Using long exposures to blur moving water is a classic technique, but a similar technique can also be used to great effect to blur scudding clouds. In this tutorial we show you how to calculate exposures with your ND filter to blur moving clouds, giving more impact to your landscape photography.
The principal of using a long exposure to capture motion blur is the same for clouds as it is for water, but there are two important differences.
Firstly, the length of the exposure required to blur moving clouds is much longer: typically 30 seconds or more.
And secondly, the light levels are usually much brighter, making it difficult to achieve the slow shutter speeds necessary for a correct exposure.
This is where neutral density (ND) filters come in – they basically reduce the amount of light reaching the lens, allowing you to set longer and longer exposures.
How to blur moving clouds: 01 Choose a filter
ND filters range from weak to strong in terms of the amount of light they filter out – the stronger the filter, the longer the exposure you can set.
A 0.3ND filter will enable you to increase the exposure time by a stop (e.g. from 1/30 to 1/15 sec), but this is still too short to blur clouds, even if they’re moving fast.
To get exposure times as long as 30 seconds, you’ll need a high-density filter, which will enable you to extend exposure times by up to 10 stops (e.g. from 1/30 sec down to 30 seconds).
How to blur moving clouds: 02 Compose and calculate
The first thing you’ll notice is that high-density filters are so dark that they’re almost impossible to see through, so you’ll need to frame up your shot before fitting one to the front of your lens.
More of an issue, though, is working out the correct exposure time.
Most filters come with a conversion table to help you calculate the correct exposure, based on the exposure time without the filter fitted.
As mentioned, a shutter speed of 1/30 sec would become 30 secs when using a 10-stop ND filter.
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