Variable ND Filters: How They Work and How to Choose One

Sooner or later every photographer and filmmaker runs into a variable ND filter: one piece of glass that replaces a bagful of fixed ND filters, and lets you dial the exposure down by turning a ring instead of swapping and re-composing.

It is a genuinely useful tool. It is also the filter where the gap between a good one and a cheap one is most brutal, because a variable ND does not fail quietly — it fails by putting a dark cross across your frame, or a colour shift through your whole edit. This page explains what it actually is, why those failures happen, and what separates a filter that survives real work from one that does not.

What is a variable ND filter?

A neutral density filter is a piece of dark glass that cuts the light reaching the sensor without changing the colour of it. A variable ND does the same job, but over a range: instead of a single fixed strength, one filter covers several stops, and you choose where you sit in that range by rotating it.

The practical difference matters more than it sounds. With a strong fixed ND you compose, then fit the filter, because you cannot see through it well enough to frame. With a variable ND you keep it on the lens, frame at its lightest setting, and rotate down to the exposure you want — without touching the shutter speed or the aperture. That is why it became the standard tool for video, where the shutter speed is locked by the frame rate and the aperture is locked by the look.

How does a variable ND filter work?

A variable ND is two polarising filters stacked and set in opposition. Rotating the front element changes the angle between them, and that angle is what controls how much light gets through.

The first polariser lets through only light vibrating in one plane. The second one then blocks a share of what is left, depending on how far it is turned relative to the first. Line the two up and most of the light passes. Turn them towards ninety degrees to each other and almost nothing does. The transmission does not fall away in a straight line either — it follows the square of the cosine of the angle, which is why the last part of the range gets dense very quickly while the first part feels gentle.

Two consequences follow directly from that design, and they are not defects to be engineered away — they are the physics.

• It cannot start at zero. Even at its most open setting, you are still looking through two polarisers, so every variable ND has a minimum of roughly one to two stops. There is no such thing as a variable ND that does nothing at one end.
• It polarises. A variable ND is made of polarisers, so it behaves like one: reflections on water, glass and wet foliage change as you rotate, and skies can deepen. Sometimes that is a bonus, sometimes it is something to watch for on a wide lens.

The X pattern: what it is and why it happens

The X pattern — also called the cross effect, or cross-polarisation — is a dark X that spreads diagonally across the frame when a variable ND is pushed towards its densest settings. Once it appears, the shot is gone. It cannot be corrected in post, because the exposure is genuinely different in different parts of the frame.

Here is why it happens. The density of a variable ND depends on the angle between the two polarising elements — and the light arriving at the edge of the frame does not hit the glass at the same angle as the light arriving at the centre. On a normal or long lens that difference is tiny and invisible. On a wide lens the rays at the corners arrive at a steeply different angle, so the effective angle between the polarisers is not the same everywhere on the filter.

At mild settings this does not matter, because the transmission curve is fairly flat there. But as the two elements approach perpendicular, that same curve becomes extremely steep: a couple of degrees of difference stops being a rounding error and becomes a visible difference in brightness. The parts of the frame where the two polarisers are closest to crossed go almost black, and because of the geometry they form a band across the frame — and, at the extreme, a cross.

This is where the marketing on a spec sheet works directly against you. Many manufacturers push their variable ND range well past seven stops, up to nine or ten, because a bigger number sells better. Those extra stops are exactly the part of the range where the X appears. The filter technically reaches ten stops; it is simply not usable there on anything wide.

NiSi took the other decision and stops the ND-VARIO range at five stops, which keeps the whole range on the safe side of the curve, and then lets you go further by stacking a fixed ND on top rather than by twisting the polarisers closer to crossed. You get to nine stops or beyond without ever entering the region where the cross forms. It is a less impressive number on the box and a better filter to actually shoot with.

Colour cast, and what True Color means

The second thing that separates variable NDs is whether the image stays neutral as you rotate. A cheap one typically drifts — usually warm or magenta — and the drift changes with the density, which is the part that hurts. A constant cast you can white-balance out once. A cast that shifts every time you adjust the filter means every clip in the sequence needs its own correction, and matching them becomes the edit.

Two things drive it. The first is the polarising film: it is the component most often left unspecified on a spec sheet, precisely because it is where cost is cut. NiSi uses Nitto film, one of the recognised makers, and says so. The second is the glass itself — lens-grade optical glass, the same class of material used in the lens you are putting it on, rather than resin or unspecified sheet glass.

