ND Filter Exposure Calculator

Transmission ratio: an ND8 passes 1/8 of the light (3 stops, since 2³ = 8); ND64 is 6 stops; ND1000 is nominally 10 stops though 2^10 is 1024 — makers round the label, and the real density varies a third of a stop between brands. The other labeling system, optical density (0.9 = 3 stops, 3.0 = 10 stops), is just stops × 0.3. This tool uses honest exponent values, which is why ND400 shows 8.6 stops rather than a tidy integer.

Meter without the filter, then compute — that's this page's whole job: compose, focus (switch to manual focus so it can't hunt), note the metered shutter, mount the filter, dial the computed time in Bulb with a remote or the 2-second self-timer. Above 10 stops, autofocus and most meters fail outright; mirrorless cameras with brightened live view can sometimes compose through 6 stops, never 16. Check the histogram of the first frame and adjust by full stops — long-exposure failures are rarely subtle.

The artistic menu: 1/4–1 s streaks moving water while keeping texture; 2–10 s silks waterfalls and smooths small chop; 30 s–2 min flattens the sea to glass and smears clouds into brushstrokes; 4+ min erases moving pedestrians from architecture shots entirely. Working backward: pick the effect time, take this calculator's stop output, and choose the filter that bridges from your base meter reading — the reason landscape kits standardize on 6- and 10-stop filters.

Two separate physics taxes: cheap (and some expensive) ND glass transmits unevenly across the spectrum — the notorious 10-stop warm/magenta casts — correctable in RAW white balance or with a custom WB frame. Noise rises because sensor heat accumulates over minutes: amp glow and hot pixels appear, which long-exposure noise reduction cancels by shooting a matching dark frame (doubling total time). Cool the camera between frames, shoot RAW, and budget the LENR wait into blue-hour timing.