Recently we explained how to use an ND grad filter to rectify murky foregrounds or over-exposed skies. This week we thought we’d show how to use a variable ND filter, which you can use to capture amazing motion-blur effects.
Unlike an ND grad, a variable ND filter reduces the light across the whole image, and you can twist it to control the amount of filtering – this is what variable means.
ND variable filters come in handy for slow water effects or adding motion blur to people, and they’re essential in bright conditions.
You could even keep a variable ND on your lens all the time, so long as you don’t mind losing a stop or two. Here’s how to use this versatile filter to get cool motion blur effects on people.
How to set up and use a variable ND filter
01 Set up your camera
Blurring people walking past a scene requires a slow shutter speed, so you’ll need to put your camera on a tripod. Then either use a cable release or the self-timer in order to avoid camera shake. You can also lock up the mirror on your camera to reduce shake even more. Your camera’s manual will explain how to do this.
02 Select your settings
Attach the variable ND filter to your lens. Try an aperture of f/11 and choose a shutter speed of 1/4 sec. The filter will enable this. Set a low ISO (a higher one or auto ISO will give a faster shutter speed and spoil the effect), and zoom in about halfway if you have a standard zoom lens. Variable NDs can cause banding if you shoot wide-angle.
03 Switch to manual focus
You will need to switch to manual focus, and focus where you want to stay sharp – a building as the background, for instance. If you can’t see anything because of the filter, focus using Live View and adjust the filter until you get the desired effect. Don’t rely on autofocus, as it can struggle to focus through the filter.
04 Block stray light
Once you are happy with the focus, it’s worth covering the eyepiece to stop stray light getting in (again your manual will explain how), or just cover with your other hand. Then, wait until people pass in front of the background and take the shot. Adjust the shutter speed if people are moving very quickly or slowly.
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