In photography and optics, a neutral density filter or ND filter is a “grey” filter. An ideal neutral density filter reduces light of all wavelengths or colors equally. The purpose of standard photographic neutral density filters is to allow the photographer greater flexibility to change the aperture or exposure time, allowing for more control, particularly in extreme circumstances.

Let’s say that you’re taking a photo of some waterfalls in the middle of the day, and want to use a slow shutter speed to blur the water.  If it’s a bright sunny day, you won’t be able to.

There is a lower limit on shutter speed in the middle of the day – you can only slow it down so much before the photo becomes over-exposed.   If you put a neutral density filter on your lens (and reduce the amount of light) then you can slow the shutter speed down enough to get the right effect.  The more available light, the more powerful ND filter you must use.

When it is desirable to maintain a particular lens opening for sharpness or depth-of-field purposes, or simply to obtain proper exposure when confronted with too much light intensity, use a Neutral Density (ND) filter.  This will absorb light evenly throughout the visible spectrum, effectively altering exposure without requiring a change in lens opening and without introducing a color shift.

A Neutral Density filter reduces the amount of light passing through the camera lens without changing the color of the scene.  It is especially useful in bright light conditions to help prevent overexposure.  The neutral density filter also allows proper exposure at a wider lens opening for reduce depth-of-field to highlight a key subject by making the foreground and/or background out of focus.

Neutral Density filters are often ignored by photographers, but they have several uses and offer the possibility to achieve otherwise unachievable results. ND filters appear gray and reduce the amount of light reaching the film, yet they have no affect on color balance.

These filters are for all video, still, digital and film cameras. And  ND filters have four main uses:

First, to enable slow shutter speeds to be used, especially with fast films, to record movement in subjects such as waterfalls, clouds, cars, seas etc.

Second, to decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background.

Third, to decrease the effective ISO of high speed film (ie: above ISO400) and allow it to be used outdoors in bright situations.

Fourth, to allow video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes which would normally cause over-exposure.

A very popular tool in any landscape photographers kit is the graduated neutral density filter. When looking at landscape photography the average user doesn’t realise that many shots these days are composed utilising graduated neutral density filters.

For the somewhat more experienced photographer it may be assumed that the photograph was composed and later digitally edited in photoshop. This belief comes as a result of the difficulties which photographers have when composing a shot in camera.

Without a graduated neutral density filter, the user instead focuses on a point in the shot to gain a correct exposure. If you do this to the land of the shot then another for the sky, it will differ. Therefore when taking the photograph the sky may be over exposed while the land will be as hoped. This is where graduated neutral density filters come into the equation.