Given an exposure without a filter (fStop, ISO, exposure time) and a filter adjustment factor, this calculator will simplify finding a correct fStop, ISO and, exposure time that will provide a correct exposure with the filter. The calculator completely eliminates the guesswork out of calculating a correct exposure for a filter that is too dark to take a correct through-camera exposure.
The iPhone can be used as a light meter to take an exposure, eliminating the need to manually enter the exposure data.
If a graduated neutral density filter is needed to balance lighting (e.g., between sky and land), the iPhone can be used to find two exposures. The fStop difference is calculated automatically between the two exposures to assist in selecting the best filter to produce the intended image.
Exposure increments can be computed in 1.0, 1/3, or 1/4 fStop intervals. Exposure times can be entered as ratios (1/30), and in seconds (the default), minutes, hours, or days.