How and when to use ND filters and what the numbers mean

Have you ever wondered how professional photographers capture movement in their landscapes to produce soft, blurry clouds and misty waterfalls? Are your long exposures just not delivering the same effect?

Chances are that those professional images have been shot using a neutral density filter (otherwise known as ND filters, and not to be confused with ND grads, which only darken part of the image).

These dark filters are designed to reduce the amount of light reaching the sensor in order to increase exposure times, without affecting the colour of the image. But how do you know when to use ND filters?

How to use ND filters

You’ll find ND filters in the kit bag of any professional landscape photographer. However, they tend to be less appreciated by amateurs. This may be due to the fact that, at first glance, they’re simply a plain, grey bit of glass!

They don’t radically affect the image that the camera captures, but merely slow down the amount of time it takes for the sensor to record the image.

But if used when elements of your scene are moving, such as water, clouds or even people, they open up a world of creative possibilities. Freeze a waterfall with a regular shutter speed and it looks static and rather dull; capture the water as a blur and it conveys a sense of movement.

ND filters give you the flexibility to set the aperture and shutter speed you want, rather than what the conditions dictate (find out some of the common mistakes at every shutter speed – and the best settings to use).

An ND filter can be used on a sunny day to slow things enough to create a sense of movement, but they’re even more effective around dawn or dusk, when they can turn an already-slow exposure into one several seconds long, enabling you, for example, to turn a surging tide into a gentle mist.

There are several different types of ND filter on the market. Circular threaded screw-in filters are the simplest to use, but have the disadvantage that stacking them together soon leads to vignetting issues.

Slot-in filters require you to first attach a filter holder to your lens via a ring adapter, then slot square or oblong filters into the holder – the chief advantage is that, once set up, it’s easy to swap filters, stack them or add different kinds of filter to the mix.

A more recent innovation are variable filters, which screw into the lens but have an adjustable outer ring, which you rotate to adjust the density depending on the light conditions and the effect you want.

When to use ND filters

For waterfalls you don’t necessarily need a very long exposure to capture motion blur, because the water is moving so rapidly, so a three-stop ND filter will work fine in the middle of the day.

However, if you want to achieve a similar effect with a seascape you’re looking at an exposure that lasts 30 seconds or more, as the sea and clouds aren’t moving as quickly.

With an ND filter

Without an ND filter

Shooting during the ‘golden hours’ at dawn or dusk will help, as the lower light levels will facilitate longer exposures – and of course the quality of the light will help to produce great images!

ND filters aren’t just for blurring the elements – you can use them to make moving people disappear! You’ll need a really long exposure of several minutes, but people walking through a scene will simply vanish – architecture photographers use this trick when shooting crowded tourist hotspots.

And it’s not just slower shutter speeds that can be obtained by using ND filters. If you’re shooting portraits on a bright, sunny day, for example, you may find you can’t shoot at wide apertures to obtain a shallow depth of field because it requires a shutter speed that exceeds the fastest available.

Adding an ND filter will enable you to select a wider aperture.

Making sense of the numbers on ND filters

Confusingly, different ND filter manufacturers use different scales to denote optical density. You can use the table below to ensure you get the right filter for your needs.

Things to look for in an ND filter

A number of factors affect the quality – and the cost – of a filter. Resin, glass and polyester can all be used to manufacture filters. Polyester is the cheapest option, but gives poorer quality results. A variety of advanced coatings are used to reduce lens flare and ghosting, while black almite frames and rimmed glass further reduce reflections. Thin, shallow-profile frames help to reduce vignetting.

Screw-in or slot-in?

Circular filters are small, lightweight and easy to fit, but lack versatility as ‘stacking’ soon leads to vignetting problems, and each filter only fits a specific diameter lens. Square slot-in filters require an adapter and holder, which means extra kit to carry around. They’re initially fiddlier to set up, but they’re more adaptable as you can easily stack ND filters for increased exposure times, or add other types of filters, such as ND grads or polariser.

Adaptor & step-up rings

You can easily fit a slot-in filter system to different lenses by purchasing inexpensive adaptor rings for each lens size you own. For screw-in filters, buy a filter to match the largest lens you own and buy step-up rings so you can fit it to smaller lenses.