ND filter calculator
From the unfiltered reading to the time with ND2…ND1000 (even two stacked), with a built-in timer for long exposures.
How to use it in the field
Compose and meter WITHOUT the filter (e.g. 1/125). Mount the filter, set the metered time and the filter here: the tool gives the new time (with ND1000: 8 seconds). Beyond 30 seconds switch to bulb and use the tool's timer: it beeps when done.
The three filter notations
The same filter goes by three names: factor (ND8), density (0.9) or stops (3). The rule: each stop halves the light, so ND8 = 2³ = 3 stops. Most used: ND64 (6 stops) for waterfalls, ND1000 (10 stops) for silky seas and moving clouds in daylight.
Stacking filters
Two filters multiply: ND8 + ND64 = factor 512 (9 stops). Beware though: every extra glass costs sharpness and can add colour casts and wide-angle vignetting — one filter of the right density beats two, when possible.
Details that ruin the shot
Cover the viewfinder (DSLRs leak light from behind), use self-timer or remote, switch stabilisation off on a tripod. Past a minute the sensor warms up: a few hot pixels are normal and clean up in post.