ND filter calculator
Pick your shutter speed and the filter, and the stretched exposure time appears right below. These are the same values the app uses: 49 shutter speeds and every ND filter from ND2 to ND8192.
hours : minutes : seconds : milliseconds
How the calculator works
Every stop doubles the exposure time, so new time = original time × 2stops. An ND1024 brings ten stops, which turns 1/125 s into 8.192 s. Stacking adds stops, not factors: ND8 plus ND64 is 3 + 6 = 9 stops, a factor of 512.
Past 30 seconds you need bulb mode
Most cameras stop at 30 seconds. Beyond that the exposure runs in bulb mode, and something has to keep count. The app has the timer built in, keeps it running in the background and speaks up at the end There is an open shutter at the other end of it.
The common filter strengths
What each strength does, what it is meant for and what to watch out for. One page each, with the calculator preset.
Not sure which strength yet? Which ND filter? works from the subject to the number of stops.
Rather look it up than calculate? The ND filter table lists stops, factor and exposure time, and the FAQ answers the eleven most common questions. The same eleven answers ship inside the app; nd-filter-expert.com is the site about it.
This page can be pre-filled: ?t=1/125&nd=10 opens the calculator at 1/125 s and ten stops.