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

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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.