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

Have you ever wondered how pro photographers capture movement in their landscape shots to produce soft, blurry clouds and misty waterfalls? Are your long exposures just not delivering the same effect? Chances are that those pro 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.
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How and when to use ND filters (and what the numbers mean)