Variable ND filters

A variable ND is a grey filter whose strength you set on a ring, usually from two to five or from six to nine stops. One filter in the bag instead of three. In exchange it does things a fixed filter does not.

How it works

Inside sit two polarising filters, one behind the other. Turning the front one shifts their orientation against each other and less light gets through. It is the same effect a polariser uses to kill reflections – here it is used as a dimmer.

Everything else follows from that. A variable ND is technically not a grey filter but a pair of polarisers, so it removes reflections too, whether you wanted that or not.

The three known problems

• Cross pattern at the end stop: Turn it to the end of its range on a wide lens and a dark X appears across the frame. The fix is not a better filter but one stop less – on most models the last half turn is unusable.
• A colour cast that moves with the setting: On a fixed ND the cast is constant and can be corrected once. On a variable ND it changes as you turn, which makes a series inconsistent. For a sequence, pick a setting and leave it there.
• Softer corners: Two extra glass layers plus an air gap cost resolution, mostly at the edges. On cheap models you can see it; on expensive ones you can measure it.

When it is still the better choice

• Video: The shutter is fixed by the 180-degree rule and the light keeps changing – only the filter can give. That is the job variable NDs were built for.
• Travelling with one lens: If you are walking and counting grams, three filters plus step-up rings become one. For stills in the seconds range, a good model is good enough.
• When the strength is not clear in advance: At the water it is often the look on the screen that decides whether half a second or a full second works better. Turning a ring beats swapping glass.

When a fixed filter wins

On long exposures beyond a minute. Every optical weakness counts double there, the colour cast sits in the frame for a long time, and the cross pattern lives in exactly the part of the range you would need. For ten stops and more a single ND1000 is the clean answer.

A variable ND is also the wrong partner for stacking: two pairs of polarisers behind each other produce patterns nobody can predict. If you want to combine, use fixed filters.

For video a variable ND is the right tool, for short exposures a convenient one. For real long exposure it is neither. If you do both, a variable ND for everyday work and a fixed ND1000 for the tripod is the best split.

Every value is in the ND filter table, the FAQ covers common questions, and Which ND filter? takes you from the shot to the number of stops.