The download button opens the iTunes App Store, where you
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From Kenneth Wahl:
FilterCompute takes the guesswork out of estimating exposures with neutral density filters.Given an exposure without a filter and a filter adjustment factor, this calculator will allow you to select an fStop, exposure time, and an ISO that will provide a correct exposure with the filter. The calculator completely eliminates the trial and error that can be involved with calculating a correct exposure for a filter that is too dark to take a correct through-camera exposure. This free version computes exposure increments in 1.0 fStop intervals, and requires an exposure taken by an external camera or a light meter.