Neutral Density Filters
A neutral density filter simply reduces the amount of light that reaches the camera lens. This allows you to use slower shutter speeds or a wider aperture than would be possible without using the filter. You therefore have more creative opportunities for your landscape photographs.
Neutral density filters, commonly referred to as ND filters, come in three strengths, giving 1, 2 and 3-stop reductions in light intensity. They can be either the square gelatin-type filter or the screw-in glass type.
Since neutral density filters are not graduated, their orientation in front of the lens is not important. The down-side to using the glass type is that, unless all your lenses are the same diameter, you'll need a separate filter for each one (i.e. more expense).
Like everything that has an impact on exposure, these filters are measured in stops, but manufacturers seem to have a hard time agreeing on how to label them. The table below shows you what typical labels are used for the different strength ND filters:
|What The Numbers Mean|
|ND Type||B+W, Cokin, Hoya||Lee, Tiffen|
|1-stop||ND2, ND2X||0.3 ND|
|2-stop||ND4, ND4X||0.6 ND|
|3-stop||ND8, ND8X||0.9 ND|
Why Would You Use One?
To Reduce the Light Intensity: Some digital cameras don't provide very small apertures with their built-in lenses. For example, the smallest apertures in the Nikon Coolpix 950, 990 and 995 cameras are f/11.4, f/11 and f/10.3. Their fastest shutter speeds are 1/750, 1/1000 and 1/2000 sec.
Occasionally, it is possible that a correct exposure cannot be achieved even with the smallest aperture and fastest shutter speed. In such cases, Neutral Density filters become useful, because they can reduce the intensity of the light so that a photo can be taken within the limit of your camera.
This is actually a common situation when film cameras and high speed films are used or you've set a high ISO speed on your digital camera.
To Use Slower Shutter Speeds: This might be because you're using a fast ISO speed film or digital sensor setting (say ISO 1000 or above) or you're trying to take a photo in very bright conditions. Your camera may not be able to take a properly exposed image because the shutter speeds aren't fast enough. For example, maybe for a correct exposure, a shutter speed of 1/4000 sec is needed but the shortest shutter speed on your camera is only 1/2000 sec. Using a neutral density filter will let you bring the scene within the capabilities of your camera.
Another use for slow shutter speeds is in photographing moving objects such as water or wind-blown foliage. It's simple to achieve these effects in low-light conditions - simply use a small aperture and you'll get corresponding shutter speeds of 1 second or longer. But what if you want to achieve those effects in very bright conditions? That's where the neutral density filter comes in. You can always combine different strength filters to get the amount of light reduction you need for the photograph you're trying to take.
The image on the right shows the effect of using a neutral density filter on the same scene as the left image
To Use Wider Apertures: Again, in low-light conditions, using a wide aperture is seldom a problem, particularly if you're trying to isolate a particular feature in a landscape using a shallow depth-of-field. In bright conditions this isn't possible. A low ISO film or digital sensor setting (e.g. ISO 100 or lower - the lower the ISO the slower the film/sensor reacts to light and the wider the apertures you can use) can help but that still might not let you use the apertures you actually want. This is where the neutral density filter comes in again, cutting down the amount of light getting to the lens and letting you use wider apertures as a result.