ND filters
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Each stop halves the light. ND 0.9 is 3 stops, ND 1.8 is 6, ND 3.0 is 10.
Strong NDs block visible light much more than infrared, so the sensor sees a larger share of IR, which it records as red and magenta.
Why ND filters matter, and how they go wrong
Neutral density filters are grey glass that cut light without changing its colour, so you can keep a wide aperture or a cinematic shutter speed in bright sun. "Neutral" is the hard part: strong and cheap filters often aren't.
Stops and ND numbers
ND strength is written in stops or as optical density: 0.3 per stop. ND 0.9 cuts 3 stops (1/8 of the light), ND 1.8 cuts 6 (1/64), ND 3.0 cuts 10 (about 1/1000).
Why filmmakers need them
At 24 fps the 180° rule wants a 1/48 s shutter. In full sun at a wide aperture that can be 9 or 10 stops too much light; ND is the only way to keep both the shallow focus and the natural motion blur.
Colour cast
The dyes in budget filters don't absorb every colour equally, so strong ones tint the picture warm brown, green or magenta. The stronger the filter, the stronger the cast.
IR pollution
Many ND dyes block visible light but pass infrared. Behind a strong ND, IR becomes a big share of what reaches the sensor, and since the sensor's red channel picks it up, black fabrics turn maroon and foliage goes brownish. IR-cut or "IRND" filters fix it.
Variable ND
Two polarisers rotated against each other give adjustable density. Near their limit, polarisation varies across a wide-angle view and leaves a dark X across the frame.
Built-in ND
Many cinema cameras have internal ND wheels or electronic variable ND designed to stay neutral, which is a big reason they're favoured for documentary and run-and-gun work.