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ND Filter Calculator

PhotographyExposure and Camera Settings
Metered shutter speed as

For 1/125 s enter 125.

Shutter time with the filter (seconds) —
As a camera setting
—
Metered time without filter
—
Exposure factor (×)
—
Optical density
—

An ND filter cuts light by whole stops, and every stop doubles the shutter time. Multiply the metered time by 2 raised to the number of stops. A 10-stop ND1000 at a metered 1/30 s gives 1024 ÷ 30 = 34.13 seconds, so you set roughly 34 s in Bulb or the nearest 30 s step.

About this tool

Landscape and architecture photographers use neutral density filters to drag the shutter: silky water, streaked clouds, empty city squares. The camera usually cannot meter through a 10-stop filter, so the reliable route is to meter without it, then scale the time. Enter the shutter speed your camera chose without the filter, either as a fraction like 1/125 or as whole seconds, and pick the filter by its ND number or density. The calculator returns the new shutter time, the same time written as a camera setting, the exposure factor and the optical density. It assumes aperture and ISO stay fixed. Real filters are often a third of a stop off their label, and very long exposures can suffer reciprocity failure on film, so treat the result as the first test frame, not the final one.

How to use it

  1. Meter without the filter

    Set aperture and ISO, then note the shutter speed the camera picks before mounting the filter.

  2. Choose how to enter it

    Pick Fraction for speeds like 1/250 and type 250, or Whole seconds for readings of 1 s or longer.

  3. Select the filter

    Choose the ND number or density printed on the filter ring; the list runs from 1 to 20 stops.

  4. Read the new time

    Use the camera-setting line; above 30 s switch to Bulb and time it with a remote.

Examples

Big Stopper at 1/30 s

Metered shutter speed as
Fraction (1/x s)
Shutter speed without filter
30
ND filter strength
ND1000 / 3.0 (10 stops)

Result Shutter time with the filter (seconds): 34.13
As a camera setting: 34.1 s
Metered time without filter: 1/30 s
Exposure factor (×): 1,024
Optical density: 3.0

  1. Light reduction factor = 2^10 = 1024
  2. New time = 0.03333 s x 1024 = 34.13 s (34.1 s)
  3. Optical density = 10 x 0.301 = 3.01

A 10-stop filter turns a 1/30 s daylight reading into about 34 seconds, enough to smooth sea and blur clouds.

ND64 at 1/125 s

Metered shutter speed as
Fraction (1/x s)
Shutter speed without filter
125
ND filter strength
ND64 / 1.8 (6 stops)

Result Shutter time with the filter (seconds): 0.51
As a camera setting: 1/2 s
Metered time without filter: 1/125 s
Exposure factor (×): 64
Optical density: 1.8

  1. Light reduction factor = 2^6 = 64
  2. New time = 0.008 s x 64 = 0.51 s (1/2 s)
  3. Optical density = 6 x 0.301 = 1.81

Six stops multiply the time by 64, giving 0.512 s, which most cameras offer as 1/2 s.

15 stops at 1/60 s

Metered shutter speed as
Fraction (1/x s)
Shutter speed without filter
60
ND filter strength
ND32000 / 4.5 (15 st.)

Result Shutter time with the filter (seconds): 546.13
As a camera setting: 9 min 6 s
Metered time without filter: 1/60 s
Exposure factor (×): 32,768
Optical density: 4.5

  1. Light reduction factor = 2^15 = 32768
  2. New time = 0.01667 s x 32768 = 546.13 s (9 min 6 s)
  3. Optical density = 15 x 0.301 = 4.52

32768 ÷ 60 gives just over nine minutes, so the shot needs Bulb mode and a timer.

How it is calculated

t_ND = t × 2^n, D = n × log10(2) ≈ 0.301 × n

t
metered shutter time without the filter (s)
n
filter strength in stops
t_ND
shutter time with the filter fitted
D
optical density printed on many filters (0.3 per stop)

A stop is a halving of light. A filter that passes 1/2^n of the light needs the shutter open 2^n times longer for the same exposure. ND numbers such as ND8 or ND1000 are that factor written directly, while density figures like 0.9 or 3.0 are its base-10 logarithm. ND1000 is a rounded 2^10 = 1024, so the tool uses the exact power of two.

When not to use it

  • Variable ND filters have no fixed stop value and often vary across the frame.
  • On film exposures over about one second, reciprocity failure needs extra time beyond this figure.
  • If you change aperture or ISO after metering, the base reading no longer applies.

Common mistakes

  • Entering 125 in the seconds box when the meter read 1/125 s.
  • Treating ND1000 as 1000 stops instead of a 1000× light reduction, about 10 stops.
  • Stacking two filters and adding their ND numbers instead of adding their stops.
  • Leaving the viewfinder open on long exposures, which lets stray light fog the frame.

Frequently asked questions

How many stops is an ND1000 filter?

About 10 stops. 2^10 is 1024, and makers round that to 1000. Its optical density is 3.0, since each stop adds roughly 0.3 to the density.

How do I combine two stacked ND filters?

Add their stops, or multiply their ND factors, or add their densities. An ND8 (3 stops) on an ND64 (6 stops) gives 9 stops, close to ND500, which you can select directly in the list.

Why does my result not match a shutter speed on the dial?

Cameras step in thirds of a stop and cap timed exposures at 30 s. Pick the nearest available speed, or switch to Bulb for anything longer and time it with a remote release or the camera's interval timer.

What does density 0.9 mean on a filter?

Density is the base-10 logarithm of how much light the filter blocks. A density of 0.9 means 10^0.9, about 8 times less light, which is 3 stops, the same as an ND8.

Do I need to adjust focus with a strong ND filter?

Focus does not change, but a 10-stop filter is too dark for autofocus. Focus first without it, switch the lens to manual focus, then screw the filter on carefully so the ring does not turn.

Why is my long exposure darker or bluer than expected?

Filters are rarely exactly their rated strength, and cheaper strong filters can add a blue or magenta cast. Shoot a test frame, check the histogram, and adjust time by a third or half a stop.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 16). ND Filter Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/photography/exposure-and-camera-settings/nd-filter-calculator/

MLA

Tools Directory Editorial Team. “ND Filter Calculator.” Tools Directory, 16 Oct. 2026, https://tools.milunir.com/photography/exposure-and-camera-settings/nd-filter-calculator/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “ND Filter Calculator.” Tools Directory. Last modified October 16, 2026. Accessed October 27, 2026. https://tools.milunir.com/photography/exposure-and-camera-settings/nd-filter-calculator/.

Link<a href="https://tools.milunir.com/photography/exposure-and-camera-settings/nd-filter-calculator/">ND Filter Calculator – Tools Directory</a>