Oil Pump - North Dakota - 21 July 2008 – Panasonic LUMIX DMC LX2 w/ND400 filter.

What is an “ND400” filter? It is a neutral density filter that cuts light by 400 times. I’m guessing that most of them are sold to astrophotographers shooting very bright objects like the sun. What they do is effectively reduce the speed of the film you’re using. If you’ve got ASA:400 film – or your digital camera set at ISO:400 – an ND400 makes it ASA:001 or ISO:001. Bright sunlight ASA:100 is usually F:16 at 1/100th second. With an ND400 filter ASA:100 film becomes ASA:0.25. The “Sunny 16 Rule” delivers an exposure of F:16 at 4-seconds. Trees blowing in the wind turn to green smears. Water loses its’ individual waves. A corner of a street with hundreds of people scurrying past suddenly becomes nearly vacant. People standing still remain – but everyone else turns to a grey mist. It’s a delightful effect. Convert this image to B&W – and it still works.