neutraldensity

Silent World is a project by Paris-based artists Lucie & Simon that shows post-apocalyptic views of famous locations around the world. All but one or two of the people in each location are removed from the scene. Rather than use multiple exposures and compositing the images to remove moving objects (e.g. people and cars), they chose to use a neutral density filter -- one that's normally used by NASA for analyzing stars -- in order to achieve extremely long exposure times during the day.

Runners in broad daylight aren't often captured as motion blurs, but that's exactly how Flickr user Justin (just big feet) shot the London Marathon. Just stick a neutral density filter or two onto your lens to restrict the amount of light entering your camera, allowing you to shoot at slower shutter speeds.

What is a variable neutral density filter?

The neutral density bit means it is a filter simply designed to block some of the light getting into a camera. The variable bit means it is variable - you can control the darkness of the filter just by twisting one part of it. A proper variable neutral density filter can cost £100 or more!