If you've ever looked at a beautiful landscape photograph and wondered how it's been achieved, the answer probably lies with a neutral density filter.
Landscape photographers often use long exposures in their work, particularly to make water and clouds look soft and silky. However, it can be hard to achieve these results in daylight without overexposing the image. This is where a neutral density filter comes in useful.
The purpose of a neutral density (ND) filter is to reduce the amount of light that reaches the camera's sensor. The most popular ND filters reduce light by two (labeled as ND4x or 0.6 filters), three (ND8x or 0.9), or four (ND16x or 1.2) f-stops. It's unlikely that you'll find much use for more reduction than four f-stops, although some manufacturers make ND filters that reduce light by as many as six f-stops.
Using neutral density filters correctly can be a little tricky, so follow these tips.
- Use a tripod. Because you're aiming for long exposures, you'll need to use a tripod to keep your camera steady. If you're shooting someplace where using a tripod isn't possible, you can also try using a "pod", which is a beanbag with a tripod screw set into it. This will allow you to create a stable and soft surface for your camera wherever you are.
- Focusing. ND filters are very dense, meaning that the camera may be unable to autofocus properly. ND filters also often make the viewfinder too dark to focus manually. Obviously, the easiest way to work around this is to focus before attaching the filter. Alternatively, you can switch on Live View and focus the camera manually with the filter attached. With this method, it's best to use the magnified view to ensure accurate focus.
- Composing. As with focusing, the density of an ND filter can make it impossible to compose a scene accurately through the viewfinder. Again, your options are to compose the shot before attaching the filter or to use Live View.
- Noise. Long exposure images can be prone to excessive noise, even at low ISOs. Some cameras have a long exposure noise reduction system, where the camera takes a second "dark frame" image alongside the correctly exposed image. The camera then removes noise that is visible in this dark frame image, which will then be subtracted from the first image to leave a noise-free photo. The only downside is that the shot takes twice as long!