With the Exposure Guide for iPhone you can calibrate the darkening effect of your photographic neutral density filters. Based on the calibration the Exposure Guide calculates the correct exposure for your filters and gives you a timer to count down in seconds.
The Exposure Guide allows you to combine any or all of your filters in order calculate the right exposure for your filter combinations.
You do not need conversion tables for your filters anymore. Thru the calibration process you get exact customized conversion data for your photographic equipment which will result in correct exposures and better images.