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CalcChef

ND Filter Exposure Calculator

Find the shutter speed to use with an ND filter. Pick your base shutter speed and the filter strength in stops or ND number (ND8, ND64, ND1000). How it works

Use the ND Filter Exposure Calculator

ND Filter Stops
Calculated Exposure Time
64.0ms
Base 1/125s · Factor ×8

Equivalent shutter: 1/16s

Filter: 3 stops (ND8) — Exposure factor: ×8

Formula

new_time = base_time × 2stops

Example: 1/125s × 2³ = 1/125s × 8 = 0.064s, shown as 1/16s (1/15.6s exactly)

About the ND Filter Exposure Calculator

Each stop of ND filter doubles the exposure time: new time = base time × 2^stops. At 1/125 s an ND8 (3 stops) gives 64.0ms, shown as 1/16s, and an ND1000 (10 stops) gives 8.2s. Past about 1 second film starts to need more time than the formula gives, an effect called reciprocity failure, so on film treat the time shown as a minimum and check the film data sheet.

A 1/125 s base exposure through each filter

FilterStopsFactorExposureEquivalent shutter
ND21×216.0ms1/60s
ND42×432.0ms1/31s
ND83×864.0ms1/16s
ND164×16128ms1/8s
ND325×32256ms1/4s
ND646×64512ms1/2s
ND1287×1281.0s1.024s
ND2568×2562.0s2.048s
ND5129×5124.1s4.096s
ND100010×10248.2s8.192s

Exposure and equivalent shutter are written the way the tool shows them. Up to 1 second the equivalent shutter is a listed speed within 1 ms, otherwise 1 over the nearest whole number, so it can differ from a speed your camera offers. From 1 second it is the seconds to 3 decimals.

An ND1000 (10 stops) by base shutter speed

Base shutterExposure with ND1000
1/10001.0s
1/2504.1s
1/1258.2s
1/6017.1s
1/3034.1s
1/151m 8s
1/44m 16s

Each row is the base speed × 1,024.

How to use this tool

  1. Pick the base shutter speed you metered without the filter.
  2. Choose Stops (1 to 10) or ND Number (ND2 to ND1000).
  3. Read the new exposure time and an equivalent shutter value.

Frequently asked questions

How do I calculate exposure time with an ND filter?
Multiply the base shutter time by 2 for each stop of the filter. A 1/125 s base through 3 stops is 1/125 × 8 = 0.064 s. Through 10 stops it is 1/125 × 1,024 = 8.192 s.
How many stops is an ND1000 filter?
10 stops. Its exact factor is 2^10 = 1,024, and the name rounds that to 1,000. The tool uses 1,024, so 1/125 s becomes 8.192 s.
What shutter speed do I need with an ND64 filter at 1/60 s?
An ND64 is 6 stops, a factor of 64. 1/60 s × 64 = 1.067 s, which the tool shows as 1.1s.
Can I stack ND filters?
Stacked filters add their stops: an ND8 (3 stops) plus an ND64 (6 stops) is 9 stops, a factor of 512. Enter the total as stops. The tool’s limit is 10 stops, so for a 13-stop or 15-stop filter, or an ND1000 plus an ND64 (16 stops), work it by hand: base time × 2^stops. 1/125 s × 2^16 = 524.3 s, about 8 min 44 s.
What is the longest exposure it can show?
A 30 s base through 10 stops is 30,720 s, shown as 8h 32m. Anything over 1 second carries a note that the time is nominal. On film the real exposure at that length is longer, by an amount the film maker publishes; a digital sensor mostly gains noise instead.

Slowing a shutter in daylight

An ND filter lengthens the exposure when the light would otherwise force a fast shutter, for smoother water, streaked clouds or blurred crowds in daylight or twilight. Pick the shutter speed you metered without the filter, then the filter by stops (1 to 10) or by ND number (ND2 to ND1000). The page gives the new exposure time and an equivalent shutter fraction, which is not always a speed your camera offers. It changes shutter time only and leaves aperture and ISO alone. The base list runs from 1/8000 s to 30 s. Meter the scene without the filter, mount the filter, then set the calculated time in manual or bulb mode.

Stops and exposure time

new time = base time × 2^stops

Each stop halves the light, so each stop doubles the time. An ND number is the factor: ND8 is 3 stops (×8) and ND64 is 6 stops (×64). ND1000 is the 10-stop filter. Its exact factor is 1,024, which is what the page uses, so 1/125 s becomes 8.192 s and is shown as 8.2s. Example: 1/125 s with an ND8 gives 1/125 × 8 = 0.064 s, shown as 64.0ms and as 1/16s on the equivalent-shutter line (the exact value is 1/15.6 s). Filters stacked on one lens add their stops: ND8 plus ND64 is 9 stops, a ×512 factor. The page offers up to 10 stops; a 16-stop stack works the same way by hand, ×65,536. Above 1 second the page marks the time as nominal, because film then needs extra exposure that the formula does not give.

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