ND Filter Calculator
Tell it the shutter speed you metered without the filter and how many stops your filter cuts, and it works out the shutter speed you need with the filter on — plus the times one stop either side for quick bracketing. Instant, in your browser, no sign-up.
With a 6-stop filter, your shutter speed becomes 1/2 s.
| Base shutter speed | 1/125 s |
| Filter strength | 6 stops · ND64 |
| New shutter speed | 1/2 s |
| One stop less (5 stops) | 1/4 s |
| One stop more (7 stops) | 1 s |
How to read this
Meter without the filter first. Compose, focus and take a test shot with no filter on the lens, and note the shutter speed that gives you a correct exposure — that is your base speed here. A dark filter makes metering and focusing through the viewfinder somewhere between difficult and impossible, so the honest order is: get the exposure right bare, then do the maths, then fit the filter.
Each stop doubles the time. One stop of neutral density halves the light reaching the sensor, so the shutter stays open twice as long to collect the same exposure: 1/125 s becomes 1/60 s, then 1/30 s, and so on down the line. That is all this calculator does — it doubles your base speed once for every stop you dial in, and shows the times one stop either side so you can bracket without working anything out in the field.
Stacking two filters adds their stops. An ND8 (3 stops) in front of an ND64 (6 stops) behaves like a single 9-stop filter, so add the stops together and enter the total. The exposure maths stays the same — though stacked filters vignette sooner, especially on wide lenses, which is a reason to test the combination before it matters.
What an ND filter is
A neutral density filter is sunglasses for your lens: a piece of darkened glass that cuts the light reaching the sensor without — in theory, at least — changing its colour. That sounds like the last thing you would want in bright daylight, until you realise what the lost light buys you: time. With several stops held back, a shutter speed that would have been 1/250 s in sunshine stretches to seconds, and moving things smear into the effects photographers chase ND filters for — waterfalls and rivers going silky, crowds in a busy square ghosting away to nothing, clouds streaking across the sky. The same trick works at the other end of the scale: video shooters use ND to hold a cinematic 1/50 s shutter at a wide aperture in full sun, and portrait photographers use it to shoot at f/1.4 outdoors without the highlights blowing out. If you are new to how shutter speed trades against aperture and ISO, understanding the exposure triangle is the place to start — an ND filter is simply a way of pulling the shutter-speed corner of that triangle much further than daylight normally allows.
Stops and ND numbers, explained honestly
Filters are sold under two naming systems, and they cause endless confusion. The honest unit is the stop: each stop halves the light and therefore doubles the exposure time. The other system is the ND number, which is meant to say how many times darker the filter makes the scene. The two convert like this: ND8 is 3 stops (2³ = 8), ND64 is 6 stops, and the famous ND1000 is 10 stops. Note the naming mess in that last one — 2¹⁰ is 1024, not 1000, so an “ND1000” is not exactly 1000× darker. Manufacturers rounded to a tidier number and the name stuck. It makes no practical difference to your photograph, but it is worth knowing so the arithmetic on this page — which works purely in stops — does not look like it disagrees with the box your filter came in. This calculator takes stops because stops are what your exposure actually does: tell it 10 for an ND1000, 6 for an ND64, 3 for an ND8, and stack filters by adding their stops together.
The working method
Fit the filter last. Compose and meter without it: set your aperture and ISO, focus carefully (autofocus works fine before the glass goes dark), and take a test shot. Note the shutter speed that exposes correctly — that is your base speed in the calculator above. Add your filter's stops, read off the new time, then fit the filter, switch the camera to manual so nothing re-meters, cover the viewfinder eyepiece if your camera leaks light through it, and set the new shutter speed — or bulb mode if the answer is past 30 seconds. Two habits save most spoiled long exposures. The first is a solid tripod and a hands-off shutter: our guide to using a tripod for long-exposure shots covers the basics, and if you are shopping, our travel tripod picks are the ones that survive being carried. The second is watching for the cheap-filter tells: strong NDs, especially bargain variable NDs, can add a colour cast — often magenta or muddy green — and variable designs pushed to their darkest end show a dark X-cross across the frame. Shoot RAW so a cast can be corrected, and test a new variable ND at every strength before trusting it on a real shoot.
Planning the rest of the exposure
An ND filter decides how long the shutter stays open, but aperture still decides what stays sharp while it is open — pair this page with the depth of field calculator when you are stopping down for a landscape and want the foreground and horizon sharp together. And if you are out after dark rather than fighting daylight, you may not need a filter at all: shooting the night sky instead? The sharp-stars calculator works out the longest exposure your lens allows before the stars start to trail.
Frequently asked questions
How many stops do I need for waterfalls?
Typically 3–6 stops in shade, and more in sun. What you are aiming for is a shutter speed of roughly half a second to a few seconds — long enough for falling water to blur into silk, short enough that it keeps some texture. In the shade of a gorge, an ND8 or ND64 usually gets you there from a typical daylight reading. In full sun on a bright river, expect to need the ND64 or to stack towards 10 stops. Check the calculator from your metered base speed rather than guessing: if the answer lands in that half-second-to-several- seconds window, you have the right filter on.
Does the calculator work for video?
Yes — it is the same maths. Filmmakers usually want a shutter speed near twice the frame rate (1/50 s at 25 fps), held at a wide aperture even in bright sun. Meter the scene without the filter, then use the calculator to find how many stops take your base speed down to your target: if the camera wants 1/800 s and you need 1/50 s, that is four stops — an ND16. A variable ND makes this easier in changing light, but the stops it needs are the stops this page adds up.
What if my time is over 30 seconds?
Use bulb mode. Most cameras top out at 30 seconds on the dial, so for anything longer you hold the shutter open yourself — ideally with a cable release or remote so your hand is not on the camera — and time the exposure with a phone. The calculator flags it when your result passes 30 seconds. On bulb, a few seconds' error in a multi-minute exposure barely matters: five seconds over or under on a three-minute frame is a fraction of a stop.
Does ISO or aperture change the maths?
No — the calculator assumes you keep them fixed. The doubling rule only converts shutter speeds: whatever your aperture and ISO, the filter multiplies the exposure time by the same 2^n. If you change ISO or aperture after metering, you have moved the base exposure, so re-meter without the filter, note the new base shutter speed, and run the calculator again from there.