

A demonstration of the effect of a neutral density filter. Note that the photograph was exposed for the view through the filter, and thus the remainder of the scene is overexposed. If the exposure had instead been set for the unfiltered background, it would appear properly exposed while the view through the filter would be dark.
A set of ND filters
Comparison of two pictures showing the result of using an ND filter at a landscape. The first one uses only a polarizer, and the second one a polarizer and a 1000× ND filter (ND3.0), which allowed the second shot to have a much longer exposure, smoothing any motion.
Neutral-density filters are often used to achieve motion-blur effects with slow shutter speeds.