Making long exposures
Use ND filters to give you magical motion blur on sunlit days
In order to drag out the exposure for multiple seconds or minutes, you need to cut the amount of light coming into the camera. While you can set a narrower aperture to do this, that’s unlikely to give you the sort of ultra-slow shutter speeds you typically need to blur clouds moving across a landscape – especially if you’re shooting in the middle of the day. This is where a neutral-density filter comes in handy.
An ND filter is essentially a very dark piece of plastic or glass that reduces the amount of light entering the lens. There are various densities or strengths available, each measured by the ‘stops’ of light that they block. Each stop requires the duration of the exposure to be doubled. So a ‘naked’ exposure of 1/60 second becomes an exposure of 1/8 sec with a three-stop ND filter in place (the first stop is 1/60 sec to 1/30 sec, followed by 1/30 sec to 1/15 sec, and 1/15 sec to 1/8 sec).
Your camera can automatically take care of exposures up to 30 seconds long, but you’ll need to work out exposures longer than this. There are plenty of smartphone apps available that will spit out a suggested exposure once you’ve entered the current exposure and filter strength, though.