The basic function of neutral density filter or ND filter is cutting down the light entering the camera without introducing any color shifts. They have many uses in photography. Neutral density filters can be categorized into two main types: graduated neutral density filters and regular neutral density filters. The former has graduated distribution of the light reduction levels across the filter. The latter has simple uniform light reduction across the whole filter.
Graduated neutral density filters
Landscape photographers love to use graduated ND filters to reduce the dynamic range of the scene so the camera can capture the entire scene without losing details in the shadows or highlights. I demonstrated the effect in an article High Dynamic Range Landscape Photography at DPTnT. Typical graduated neutral density filters have darker portion near the top edge and become lighter at the center. This is good for darkening the sky while properly exposing the foreground. The following image was taken with a 2-stop graduated neutral density filter.
There are also reverse graduated neutral density filters that have darker region at the center and lighter region at the edge, which is good for capturing sunset and sunrise where the brightest part of the scene is at the middle.
To make graduated neutral density filters easier and more flexible to use, they come in rectangular shapes and are used in filter holders that allow the filter to slide up and down so the dark-to-light transition can be exactly where it is needed.
Regular neutral density filters
The regular neutral density filters that have uniform light reduction across the whole frame are also useful. For example it can be used to reduce shutter speed. It is a well-known technique to use ND filters to slow the shutter speed down so moving waters or clouds can be rendered silky smooth. One example is shown in the photo below.
This typically requires a very long exposure that may not be possible during day time even at a very small aperture such as f/22. Even if it is possible, image quality at such small aperture may suffer due to diffraction. To make it possible to shoot at long exposure and/or with an aperture that is the best for sharpness, a neutral density filter comes in handy.
Another common application for neutral density filter is for outdoor flash photography. ND filter allows photographers to use a large aperture to obtain good background blur while staying below the camera’s sync speed. The alternative method is to use so-called high speed sync. However not all flashes are capable of high speed sync and the light loss may be too much to make the flash useful.
For people who shot videos using their DSLR cameras, a neutral density filter will allow them to shoot at a large aperture to get a nice shallow depth-of-field in bright daylight.
Variable neutral density filters
Not all shooting situations are the same, the required light reduction varies depending factors such as ambient light level, moving speed of the subject, and desired effect. Using conventional ND filter means the photographer will have to buy ND filters with different levels (stops) of light reduction. Stacking several ND filters together can reduce the number of ND filters needed but it adds to complexity and the chance for vignetting becomes higher. The solution is so-called variable neutral density filters.
Some say these are two stacked circular polarizers. Rotating one relative to another will lead to different levels of light reduction. The best known variable neutral density filter is the Singh-Ray Vari-ND filter. The Singh-Ray offers 2-8 stops variable light reduction. Singh-Ray Vari-ND filter is quite expensive. Fortunately there are quite some inexpensive ones available from eBay or amazon. The video below shows you how a cheap variable neutral density filter is used for capturing videos using a large aperture.
Keywords: Filters, Graduated neutral density filter, ND, Neutral Density, Variable ND Filter