How the ND filter calculator works
The doubling ladder
ND filters halve light per stop: ND8 (3 stops) is 8× dark, ND64 (6 stops) 64×, ND1000 (10 stops) 1,000×. The filtered exposure is the base multiplied by the factor — a 1/500s daylight frame through ND1000 becomes 2 seconds; through ND32000 (15 stops), a full minute.
The workflow that works
Compose and focus without the filter (the viewfinder goes black at ND1000), meter the base exposure, mount the filter, multiply, shoot, and bracket ±1 stop. Long exposures also amplify sensor heat noise and any light leak — the mirror-up/interval settings and a covered viewfinder are the craft details.
Choosing filter strength
3 stops for moving-water texture in shade; 6 for blurred crowds and waves; 10 for the classic silk-water-and-streaked-clouds daylight look; 15 for minute-long exposures and creative extremes. One good 6-stop and one 10-stop cover most work — the variable NDs trade convenience for image quality (x-pattern cross-polarization is real).