Everything You Need To Know About Variable ND Filters

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New to variable ND filters? Here’s the lowdown on what they are and how they work.

Words and Photography by Urth HQ

A variable neutral density (ND) lens filter reduces the amount of light that enters a camera. While a fixed ND filter blocks out a fixed amount of light, variable ND filters allow you to adjust levels of light so you don’t have to keep swapping between individual filters.

A variable ND lens filter is very flexible as it lets you alter light exposure between around 1 f-stop (ND2) and 10 f-stops (ND1000) simply by rotating the outer element of the filter. And it does this without affecting the colour or quality of your images.  

There are plenty of situations where you might wish to use a variable ND lens filter, and they’re useful for just about any genre of photography. In particular, photographers who deal with changing light conditions, like in landscape or nature photography, find this filter a godsend.

Shot with a Canon 6D camera, a Canon 70-200mm f/2.8 lens at 70mm, and a Urth Variable ND8-128 filter at ND8.

How a variable ND filter works

Variable ND filters consist of two circular, polarizing layers of glass that are placed in opposition to each other. They are easily screwed onto a camera lens with the inner layer staying fixed as it is screwed onto the lens while the outer layer is attached to a ring at the front of the filter frame and can be rotated.

The inner polarizer cuts out light in a single plane. As the outer polarizer on the front element 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 polarizing 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.

How to use a variable ND filter in your photography

A single fixed ND filter reduces light by a set stop. But with a variable ND filter, you can adjust the range of light exposure from 1 f-stop of light (with an ND2) to 10 f-stops of light (with an ND1000) depending on which design and brand of filter you choose.

The filter shows the minimum and maximum markings on its frame which give a guideline to the individual density ranges. 

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 conditions. 

Everyone knows that empty space in your kit bag is worth its weight in gold, so these easy to use and multifunctional filters are definitely something worth considering. The handheld weight of your kit can really make the difference between a fun day out shooting and a gruelling time out with a sweaty back and aching shoulders.

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 strong ND filter such as an ND1000 version, which can make seeing through the camera’s viewfinder quite tricky given how much light it blocks. The last thing you want to happen on your shoot is not being able to properly see what you’re shooting, and whilst electronic viewfinders and backlit LED displays might come in handy it’s definitely something you should keep in mind.

When should I use a variable ND lens filter?

VARIABLE ND FILTERS HAVE MANY CREATIVE APPLICATIONS

By simply rotating the filter, you can slow down shutter speeds to create beautiful effects such as motion blur. You can also make something stand out from its background by widening apertures to produce a shallow depth of field. The creative applications are endless given the relative ease of use, and these filters are particularly great for taking shots of moving water, vehicles and people. 

You can easily stack ND filters to account for different levels of light but many photographers find that using a variable ND lens filter allows for easier experimentation. 

Using them with a digital camera allows you to see their results immediately. With a variable ND filter, you can adjust your exposure while you shoot, as the lighting changes in a scene with a technique commonly referred to as ‘riding iris’. You rotate the filter to change the exposure as the light alters, keeping the same f-stop and depth of field. 

They’re pretty straightforward to use and having just one filter to carry about offers convenience for travel and outdoor photographers. If you need to shoot on a bright day, where the sun keeps moving behind the clouds and back out again, this filter will be invaluable for adapting to changing light quickly and easily.

VARIABLE ND FILTERS ARE GREAT FOR FILM OR VIDEO

One scenario where variable ND filters come into their own is during film or video shooting. Since you have less flexibility to alter your settings in cinematography, this filter is really handy for allowing quick and precise exposure adjustments. Time is of the essence whenever filming, so it’s definitely worth exploring solutions that work smoothly and efficiently so that you get the best out of your filming time.

The video below was shot with an Urth Variable ND2-400 to keep light balanced:

VARIABLE ND FILTERS MEAN LESS CLEANING

Variable ND filters are very useful if your filters get dirty quickly, particularly if you’re into coastal photography, outdoor photography or travel photography.

With this filter, you only have to clean one filter instead of multiple fixed ND filters. This saves time, letting you fully concentrate on getting your exposure settings spot-on, just at the right crucial moment.

Things to keep in mind when using variable ND lens filters

KEEP AN EYE ON MINIMUM & MAXIMUM SETTINGS

If you go beyond the minimum and maximum settings 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, regardless of make, brand or price tag as the angle of light becomes too varied between the two layers of glass. To avoid this occurring, check to make sure that you’re within the minimum and maximum setting on the filter and don’t rotate the rings beyond their rating to keep within the density parameters.

BE AWARE OF COLOR CASTS & VIGNETTING

Another undesirable effect of using a variable ND lens filter, which is dependent on its quality, is that color casts in your image can sometimes occur. For this reason alone, always buy a high quality variable ND lens filter, and look for a thin one if you want to minimise the risk of vignetting on wide-angle lenses.

A VARIABLE ND FILTER DOESN’T PROVIDE POLARIZATION

In many ways a variable ND filter works like a circular polarizing filter, however, it is no substitute. A variable ND filter doesn’t have any impact on polarization (unlike a circular polarizing filter) and it won’t affect color balance. If you want to achieve a polarizing effect, you need a polarizing filter for this task or you can check out our polarizing ND filter. 

Once you understand how variable ND filters work, you’ll appreciate the importance of buying a high quality filter to achieve the best results possible.

Still wondering whether to buy a fixed ND or a variable ND filter? Read this article. 

Did you know Urth makes variable ND Filters that reforest the world? Five trees are planted in deforested areas for every filter purchased. Shop our range of variable ND filters here.

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