How to Calculate Your ND Filter Exposure Time

A 10-stop ND filter doesn’t cut your light by 90%, or even 99%. It cuts it to about 1/1,024th of what your sensor read before you screwed the filter on — which is why a bright-day exposure of 1/125s at f/11 turns into a shutter speed measured in seconds, not fractions.

Most photographers handle this with a printed chart taped inside a filter pouch, lining up a starting shutter speed against a filter strength to read off the new one. The chart works, but it only covers the combinations someone bothered to print. The stop-doubling math behind it works for every combination, and once you’ve done it twice you stop needing the chart.

The filter number is a stop count in disguise

ND filters get sold under two labels that mean the same thing. The “ND” number is a light-transmission factor: ND8 lets through 1/8th of the light, ND64 lets through 1/64th, ND1000 lets through roughly 1/1000th. The “stop” number just counts how many times you halved the light to get there — each stop is one halving, so:

• 1 stop = ND2 (1/2 the light)
• 3 stops = ND8 (1/8)
• 6 stops = ND64 (1/64) — the common “sunset” strength
• 10 stops = ND1000 (really 1/1,024, rounded down to a friendlier “1000” for the label)

That rounding is why the math and the sticker on the filter don’t quite agree. Use 1,024, not 1,000, if you want the exact shutter speed rather than one close enough to not matter.

The doubling math

Every stop of ND doubles your shutter speed, because that’s what a stop of light is: with aperture and ISO held constant, halving the light means doubling the time needed to collect the same exposure. The conversion is one multiplication:

new shutter speed = metered shutter speed × 2^(filter stops)

Meter the shot first, without the filter on — that’s the only way to know your real starting exposure for that light. Say you meter f/11, 1/125s, ISO 100 on a bright afternoon and you’re reaching for a 6-stop filter. 2^6 is 64, so:

1/125 × 64 = 64/125 ≈ 0.51s

Swap to a 10-stop filter and 2^10 is 1,024:

1/125 × 1,024 ≈ 8.2s

That’s the whole calculation. No chart required — and if you’d rather not run it in your head on a windy headland, our ND filter calculator does exactly this: pick the shutter you metered, pick the filter you’re fitting, and it returns the exposure time, stacked filters included. It also flags the moment the answer crosses 30 seconds, which is where your camera’s own timer stops and bulb mode starts.

Stacking two filters

Stack a 3-stop and a 6-stop filter and their transmission factors multiply: ND8 × ND64 = ND512, the same 9 stops you get by simply adding 3 + 6. Adding stops is the fast way to do it in your head, and it’s always correct — what catches people out isn’t the arithmetic, it’s forgetting how quickly the numbers get impractical. Two stacked 10-stop filters is 20 stops, and 2^20 is over a million — a 1/125s exposure becomes roughly a 2.3-hour one. Vanishingly few shots need that. A 6-stop and a 10-stop in the same bag covers nearly everything a landscape photographer will shoot; there’s rarely a reason to double them up.

A field example: waterfall at midday

Waterfalls and rivers are usually shot for the silky-water look, and the timing problem is real: the water blur wants a slow shutter, but midday sun wants a fast one, so the ND filter is what buys back the time. Meter the scene unfiltered first — say f/8, 1/500s, ISO 100 in open shade. You want at least half a second of blur. Counting doublings from 1/500s down to 1/2s (1/500 → 1/250 → 1/125 → 1/60 → 1/30 → 1/15 → 1/8 → 1/4 → 1/2) is 8 stops, so an 8-stop filter — or a 6-stop plus a 2-stop stacked — gets you there without touching aperture or ISO. Carrying only a 10-stop instead means overshooting to roughly 2 seconds, fine for smoothing water but risky for blowing the texture out of any cloud or foliage that’s also moving.

Timing the light matters as much as the filter strength here. Golden hour already hands you two or three stops less light than midday, so the same silky-water shot needs a lighter filter — or a shorter exposure at the same filter strength — right when you also get better color and softer contrast on the rocks. Reaching for the strongest filter in the bag out of habit at golden hour can push you well past the exposure the water blur actually needed; check the metered numbers first.

Skip the chart, keep the number

Once the doubling logic is in your head, a chart becomes a backup instead of a requirement — useful for double-checking a value fast in the field, not for understanding what’s happening. It’s the same reasoning behind why golden hour runs a different length depending on where you’re standing: stops and minutes both come down to doubling and halving, once you know where to look. If you shoot long exposures often enough across seasons and locations that this kind of light math is worth tracking, it’s the sort of detail that turns up occasionally in our field-notes emails.