Camera filters: ND, polariser and more

Why use a filter at all?

Most of what a filter does could not be done later in editing. A filter changes the light itself before it hits the sensor: how much arrives, which reflections get through, or how bright one part of the frame is compared with another.

Software can fake some effects, but it cannot recover a reflection the polariser would have removed, or turn a 1/250 s exposure into a 30 s one. That is why landscape and video shooters still carry a few pieces of glass.

Protective UV or clear filters are a different story: digital sensors already block ultraviolet, so these mostly guard the front element against scratches and spray. A cheap one can add flare and soften the image, so if you use one, buy a good multi-coated one.

ND filters and their strength names

A neutral-density (ND) filter is a sunglass for the lens. It darkens the whole image evenly, ideally without changing colour, so you can use a slower shutter or a wider aperture than the light would otherwise allow.

The number after “ND” is the light reduction factor. Each doubling equals one stop: ND2 is 1 stop, ND4 is 2 stops, ND8 is 3 stops, ND64 is 6 stops and ND1000 is about 10 stops (1000 is close to 2¹⁰ = 1024).

Some makers use optical density instead: 0.3 means 1 stop, 0.9 means 3 stops and 3.0 means about 10 stops. Same filter, different label.

To find the new shutter speed, double the base time once per stop. A 1/30 s exposure becomes 1/15 s with ND2, about 1/4 s with ND8, and about 30 s with ND1000. The calculator below does this for you.

Long exposures: water and clouds

With a strong ND, moving things blur into smooth shapes while still things stay sharp. Waterfalls turn silky at about 1/2 s to 2 s; the sea flattens into mist at 10 to 30 s or more; clouds streak across the sky at 1 to several minutes on a windy day.

A tripod is essential, and a remote release or the self-timer avoids shaking the camera when you press the button. Focus and compose before screwing on a very dark filter, because autofocus and the viewfinder may struggle through it.

Cover the viewfinder eyepiece on a DSLR during long exposures; light leaking in from behind can fog the frame.

Video and the 180° shutter rule

In video the shutter speed controls motion blur in each frame, and viewers are used to a certain amount of it. The traditional guide, the 180° shutter rule, says to use a shutter speed of about 1 divided by twice the frame rate: 1/50 s for 24 or 25 fps, 1/60 s for 30 fps, 1/120 s for 60 fps.

On a sunny day that slow shutter would badly overexpose, even at the lowest ISO and a small aperture. A variable ND (two polarisers that rotate against each other, roughly 2 to 8 stops) lets you dial exposure smoothly while keeping the shutter fixed.

The rule is a convention, not a law. Faster shutters give a crisp, staccato look that some action scenes use on purpose.

Circular polariser: reflections and skies

A polariser passes light vibrating in one direction and blocks the rest. Light reflected from water, glass and wet leaves is strongly polarised, so rotating the filter can remove glare and let you see into a lake or through a shop window.

It also deepens blue sky, because skylight is polarised most strongly at about 90° to the sun. Point your index finger at the sun and stick your thumb out: the sky your thumb sweeps across is where the effect is strongest. Looking towards or away from the sun, it does almost nothing.

With wide-angle lenses, roughly 24 mm and wider on full frame, the frame covers so much sky that some of it is near 90° and some is not. The result is an uneven, dark band across the blue. Rotate the filter for less effect or crop wider skies carefully.

Expect to lose about 1 to 2 stops of light (approximate). Use a circular polariser rather than a linear one, because linear types can confuse autofocus and metering.

Graduated ND for horizons

A graduated ND (GND) is dark at the top and clear at the bottom. It holds back a bright sky so the land below is not lost in shadow, which was the classic sunset solution long before HDR.

Hard-edge grads suit flat horizons such as the sea; soft-edge grads suit hills and trees that break the horizon line; reverse grads are darkest in the middle for a sun sitting right on the horizon.

Anything that sticks up through the horizon, like a mountain or a building, will also be darkened. In those scenes, bracketing two exposures and blending them may look more natural.

Screw-in vs square systems, and colour casts

Screw-in filters thread onto the front of the lens. They are compact, sealed against stray light and cheap per piece, but you need one per thread size (or step-up rings), and graduated screw-ins cannot move the transition line.

Square or rectangular systems use a holder clipped onto an adapter ring. One set of 100 mm filters fits many lenses, you can slide a grad up and down and stack an ND and a grad together. They are bulkier and can let light leak in at the edges.

Strong NDs, especially cheaper ones, often add a colour cast: blue, magenta or warm brown. Shoot RAW and set a custom white balance, or correct it later. Very wide lenses with stacked filters may also show dark corners (vignetting).