Neutral Density Filter
A grey filter that reduces the amount of light entering the lens uniformly across all wavelengths, without altering colour.
Formula / rule
Stops = log₂(filter factor)
Measured in
stops (EV)
In depth
An ND filter is the only way to reach long shutter speeds or wide apertures in bright light once ISO is already at base. Density is quoted three ways — as stops, as an ND number such as ND1000, or as an optical density such as 3.0 — and cheap filters often fail the 'neutral' part, introducing a colour cast that has to be corrected in processing.
Examples in practice
A six-stop ND turns a 1/60 s exposure into one second, smoothing a river into silk in daylight.