Instantly it tells you exactly how long to leave your shutter open, and it even has a built-in timer countdown clock. Features: - User friendly UI - Includes a countdown timer if exposure time is more than 60 seconds (bulb) - Option to dim the screen while countdown - Option to enable disable sound once timer reach - Neutral density or Optical density unit selection - Supports up to 20-stops of ND filter - Base Shutter speed from 1/8000 - 60 sec - Show Fractional transmittance data with selected ND filter Steps: 1 - How many stops your ND filter is (like a 3-stop, 4-stop, 10-stop, etc.) 2 - Base shutter speed - What your current shutter speed is (before you put your ND filter on your lens) 3 - Final time are calculate base on your input, you can press the start button sync with DSLR/DSLM camera shutter at the same time. And the timer will remind you once timer reach to release your DSLR/DSLM camera shutter. Thanks for your support and do visit nitrio.com for more apps for your iOS devices.