Variable ND filters for video: how to choose the right ND filter

The first summer I shot exteriors on a fast prime, I spent a week of evenings wondering why the footage felt wrong. Faces were sharp, colour was fine, and the waves at Spanish Banks moved in stiff little jerks, like a flip book with pages missing. I had pushed the shutter to 1/2000 of a second to stop the picture blowing out at f/1.8. That fixed the exposure and wrecked the motion. An ND filter would have solved it in the time it takes to screw one on, and I did not own one.

This guide covers what an ND filter does for video, how to read the numbers on the box, what goes wrong at the strong end of a variable ND, and how thread sizes and polarizers fit in. The three picks are variable NDs in three thread sizes.

Why video needs an ND filter

A neutral density filter cuts the light reaching the sensor without changing its colour. That is the whole job. It adds no contrast and does not rescue a white sky. What it buys you is the freedom to leave aperture and shutter where you want them when the sun hands you far too much light.

A stills photographer can answer bright light with a fast shutter. In video that is a poor trade, because shutter speed shapes motion. The long-standing convention is the 180-degree shutter: a shutter speed of about one over twice the frame rate. At 24 frames per second that is 1/48, which most cameras round to 1/50. At 30 it is 1/60, at 60 it is 1/120. The name comes from the spinning half-disc shutter of film cameras, open for half of every turn. The shutter angle calculator converts between angle and speed for any frame rate. (Despite the number, none of this has anything to do with the 180-degree rule about screen direction.)

At that speed each frame carries a little motion blur, and the blur is what makes movement read as natural. Push the shutter much faster and every frame freezes its own instant, so rain and gesturing hands go crisp and jittery. Sometimes that is a choice. More often it is an accident caused by the sun.

The arithmetic is short. The old sunny 16 rule says that in full sun at ISO 100, a correct exposure is about f/16 at 1/100. Open to f/2.8 and you let in five more stops; slow the shutter to 1/50 and that is a sixth. ISO is probably at its floor already, so the only way to take the light back is glass in front of the lens. Many cameras have a higher base ISO in log profiles, which widens the gap.

Stops and ND numbers, explained

ND filters are labelled in two or three ways. The commonest is the filter factor: ND2, ND8, ND64. The number tells you what the light is divided by. ND2 passes half the light, which is one stop. ND4 halves it again, two stops. Every doubling of the number adds one stop, so ND8 is three stops, ND16 four, ND32 five and ND64 six.

Numbers that are not powers of two cause the confusion. ND400 divides the light by 400. Eight doublings make 256 and nine make 512, so ND400 lands at about 8.6 stops. ND1000 is close to ten.

Cinema filters are usually marked in optical density instead, at 0.3 per stop: ND 0.3, 0.6, 0.9, 1.2. An ND 0.9 and an ND8 are the same strength under different names. Variable filters print a range on the box, such as ND2 to ND400, and many have a scale of dots or numbers on the ring. Treat that scale as a rough guide and trust your exposure tools.

For the sunny exterior above, six stops means ND64, or a wide-range variable ND turned to somewhere near its middle. A grey overcast day needs two or three stops less.

Fixed or variable ND, and what makes a good one

A fixed ND is one strength in one piece of glass. A good one stays very even in colour and across the frame, which is why planned productions rely on them, often as square filters in a matte box. The cost is speed. When the light changes you swap filters, and by then the moment has walked off.

A variable ND is two polarizing layers in one mount. Turning the front ring changes the angle between them and lets through more or less light, so you can ride exposure while a cloud passes or a subject walks out of shade, without cutting. That is why variable NDs, sometimes called faders, sit on so many documentary lenses. Some camcorders and cinema cameras have ND built in. Most mirrorless cameras do not, so the filter goes on the lens.

The best variable ND filter for you is the one whose range matches your light and whose thread fits your largest lens. A wide range is flexible but tempts you into its strongest, least clean end. A narrower range stays clean and runs out of strength on a beach at noon. After range, look for a ring that turns smoothly and a mount slim enough not to darken the corners of your widest lens.

The K&F Concept 77 mm variable ND runs from ND2 to ND400, about one to nine stops, which covers nearly any daylight exterior at a wide aperture. Use the last part of that range with the caution in the next section. The Tiffen 52 mm variable ND is a long-running design in a small thread size, two to eight stops, sized for compact lenses and small primes.

How I chose the filters in these bags: from the makers' published specifications and documentation, and from how long each design has been in use on small shoots. I have not tested these particular filters for this guide, so check how yours behaves on your own widest lens the day it arrives.

The X pattern and colour casts at the strong end

Two polarizers do not dim light evenly all the way down. As the layers approach a crossed position, the darkening turns patchy, and on wide lenses in particular a dark cross or X can appear across the frame, with lighter quarters between its arms. Clear sky shows it first. The strongest end of any variable ND is where the trouble starts. Some filters have hard stops that keep you out of that zone; others let you turn straight into it.