True Color is the NiSi range where colour neutrality across the full rotation is the design target rather than an afterthought. That is what the name refers to: not a coating you can add later, but the combination of film and glass chosen so that the image at five stops matches the image at one.

Vignetting on wide lenses

A variable ND contains two elements instead of one, so its frame is inherently deeper than a single fixed ND. Depth in front of a wide lens is what causes mechanical vignetting — the frame physically intruding into the corners of the image.

Two design choices decide whether it shows. The first is how thin the manufacturer can make the frame while keeping the rotation smooth and the alloy stable through temperature changes. The second is how you stack. Threading a fixed ND into the front of a screw-in variable ND adds the full depth of a second frame; magnetic filters that snap onto each other add far less, which is why the JetMag Pro system allows two stacked filters without vignetting at 15mm.

If you shoot at 16mm or wider and expect to stack, this is the specification that will actually affect your footage — more than the maximum number of stops on the box.

Variable ND or fixed ND: which do you need?

	Variable ND	Fixed ND
Strength	A range, set by rotating	One density, exact and repeatable
Optical purity	Two polarising elements in the path	A single piece of glass — the cleaner option
Best for	Video, changing light, run-and-gun, anything where you cannot stop to swap filters	Long exposures, controlled light, and any shot where absolute quality decides
Weather	Not weatherproof: two elements rotate against each other	Sealed single element — the safer choice in rain and spray
Watch out for	Cross effect and colour drift at the dense end	Carrying the right one, and refitting after composing

In practice most people end up with both, and the split is clean: the variable lives on the lens for video and for daylight work where the light keeps changing, and a fixed ND comes out for long exposures. If you are choosing a fixed density, we go through that separately in which ND filter to buy.

What makes a good variable ND

Beyond the two big failure modes above, five things separate filters that look identical in a product photo.

• The range it is honest about. A stated 1-5 stops that is clean throughout beats a stated 1-10 that is unusable past six.
• The polarising film. If the maker will not name it, that is information in itself.
• The frame. Thin enough to avoid vignetting, in an alloy that does not bind or loosen as the temperature swings — the reason a filter that turned smoothly in the shop can seize on a cold morning.
• The coatings. Nano-coating makes the surface water-repellent and resistant to skin oils, so it cleans in one wipe instead of smearing; anti-reflective coating controls the flare and ghosting that extra glass surfaces invite.
• The rotation. On video you adjust it mid-take. It has to move smoothly, without stiction and without wobbling the framing — which is why a grip or a knurled ring is a functional feature, not decoration.

Since a variable ND is built from polarisers, it is worth understanding how a polarising filter behaves as well — the two interact more than most people expect.

So — are variable ND filters any good?

Yes, with one honest caveat that we would rather state ourselves than have you discover later.

A variable ND is not a true ND filter — not ours, not anyone’s. It is always built from paired polarisers, and that means an extra element, extra surfaces and the behaviour of a polariser on top of the density. NiSi designs, manufactures and distributes its own filters, uses lens-grade optical glass and named polarising film, and keeps the range inside the region where the cross effect does not form. It is still, by construction, a different tool from a single-element fixed ND. Where the flexibility of a variable is not essential, we will tell you to use a real ND.

For video, that flexibility usually is essential, and the trade is worth it. For a two-minute exposure of a seascape at blue hour, it is not.

Which NiSi variable ND

Both NiSi variable NDs cover 1 to 5 stops, use True Color glass and Nitto film, and stop short of the cross effect. The difference is how they mount.

• The True Color ND-VARIO screws into the filter thread, in the usual sizes from 67 to 95mm. Simple, and the lower-cost way in.
• The JetMag Pro ND-VARIO is magnetic: one adapter ring per lens, then the filter snaps on and off in a movement. It also polarises on demand — rotate the whole filter in its ring and it behaves as a CPL as well — and it is the one to have if you intend to stack.

We compare the two properly — cost over time, vignetting, stacking, which one suits which kind of shooting — in a dedicated article.

Not sure which size or system you need?

Tell the Filter Finder what camera and lenses you shoot with and what you photograph, and it will work out the thread sizes, the adapter rings and the system that fits — with two alternatives, so you can see what you are choosing between.

Or describe your setup and what you are trying to shoot, and we will tell you what fits. We answer every message ourselves.