The B+W ND Calculater App is available for download on both iOS and Android.The handling is very simple. Decide which shutter speed you think is best for the subject, measure the light with an exposure meter or with a camera without an ND filter and set the measured exposure time by turning the B + W logo App. Now you can select different filter densities and in the middle of the B + W logo is immediately shown what shutter speed the filter gives.