How to Use a Variable ND Filter for Photography

VND filters don’t work the same way as typical ND filters, but that doesn’t mean they can’t be used for photography and long exposure. Let’s take a deep dive into how they work and how you can use them.

Variable ND filters, or VNDs, have always seemed like accessories with a lot of limitations for photography, especially for photographers who use filters to shoot long exposure images. Unlike other types of filters, variable NDs require physical adjustments by rotation to increase their effect. In this article, let’s talk about how VND filters work and how you can use them for different photography styles and genres.

What Is a Variable ND Filter and How Does It Work?

Variable ND filters, just like regular (constant) ND filters, work to reduce the light exposed to the camera’s sensor within a set exposure time. By doing so, one can adjust either the shutter speed/exposure time, aperture, or ISO to make up for the resulting darkening of the image. What makes variable neutral density filters special is that they can be adjusted in terms of how dark they make the image or how many stops of light they reduce the exposure by.

The reason why variable ND filters can be adjusted is that they reduce light exposure in a notably different way from traditional/constant ND filters. VNDs are made up of two layers of glass instead of the usual single layer that traditional ND filters have. This is because the filters are made up of two layers of polarizers that synergistically redirect and filter the light coming into the camera through polarization. Circular polarizers work by reducing the amount of polarized light, which is why a CPL can reduce glare and control reflections on various surfaces seen in the frame. Two layers of polarizing filters work by exponentially achieving that effect, virtually polarizing unpolarized light, and potentially cutting off the entry altogether.

What Can You Do With VND Filters?

Typically, any genre of photography that deals with abundant light that needs to be reduced to achieve either slower shutter speeds or larger apertures for visual effects. When shooting portraits outdoors in bright conditions, a variable ND filter can be used to reduce the light and enable shooting with larger apertures to achieve background blur. Alternatively, they can also be used to slow down the shutter speed when using flash to get into the maximum sync speed with the desired exposure settings.

Another possible use of variable ND filters would be in shooting sports and action. Sports generally require fast shutter speeds to capture fast-moving subjects, but there are creative ways to introduce motion blur into the mix. In very bright conditions, an ND filter or a variable ND is necessary to allow a slow shutter speed even with the other exposure settings maxed out.

Ironically, the kind of photography where VNDs have very limited usability is the one that uses ND filters the most—landscape photography, including architecture and other related workflows that require significantly longer exposures. There are ways to work around these limitations, but to understand them, we first need to understand why variable NDs have these limitations.

Limitations of Variable ND Filters

Because of the way variable ND filters achieve light reduction, there are also some limitations in certain use cases. In this case, one would see that VND filters are more comparable to circular polarizing filters than regular ND filters, especially when being used with ultra-wide angle lenses. When CPL filters are used with wide angle lenses, there is a possibility of excessive polarization, causing a dark line or band to appear in the resulting image. This dark band can be thought of as an overlap of the effects of the filter and can be reversed by simply pulling back on the filter’s effect.

Similarly, variable neutral density filters are limited by the appearance of what is commonly called over-polarization, cross-polarization, or cross artifact, wherein two intersecting bands appear across the frame, resembling an "X." Of course, the appearance of this makes the image unusable and the intended effect unachievable. The extent of the limitation is governed by three factors: focal length, aperture (mainly concerning depth of field), and the VND filter’s actual light reduction expressed in stops of light. These limitations and governing factors affect variable ND filters regardless of brand or glass composition.

To put it simply, the cross artifact becomes visible when the filter reaches a threshold. In general, it appears at around seven stops (ND128) and beyond. However, when used with longer focal lengths and larger apertures, the frame is focused on a fraction of the filter’s surface where the darkening is still relatively even, hence the lines are not visible. At seven stops, generally, any focal length beyond 70mm will not have any visible marking, and at 105mm, the full range of a 1.5-10-stop VND can be used without any artifacts.

Generally, the limitation occurs when using wider lenses with the VND close to the maximum effect. From personal testing, variable ND filters can be used up to six stops at a 16mm focal length, and anything beyond that will exhibit the cross-polarization artifact. That being said, the key to using variable ND filters is knowing the limitations and how to work around them with other filters in combination.

Working Around Limitations of VNDs

Using ND filters should always be guided by the intended effect, which is generally either reducing light to extend the exposure time or opening up the aperture in bright conditions. As mentioned previously, the limitations arise primarily when using wide angle lenses because the wide focal length limits the filter’s maximum setting.

For example, if shooting at 16mm and the filter can only be set up to six stops (ND64), another solid ND filter can be used in combination to push it past six stops, achieving an additive effect. For instance, if the VND is set to six stops and another four-stop ND filter is mounted, the combination achieves ten stops of light reduction, or 1/1024th of the original light.

Generally, most VND and ND filters can be stacked together using the filter threads however one convenient option that I’ve been using is the H&Y Revoring VND + CPL and the magnetic clip-on filters. Since I shoot wide angle perspectives often, I always consider the VND to be limited to six stops and stack either an ND32 or an ND400 magnetic clip-on to get a total of 10 or 14.5 stops, respectively. This allows me to shoot longer exposures (or sometimes with wide-open apertures) without any optical artifacts or color shifts in the frame.

Using Variable ND Filters for Long Exposure Photography

When using VNDs for landscape photography, architectural photography, and other related long exposure workflows, the optical limitations of the filter have significant implications, particularly during the day. With an absolute maximum of six stops for variable ND filters on wide angle lenses, using them at maximum effect is only feasible during twilight or around sunrise and sunset. However, in bright conditions, when attempting 30-second long exposures or more, maxing out at six stops will likely not be sufficient.

By using supplementary ND filters in combination with the VND, the effect can be increased, and the shutter speed can be prolonged significantly while still allowing for adjustment by rotating the filter. While this is not entirely the work of the variable ND alone, it still makes the stack of filters variable but with a higher minimum effect. If the filters being used are relatively thin, they can serve as a convenient, versatile on-the-go ND and CPL filter set in one.

All things considered, variable ND filters are just like other ND filters that can be used in combination with additional filters. The most important aspect is knowing what you intend to achieve and understanding how they work to overcome their limitations. Just like any other photography workflow, there are multiple ways to achieve the same effect, and variable ND filters are simply one of the more convenient options.