Fixed ND vs. Variable ND for Video

Whether you’re new to videography or a seasoned cinematographer, you’ve likely heard of a little videography accessory called the neutral density (ND) filter. In fact, some might even say that an ND filter is the most essential videography accessory you can buy. But, like most things, not all ND filters are created equally - and not all ND filters function the same way either. That’s right. There are actually two different types of ND filters: fixed ND filters and variable ND filters. While both of these ND filters are designed to block light from hitting your camera’s sensor, they’re both also constructed differently, work differently, and even behave differently. So, when it comes to shooting videos, which type of ND filter is actually best? Let’s find out.

But First, Why Do You Need an ND Filter For Video?

To achieve a natural motion blur when shooting videos, you need to follow the 180 degree rule, which means you need to set your camera’s shutter speed to double your chosen frame rate. So, if you’re filming at 24 frames per second, your shutter speed should be set to around 1/50.

The problem with the 180 degree rule is that, once you have your frame rate and shutter speed set, you can only use your aperture or ISO to keep the scene properly exposed. This is fairly simple when you’re filming indoors, but, when filming in bright sunlight outside with a wide open aperture of, say, f2.8, your ISO won’t be able to help you much. To keep your aperture wide open and maintain a shallow depth of field, you need to stop a certain amount of light from entering your camera so that your scene can be properly exposed. That’s where ND filters come in.

Think of ND filters as the “fourth” element of the exposure triangle. The standard exposure triangle is comprised of shutter speed, aperture and ISO. These three elements, in normal photography and videography circumstances, work together to help you achieve a proper exposure. When shooting videos using the 180 degree rule, an ND filter essentially replaces the shutter speed on the exposure triangle, providing you with another option for reducing the amount of light that reaches your sensor.

Related: 5 Tips for Creating Cinematic Travel Videos

The Difference Between a Fixed ND and a Variable ND

While both fixed ND Filters and Variable ND Filters stop a certain amount of light from reaching your camera’s sensor, providing you with more control over exposure when shooting videos, they both achieve this in different ways.

A fixed ND filter uses one piece of darkened glass to reduce a specific, “fixed” amount of light from entering your camera. For example, an ND8 filter provides 3 stops of light, while an ND64 provides 6 stops of light. This means that, to reduce different stops of light, you need to own multiple different fixed ND filters.

A variable ND filter, on the other hand, uses two pieces of polarizing glass to provide multiple different densities all in one filter. To achieve this, the back piece of polarizing glass is fixed to your camera’s lens when you screw it on, while the top piece rotates freely. When you rotate the top piece of glass, the density of the filter increases or decreases, depending on the direction that you turn it. This means that you can reduce multiple stops of light all in one filter. For example, some variable ND filters provide 1-5 stops of light.

Now, on paper (or on this blog), a variable ND filter might seem like the better option due to its versatility and the fact that you may only need to buy one variable ND instead of multiple fixed NDs. But, if it were that simple, there’d be no need for a blog article like this, and we’d all just go and buy one variable ND filter and be done with it. But, alas, it’s not quite that simple.

Fixed ND Filter: Pros and Cons

When you put a filter on the end of your lens, there’s always a chance that the colours in your video will shift a bit. This is known as a colour cast, and it can typically be corrected in post production.

Neutral density filters, however, are meant to be, well, neutral. And, for the most part, fixed neutral density filters are very neutral. This is due to their simple, single piece of glass construction, as the light entering your camera doesn’t need to pass through multiple different glass elements to reach the sensor. Cheaper neutral density filters are more prone to colour casts, however, because they tend to use lower quality glass. They also typically lack the special coatings that higher ND filters use to help avoid colour casting. If you’re able to shell out a bit of extra money on a higher quality set of fixed NDs, you can typically achieve perfect, or near perfect, colours right in camera.

Sounds perfect for shooting videos, right? Well, not so fast. The problem with fixed NDs is that, in constantly changing lighting conditions, you need to switch between different fixed ND filters of varying strengths to maintain the exact same camera settings throughout an entire shoot. In a controlled environment where you have a lot of time to set up each shot, this isn’t likely going to be a problem. But, if you’re someone who shoots a lot of travel videos outside where the sun can poke in and out of the clouds, you might find it cumbersome to switch your fixed ND filter constantly to maintain a good exposure. And, as special moments tend to happen quite quickly while travelling, you also run the risk of “missing the shot” when using fixed NDs, as it takes a bit of time to remove one fixed ND, put it safely away in your camera bag and then attach another one to your lens.

So, while fixed ND filters provide perfect (or near perfect) colours straight out of camera, they can really slow you down when shooting videos on the move, but is there actually a better solution?

Variable ND Filter: Pros and Cons

As mentioned above, variable ND filters use two pieces of polarizing glass stacked together, allowing you to dial in the exact amount of density required for your shot. This, of course, is highly convenient when shooting travel films, as you can simply adjust the filter’s density as the sun goes in and out of the clouds, leaving all of your other camera settings as is.

Sounds perfect right? Why would anyone even consider using a fixed ND filter when variable NDs provide that much convenience and ease of use? Well, like most things that claim to make life easier, there’s typically a trade off somewhere, and that trade off is with colour casting.

Because variable ND filters use two pieces of polarizing glass to block varying amount of light, they also tend to act like a circular polarizing filter a bit, eliminating reflections from your scene as you increase the density. This also means that, similar to using a circular polarizer, your position in relation to the sun will potentially affect the appearance of the sky within your video and create a colour cast.

In contrast to fixed ND filters, light also has to travel through two pieces of polarizing glass to reach your camera’s sensor, which, again, increases the risk of colour casting. Higher priced variable ND filters that use higher quality glass typically have less colour casting issues, but, due to the nature of their design, there is no way to avoid the potential polarization or colour casting issues that come with variable NDs. It’s, quite simply, the trade off for being able to adjust the amount of light reaching your camera’s sensor very quickly and easily.

Conclusion

So, is a fixed ND filter or a variable ND filter better for video? It depends. If you’re someone who absolutely must have perfect colours straight out of camera, or if you typically shoot in a controlled environment where you have lots of time in between shots, fixed ND filters might be for you. If you are more of a run-and-gun filmmaker, however, and shoot in constantly changing lighting conditions, the convenience of a variable ND filter may work better for you. If you do go this route, just remember that you’ll likely need to adjust the colour in post production, and you may also end up with some scenes that aren’t salvageable due to the potential polarizing effect in the sky.

If you are a run-and-gun filmmaker and the idea of colour casting and polarization scares you, you could also try just using a fixed ND8 (3 stop) filter only. In most bright conditions, and ND8 will block enough light to allow you to shoot at a reasonable aperture. However, as lighting conditions change, just know that you will need to change your aperture to compensate and maintain a good exposure. Shooting with one fixed ND filter does create a bit more convenience, as you don’t need to change fixed ND filters constantly, but, again, there is a trade off, and that trade off is control over aperture.

The best way to figure out what type of ND filter works best for you is to try them out in the field. If you can afford to buy both a set of fixed ND filters and a variable ND filter, give it a go. It never hurts to have more filter options in your kit. If you’re on more of a budget, however, try renting a few. Either way, both filters achieve the same light stopping effect, so pick the one that makes the most sense for your videography style and budget, and have fun filming!