TL;DR: VNDs are a clever solution, but their compromises are a particularly bad fit for the way action cameras are typically used.
VNDs are a great idea
Swapping out ND filters constantly can be cumbersome, especially when you need to get a shot quickly. Variable NDs solve a real problem. Instead of changing ND filters, you can rotate one filter and continuously adjust exposure. On conventional cameras, that can be extremely useful.
The issue isn't that variable ND filters are inherently bad. It's that action cameras tend to combine nearly all of the situations where their compromises become most visible.
What a variable ND actually is
A VND works differently than a fixed ND filter. It uses two polarizing elements stacked together.
A polarizer only passes light polarized in a certain direction. Reflections from things like water and glass are often partially polarized, so cinematographers and photographers use polarizers to reduce those reflections and glare.
Place another polarizer behind it and rotate one relative to the other - think of the shaker lid on a container of Parmesan cheese, with two layers of holes that can overlap:
- When they are aligned, more light passes
- As you rotate them apart, less light passes
- As they approach 90°, very little gets through
The relationship is described by Malus's Law, discovered in the early 1800s. The optical principle behind variable NDs is more than 200 years old.
Polarizers already have some well-known quirks even when you're only using one, but a variable ND filter uses two of them, which can compound the side effects.
1. #1 - Skies are not polarized evenly
Blue skylight is naturally polarized by scattering in the atmosphere, but it isn't polarized equally everywhere. The polarization is strongest roughly 90° away from the sun. This usually isn’t much of a problem with narrower lenses, since a normal lens sees a relatively small chunk of sky.
Because action cameras typically have a very wide field of view, they see an enormous portion of the sky at once. This means that one area of the sky can be strongly affected by the polarizer while another area isn't.
This results in parts of the sky that look darker than others, and large blue patches can look uneven with dark spots or gradients.
This is already a known issue with ordinary polarizers on wide-angle lenses.
With a normal polarizer, you rotate the filter specifically to control the polarization effect. With most VNDs, that same rotation is being used to control density, so you don't have independent control over both.
2. #2 - Camera movement makes it much more noticeable
With a landscape photograph, uneven polarization may simply sit there in one place. This can even be used creatively by rotating the polarizer to control how different parts of the scene are affected.
With an action camera, the camera is often constantly moving, and as its orientation changes, the relationship to the polarized sky and reflective surfaces changes too. This means that darker areas can move through the image, reflections can appear and disappear, and parts of the sky can darken and lighten.
Something that might be barely noticeable in a still image can become very distracting when you see it shifting from frame to frame.
Using an action camera for video doesn't just expose optical artifacts… it animates them!
#3 - The famous VND “X”
When you rotate a VND to make it darker, the system becomes increasingly sensitive to the angle at which light passes through it. With wide angle lenses used on most action cameras, light rays from the edge of the image are passing through the filter at significantly different angles than rays near the center, so they are not blocked in a uniform way.
This is where a “dark X” or cross pattern can appear. This isn't actually a defect or something limited to lower-quality VNDs. It's just physics..
To deal with this “defect”, manufacturers usually limit the usable rotation range to stop users from turning the filter far enough to reveal the worst of it.
#4 - Colour can change too
Real polarizers don't behave perfectly identically at every wavelength or angle. As a VND approaches its darkest settings, those small differences can become much more visible. The result can be blue, green or magenta colour casts, or uneven colour across the frame.
#5 - Polarizers can reveal some genuinely weird things
Polarizers can also reveal something called birefringence. Stress inside some glass and plastics changes how polarized light passes through them, which can produce strange rainbow bands and coloured patches. It's the reason you sometimes see weird patterns in car or airplane windows when you're wearing polarized sunglasses.
This principle is actually used scientifically to study stress inside transparent materials.
#6 - A mechanical problem: waterproofing
A fixed ND made for action cameras can be built as a single rigid, sealed assembly. A conventional VND needs optical elements that rotate relative to one another, which makes weather sealing much more difficult.
A lot of manufacturers describe their VND filters as waterproof, hydrophobic or water resistant, but that usually refers to just the surface coating on the glass. It doesn't necessarily mean the actual filter assembly is waterproof.
On a cinema or mirrorless camera, it might not be a big deal, but on a camera designed to go surfing, skiing or mountain biking, it's much more significant.
Manufacturers have ways of reducing these problems
To reduce these side effects, manufacturers can limit the rotation range, put a stop on the mechanism to avoid letting the filter reach the worst crossed-polarizer angles, and combine fixed ND with a smaller variable range.
One workaround is to combine a fixed ND with a lower-range VND, so the polarizers don't have to do all the work. The less work the polarizers have to do, the easier it becomes to avoid the worst VND artifacts. In other words, one way to make a VND behave better is... to use less of the VND!
What about electronic variable ND?
There are other ways to make an ND variable. Sony, for example, uses an electronic ND in some of its cinema cameras. Instead of rotating two conventional polarizers, it uses a liquid crystal layer combined with light-absorbing material and changes its transmission electrically. Other electronic ND technologies use different approaches.
They avoid some of the problems of a conventional VND, but they create a different engineering problem: you now need electronics, power and a variable optical element that remains uniform and colour neutral across its entire range. Sony has made this work extremely well in professional cameras, but packaging the same idea into something as small, rugged and weatherproof as an action camera is another challenge.
It's entirely possible that electronic ND will eventually become a very good fit for action cameras, but we aren’t quite there yet.
9. Fixed ND or variable ND?
None of this means VNDs are bad filters. A good VND can produce excellent images, and on a conventional camera the convenience can easily outweigh the compromises, especially when speed of adjustment is essential.
Action cameras are just a particularly tough use case. They combine extremely wide lenses, huge skies, reflective surfaces, constant camera movement and often rain, snow or water. A fixed ND is optically and mechanically much simpler, doesn't introduce polarization side effects of its own, and is much easier to seal.
For conventional cameras, the convenience of a VND often makes perfect sense. For action cameras, the simplicity of fixed ND often makes more sense.