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Video basics3 min read

LESSON 93

Variable ND for video

Keep the shutter, keep the aperture.

IN THIS LESSON

  1. Why video needs ND
  2. How a variable ND works
  3. Ratings and stops
  4. Practice: avoiding the X

Why video needs ND

A neutral-density filter cuts the amount of light entering the lens without changing its colour, ideally. Stills photographers use it to get long exposures. Video needs it for a different reason: the shutter is fixed by the frame rate, at around 1/50 for 25 fps, so on a sunny day you cannot simply use a faster shutter to control brightness. The only other ways are to close the aperture, which deepens focus and can soften the image through diffraction, or to lower ISO, which has a floor. ND lets you keep a wide aperture and a normal shutter outdoors.

How a variable ND works

A fixed ND filter has one strength. A variable ND is built from two polarising filters, at least one of which rotates. Turning the front element changes how much light passes, giving almost stepless control from one ring, so you do not need a box of separate filters for each lens size. The advantages are lower bulk and lower cost. The drawbacks are some loss of image quality, because two glass elements and two polarisers are in the path. Even fixed NDs are not perfectly neutral, and cheap ones can add colour casts.

Ratings and stops

ND filters are labelled in different ways. Some makers use ND2, ND4 and ND8, where each doubling is one stop; others write 0.3, 0.6 and 0.9 optical density, where each 0.3 is one stop; Leica writes 1x, 4x and 8x. The table converts between them. Many filters do not block infrared or ultraviolet in proportion, which can cause colour shifts and, much more seriously, means an ND alone is never safe for looking at the sun. When you stack filters, add the stops together rather than the factors: an ND4 and an ND8 together give five stops, not twelve.

Practice: avoiding the X

This is working practice. Variable NDs have a limit: turned too far, the two polarisers cross and a dark X or uneven patches appear, most visibly on wide lenses. Stay inside the marked range and check the frame at full size. Set white balance after fitting the filter, since even good ones shift colour slightly. Buy one for your largest lens thread and use step-up rings for smaller lenses. Keep a separate strong fixed ND for very bright scenes. A matte box with slide-in glass filters avoids the X entirely and suits bigger crews.

StopsND factorOptical density
1ND20.3
2ND40.6
3ND80.9
6ND641.8
10ND10003.0

Try it today

  1. 01Film a sunny scene at 25 fps, 1/50 and your lens's widest aperture.
  2. 02Watch it overexpose, then add the variable ND and turn until it is right.
  3. 03Turn the ring to its maximum on a wide lens and look for the X.
  4. 04Note the ring position where it starts, and mark it on the filter.
  5. 05Redo white balance with the filter on.