ND Filter Calculator for Long Exposures
This ND filter calculator converts the shutter speed you meter without a filter into the long exposure you need with it — for any neutral density filter rated in stops, as an ND number or by optical density — and times Bulb exposures for you.
Meter without the filter, then find the shutter speed to use once it's on — with a timer for exposures longer than 30 seconds.
Keep the same exposure while changing settings: pick the one to work out, and enter the other two.
How to Use the Exposure & ND Filter Calculator
ND filter
- Meter the scene without the filter and note the shutter speed at your chosen aperture and ISO
- Enter the filter as stops, an ND factor or an optical density — or tap a common filter
- Set the new shutter speed. Beyond 30 seconds, switch to Bulb and use the timer: press Start as you open the shutter
Equivalent exposure
- Enter a correct exposure — aperture, shutter speed and ISO
- Choose what to work out and enter the two settings you want to change to
- Read the third setting, the nearest value your camera offers, and the scene's exposure value
How to Calculate ND Filter Exposure
Each stop of neutral density halves the light reaching the sensor, so each stop doubles the exposure time. An ND filter calculator simply applies that doubling as many times as your filter has stops.
The ND filter formula
New shutter time = metered time × 2^stops
Stops = log₂(ND factor) = optical density ÷ 0.3
Worked example: a 10-stop filter at 1/60 second
- A 10-stop filter (ND1000, density 3.0) multiplies the time by 2¹⁰ = 1,024
- 1/60 × 1,024 = 17 s
- At 1/15 second the same filter needs 1 min 8 s — Bulb mode and a remote release
ND filter chart
Long-exposure times for common filters, from the shutter speed you meter without the filter. Filters are listed by ND factor, stops and optical density. Times up to 30 seconds are rounded to the nearest setting on your camera; longer ones are for Bulb mode.
| Filter | Metered at 1/1000 | Metered at 1/250 | Metered at 1/60 | Metered at 1/15 | Metered at 1/4 |
|---|---|---|---|---|---|
| ND2 · 1 stop · 0.3 | 1/500 s | 1/125 s | 1/30 s | 1/8 s | 0.5 s |
| ND4 · 2 stops · 0.6 | 1/250 s | 1/60 s | 1/15 s | 1/4 s | 1 s |
| ND8 · 3 stops · 0.9 | 1/125 s | 1/30 s | 1/8 s | 0.5 s | 2 s |
| ND16 · 4 stops · 1.2 | 1/60 s | 1/15 s | 1/4 s | 1 s | 4 s |
| ND64 · 6 stops · 1.8 | 1/15 s | 1/4 s | 1 s | 4 s | 15 s |
| ND400 · 8.6 stops · 2.6 | 0.4 s | 1.6 s | 6 s | 25 s | 1 min 40 s |
| ND1000 · 10 stops · 3 | 1 s | 4 s | 15 s | 1 min 7 s | 4 min 10 s |
| ND32000 · 15 stops · 4.5 | 32 s | 2 min 8 s | 8 min 53 s | 35 min 33 s | 2 h 13 min |
ND1000 is sold as a "10-stop" filter; its exact strength is 9.97 stops, which is why its times are a little shorter than a true 10-stop filter's.
Understanding Exposure
Every exposure balances the same three settings. Thinking in stops makes the trade-offs simple.
Stops
A stop is a doubling or halving of light. Shutter speed, aperture and ISO can each be traded one stop for another without changing the exposure.
Example: f/8 at 1/125 is the same exposure as f/5.6 at 1/250.
ND Filter Ratings
Neutral density filters cut light evenly across colours. They're sold by stops (10-stop), by factor (ND1000) or by optical density (3.0). All describe the same thing.
- 3 stops: ND8, 0.9
- 6 stops: ND64, 1.8
- 10 stops: ND1000, 3.0
Why Use an ND Filter?
To slow the shutter in bright light: silky water, streaking clouds, empty-looking busy streets. Lighter filters let you open the aperture wide in sun or keep video shutter speeds natural.
A 6-stop filter turns 1/125 into half a second; a 10-stop turns it into 8 seconds.
Exposure Value
EV sums up an aperture and shutter pair in one number. Higher EV means brighter light. At ISO 100, full sun is about EV 15 — the basis of the "Sunny 16" rule.
Sunny 16: in bright sun, f/16 at 1/ISO gives a correct exposure.
Reciprocity
Digital sensors follow the stop maths exactly, however long the exposure. Film doesn't: beyond about a second it needs extra time, so check your film's reciprocity chart.
Long digital exposures can show more noise and hot pixels; long-exposure noise reduction helps.
Light Leaks and Colour Casts
In long exposures, light entering through the viewfinder can fog the image — close the eyepiece shutter or cover it. Strong ND filters also add a slight warm or cool cast, easily fixed with a custom white balance.
ND Filter and Long Exposure FAQs
How long should my exposure be with a 10-stop ND filter?
Multiply your normal shutter time by 1,024. If the scene meters at 1/60 second without the filter, the exposure with it is about 17 seconds. At 1/15 second it becomes a little over a minute, so you'll need Bulb mode and a remote release.
What do ND8, ND1000 and 3.0 mean?
They are three ways of rating the same thing. ND8 lets through 1/8 of the light (3 stops); ND1000 lets through 1/1000 (about 10 stops). Optical density ratings such as 0.9 and 3.0 add 0.3 per stop, so 0.9 is 3 stops and 3.0 is about 10.
Why meter before fitting a strong ND filter?
A 10-stop filter is too dark for most cameras to meter or focus through reliably. Meter and focus without it, switch to manual focus, fit the filter, then set the shutter speed this calculator gives you.
Which ND filter should I buy first?
A 6-stop filter is the most versatile: it turns daylight shutter speeds into fractions of a second to a few seconds, enough to smooth waterfalls and waves. A 10-stop filter is the classic choice for minute-long exposures of clouds and seascapes.
Can I stack two ND filters?
Yes — add their stops together: a 6-stop and a 3-stop make 9 stops. Stacking increases the chance of a colour cast and vignetting on wide lenses, so a single stronger filter is usually better.
What is exposure value (EV)?
A single number for a combination of aperture and shutter speed that gives the same exposure. Each step of 1 EV is one stop. At ISO 100, bright sun is around EV 15 and a typical living room at night around EV 6.