ND Filters Explained: Stops, Long Exposures and Stacking

A neutral density (ND) filter is a piece of dark, colour-neutral glass that reduces the light reaching the sensor. It does not change colour or contrast - it simply buys you longer shutter speeds in bright conditions, so you can blur water, streak clouds, or show motion in daylight that would otherwise be far too bright.

Reading ND numbers

ND strength is measured in stops, and each stop doubles the exposure time. An ND8 is 3 stops (1/125 s becomes about 1/15 s); a 6-stop ND64 is popular for flowing water; a 10-stop ND1000 multiplies time by 1024, turning 1/125 s into roughly 8 seconds for glassy seascapes. Manufacturers label filters by ND number, optical density, or stops, so it helps to know all three.

Stacking and workflow

You can combine filters by adding their stops - an ND8 plus an ND64 gives 9 stops. Because strong filters make the viewfinder go dark, focus and compose first, then attach the filter and switch to manual focus. Always use a tripod and a remote or self-timer, since the whole point is a long, deliberate exposure.

Graduated vs solid

Solid NDs darken the whole frame for long exposures; graduated NDs darken only the sky to balance a bright horizon against a darker foreground. Many landscape photographers carry both.