Neutral density filter reducing light uniformly across all wavelengths — e.g. ND 0.9 always cuts 3 stops. Use: control shutter speed without changing aperture.
On set, you often need longer shutter speeds without the aperture being able to follow. This is exactly why you place a fixed density filter in front of the lens — a neutral density filter that blocks light uniformly across all color ranges. The result: constant, predictable light level reduction without color distortion.
The practice is simple. Your ND 0.9 (three stops) or ND 1.2 (four stops) works identically in sunlight and artificial light — always the same reduction factor. You set the aperture where you want it (f/2.8 for the depth of field you need), and the fixed density simply takes away light without altering the optics. This is the crucial difference from a variable ND screw filter: no color casts, no vignetting at extreme positions, no quality loss from glass distortion.
Practically on set, this means: If you want to shoot at 24fps and T/2 in midday sun (for motion blur in dialogue scenes), your open aperture alone isn't enough — too much light. Your fixed density filter (e.g., ND 0.6 to 1.2, depending on the sun's angle) cuts enough light so that the shutter speed remains at 180 degrees and the lens stays open. Result: natural motion blur, controlled depth of field, stable exposure for hours.
Quality depends on the filter optics. Good fixed density filters (optical glass, multi-coated) cost money — but they keep contrasts stable and don't distort. Cheap versions appear milky, creating halos around lights. On set, you notice the difference immediately in the viewfinder. The fact that fixed density filters, unlike variable NDs, do not cause polarization instability is an underestimated advantage when shooting near water or in reflective environments.
Classic mistake: Relying on the nominal stop number. On-site measurement with a light meter is mandatory — manufacturer specifications vary. Also: stacking multiple small fixed density filters instead of using one large one — this reduces glass loss and makes you more flexible in mixed lighting.