A variable neutral density (ND) lens filter controls the amount of light that enters a camera. Single variable ND filters adjust differing levels of light. This means you don’t have to swap individual filters to alter how much light you block out from your camera.
A variable ND lens filter blocks out light as a result of the way the filter is made. Variable ND filters consist of two circular, polarising layers of glass that are placed in opposition to each other. The inner layer screws onto the camera lens and stays fixed. The outer layer is attached to a ring at the front of the filter frame and can be rotated.
The inner polariser cuts out light in a single plane. As the outer polariser is rotated, it reduces an increasing amount of the available light, the nearer the front layer comes to being perpendicular to the inner layer. Essentially, by rotating the filter, the two polarising layers block out increasing amounts of light as they come into alignment. You simply twist the outer layer to increase or decrease the light exposure, until you achieve the desired effect.
A single, fixed ND filter reduces light by a set stop. But, with the variable ND filter, you can adjust the range of light exposure from around ND2 (1 f-stop) to ND400 (8 f-stops), depending on which brand of filter you choose.
The filter shows minimum and maximum markings on its frame, which give a guideline to the individual density ranges. However, these markings are not calibrated, so don’t refer to the actual ND settings, but, instead, serve as useful, index references points when you’re rotating the filter.
If you go beyond the minimum and maximum settings, however, the filters can start to interfere with each other. This creates an exposure variation pattern similar to a cross or X. This X will occur on any type of variable ND filter, no matter what the brand or its price tag.
In many ways, a variable ND filter works like a circular polarising filter, however, it is no substitute. A variable ND filter doesn’t have any impact on polarisation (unlike a circular polarising filter). Plus, it won’t affect colour balance. Basically, if you want to achieve a polarising effect, you need a polarising filter for this task.
Since you can select what range of light you want to allow to reach your camera lens, the variable ND lens filter offers great flexibility and convenience and is also easy to use. It saves you having to carry around separate filters for different light densities. Some photographers claim that it is easier to compose and focus a shot using a variable ND lens filter, particularly if you would have needed to use a very heavy ND filter such as a 1000x version, which can make seeing through the camera’s viewfinder quite tricky.
Photographers and cinematographers use variable ND lens filters to manipulate light levels, especially on a sunny day, to slow down shutter speeds to introduce motion or blur to clouds, water, traffic or people. You can also use the filter to achieve a wider aperture to separate objects from their background, giving you control over the depth of field of an image.
Once you understand how variable ND filters work, you’ll appreciate the importance of buying a good quality filter to achieve the best results possible.