ND filters for video

In photography the shutter speed is yours to choose. In video it is not: it should sit at roughly twice the frame rate, or movement looks wrong. That leaves the aperture when there is too much light – and if the aperture is to stay open, the grey filter.

The 180-degree rule

The rule comes from film cameras, where a half-open rotating shutter spun in front of the gate. Translated it says: shutter speed = 1 / (2 × frame rate). At 25 fps that is 1/50 s, at 30 fps 1/60 s, at 50 fps 1/100 s.

Stick to it and every movement gets exactly as much blur as the eye expects in a cinema. Shorter speeds make movement stutter, longer ones smear it. Both are devices you can choose; neither is a default.

Why it does not work without a filter

An overcast day at f/2.8 and ISO 100 asks for about 1/250 s. You need 1/50 s – so roughly two stops too much light. In sunshine that quickly becomes five or six.

ISO will not go lower, the shutter speed is fixed, and stopping down costs the shallow depth of field that was the reason for the fast lens. The filter is what is left.

Which strength when

• Overcast, f/2.8: two to three stops, so an ND8. The classic for grey days.
• Clear sky, in shade: four stops, ND16. The one stop an ND8 lacks as soon as the sun comes out.
• Changing light: five stops, ND32. Holds 1/50 s across sun and gaps in the cloud without a filter change.
• Full sun, wide open: six stops, ND64. The usual choice for drone video, where the aperture often cannot be changed at all.

What differs from photography

• Colour cast shows more: A still can be corrected on its own; a sequence has to hold throughout. Use a fixed white balance rather than auto, and shoot a test clip first with strong filters.
• A variable ND really earns its place here: If you track from sun into shade during a take, you cannot follow with the shutter. A variable ND is the only tool that can move with you.
• No tripod needed: The shutter stays at 1/50 s however strong the filter is. Shake is therefore no argument against more stops – unlike in photography.

For video, ND8 and ND16 are the more useful pair than ND8 and ND64: they cover an overcast sky and a clear day, which is where most footage is shot. Add an ND64 if you film wide open in the sun.

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.