Colour is the second problem. Many variable NDs tint the image as they darken, often toward blue, magenta or a warm brown, and the tint can shift as the ring turns. Strong ND of any kind can also pass infrared light that the sensor records and your eye cannot see, which tends to show up as black fabric and dark foliage turning a reddish brown. Neither fault is a disaster if you know it is coming. Set the filter first and white balance second, never the other way round, and check the balance again after any big turn of the ring.

A few habits keep a variable ND honest:

1. Find the X before the shoot. Point your widest lens at a plain sky or a white wall, turn the ring slowly toward the strong end and note where the pattern appears. Mark that point on the ring with tape or a paint pen.
2. Work in the middle of the range. If you live at the strong end, you need a stronger filter, or a fixed ND behind the variable.
3. Close the aperture by a stop instead of turning the last few degrees. Going from f/2 to f/2.8 rarely hurts a shot as much as a cross over the sky.
4. Keep it clean. A filter adds two more glass surfaces in front of the lens, and low sun finds every fingerprint.

Thread sizes: 67 mm, 77 mm or 82 mm, and step-up rings

Screw-in filters fit the thread at the front of the lens. The size is marked on the barrel or inside the lens cap, usually beside a small circle with a line through it. Standard zooms commonly take 67, 72, 77 or 82 mm filters. Small primes and compact zooms are often somewhere between 49 and 58 mm.

Buy the filter for your largest lens and add step-up rings for the rest. A step-up ring screws into a smaller lens thread and offers a bigger one at the front, 67 to 77 mm for example, so one large filter fits every lens in the bag. A step-down ring, putting a small filter on a big lens, cuts into the corners of the frame on wide lenses. Stepping up has small costs too: lens hoods rarely fit over the ring, and a big filter makes a small lens front-heavy on a gimbal.

If your main zoom takes 82 mm, buy an 82 mm ND; neither of the larger filters in this guide is that size. If your biggest thread is 77 mm, the 77 mm K&F fits it directly and fits a 67 mm lens with a ring. For a set of compact primes that share a small thread, the 52 mm Tiffen keeps a small camera small.

ND and polarizer in one filter

A circular polarizer, usually shortened to CPL, is a different tool from an ND filter even though both darken the picture. An ND cuts all light evenly. A polarizer cuts light that reflection has polarized: glare on water, shine on leaves and wet roads, the reflection on a window seen at an angle, and some of the haze in a blue sky at right angles to the sun. Along the way it usually costs a stop or two of light. So CPL versus ND filter is not a choice between equals. The ND controls exposure; the polarizer controls reflections.

A variable ND already contains polarizing layers, but you cannot steer that effect separately from its strength. A combined filter gives you both. The K&F Concept 72 mm True Color variable ND with polarizer pairs a one to five stop variable ND (ND2 to ND32) with a polarizer in a single filter. In documentary that pairing earns its keep in two places. One is an exterior on a bright but not blazing day, where you want to take the glare off a river or a windscreen. The other is an interview near a window, where the polarizer can pull the reflection off the glass behind the subject, provided the camera sees that glass at an angle. Straight on, no polarizer can do much.

Polarization changes as the camera's angle to the sun changes, so a long pan across open sky can show the blue darkening and lightening across the frame. On moving shots, keep the effect gentle.

For a daylight exterior interview I would pair the combination filter with a collapsible reflector. The filter lets you shoot at a wide aperture in strong light, and the white or silver face of a 43-inch, 5-in-1 disc throws a little of that sun back into a face that has the sky behind it. The guide to shooting documentary interviews covers where the chair and the camera go.

Questions

What is an ND filter?

A neutral density filter is a dark, colour-neutral filter that reduces the light reaching the sensor. It changes exposure only, not colour or contrast, so you can keep a wide aperture and a normal video shutter speed in bright light.

What does an ND filter do for video?

It lets you keep the shutter near the 180-degree convention, about 1/50 at 24 or 25 frames per second, in daylight. Without it, bright scenes force a very fast shutter and choppy motion.

What strength of ND filter do I need?

Full sun at a wide aperture typically needs around six stops, ND64, and more if your camera's base ISO is high. Overcast days need two or three stops less. Choose a variable ND whose clean middle covers the strengths you use most; the far end of a wide range brings the X pattern closer.

CPL vs ND filter: which do I need?

For video in daylight the ND comes first, because it controls exposure. A circular polarizer controls reflections and glare. A combined variable ND and polarizer does both jobs in one filter.

Can one ND filter fit lenses with different thread sizes?

Yes. Buy the filter for your largest thread, 77 mm or 82 mm for many zooms, and use step-up rings on smaller lenses. Avoid step-down rings, which darken the corners on wide lenses.