Should I use an ND Filter?

It's likely that if you have spent much time following the photography community you've heard of ND (Neutral Density) Filters. ND filters can be used for several purposes and are designed to decrease the amount of light entering the lens of your camera. There are several reasons why you might want to reduce the amount of light entering your lens. First, you want to shoot at a wide aperture (small F number) in bright lighting conditions. Secondly, if you want to slow down the shutter speed of your lens without exceeding the diffraction limits* of your lens for long exposures during the day or early evening/morning.

To make life a little bit more difficult there are different kinds of ND filters. This is not intended to be confusing, but to allow you to compensate for different shooting conditions. ND filters usually come in 2 stop, 4 stop and 8 stop varieties. In addition there are rectangular ND filters which allow you to only have the ND effect on the parts of the frame you want.** There is another type, which I use, a variable ND filter. These filters can be turned, like a circular polarizing filter, to increase or decrease the amount of light that enters the camera. The Cameron Fader ND Filter I use varies between 2 stops and a maximum of 8 stops.

Now I'm going to show some examples of why you might want to use an ND filter. For this example I took a few shots at the same location to demonstrate one use of the filter. In this case using the light reduction from the filter to achieve a longer exposure. Both images below were taken at 35mm, F8 and ISO 200. I shot the images with my D700 + AF-S 24-70mm F2.8G mounted on a tripod, which was placed in the creek. I used mirror lockup and a cable release to insure I didn't create any unnecessary camera shake.

The first images below was taken without an ND Filter. Even though it was a cloudy day with no directly sunlight I was still having to use a shutter speed of 1/20s.

The second image, was taken with a Fader ND Filer, with the same settings. I was able to use a 5 second exposure thanks to the filter.

By using the ND filter I was able to create that lovely smooth creamy effect on the water. The only downside in this case is that the long exposure is a little softer overall, due to the water in the creek creating vibration. If I had take these shots from land, I expect that sharpness would be nearly identical.*** I'll have more (hopefully better) examples when I post a quick review of the filter itself.

* Diffraction usually starts to appear around F10 for crop sensor cameras and F16 for full frame cameras. 

** Rectangular ND filers require a filter holder. These filters are best used for landscape photography where the sky is brighter than the foreground, but these filters and holders can prove to be expensive for hobby photographers on a tight budget.
*** Use of filters can slightly reduce image sharpness