For the maths genius amongst us it is quite a quick task to work out the increase in exposure time when you use an ND filter. For those slower mortals like myself, there's an App for that 
More importantly it is free.
Can be found at
http://itunes.apple.com/gb/app/longtime-exposure-calculator/id362297743?mt=8
More importantly it is free.
Can be found at
http://itunes.apple.com/gb/app/longtime-exposure-calculator/id362297743?mt=8
