ND or Neutral density filters come in graduated and not, and the graduated ones come in a variety of types. Graduated ND filters are used to control exposure and are covered in their own article.
In this article I want to concentrate on ND filters that are not graduated, so have the same effect over the whole of the image. They are available both as round filters that screw directly onto the lens or via stepping rings, and as square filters that fit into filter holders.
There use is in extending the exposure time. There are a variety of uses for this including:-
In the circular fit you will find ND2, 4 and 8, but if you search further you will find some more specialist items like an ND64 (B&W) and ND400 (Hoya). In the square fit Cokin have ND2 ,4.8, 100, and NDX, the 2 highest in the P holder size.
You can add more than one together, to get lager values.
The effect on exposure times can be seen below, shown both as conventional stops and also in two examples showing the comparative exposure times:-
The NDX is strong enough to allow a long enough time exposure to be made in daylight that all moving items disappear, for example you could photograph a busy motorway and get a photograph of an empty one.
To get the maximum effect you set the ISO at the minimum value your camera has. If you are a Camera Images or Photography Skills client, and have done the Exposure Masterclass or similar, you will have a laminated 3 fold EV table. If you are a Camera Images client and have done any course you can get one by going to the Camera Images website, client support section. With this you can easily work out the ideal exposure and ND value that you require.
You can also work it out as in the example above, by creating a series yourself.
A polarizer, although giving a different effect in some cases to the ND filter, also has the effect of cutting the light by 1 stop, so similar to a ND2. ND graduated filters put against each other can also produce a similar effect to a solid ND.
If you are using a very long exposure in daylight, and using a filter holder rather than a screw on filter you may find its a good idea to put something around the edge of the lens and holder to stop stray light entering the sides.
Its also a good idea to use the eyepiece blocking gadget that came with your camera (D2X or D2H and D3, flip the eyepiece switch), as this stops light entering via the eyepiece and affecting the exposure reading.
With a Nikon D200, D300, D2 or D3 camera you can split the exposure into up to 10 shots that combine, with a D80 - 3 images. This option is on the shooting menu. The advantage of this is that it cuts down noise in the image associated with long exposures. The D200, D300, D2 and D3 cameras also have a timer feature that can allow a number of images to be taken at timed intervals of a set length and if you are able to combine this, you can produce other special effects, like dots or dashes of arcs from stars. The effects of combining multiple exposures is not quite the same as a longer exposure, see the article on double and multiple exposures for more on this.
You may find that finding dealers/stockists who hold the higher value ND filters difficult to find but many will order them for you.
See also: Filter Section for more articles.