Perfect long exposure with physical ND filters

Long exposure photographs, particularly of seascapes, rivers or waterfalls, can create magical effects that allow you to express your creativity, and to show elements of the scene that the eye alone cannot discern. However, it is one of the most difficult photography techniques to master, and can be enormously frustrating. Over the years, I have developed a simple methodology to obtain perfect long exposure with ND filters. This works for all cameras. In a separate post, I cover the use of Olympus computational photography to obtain even easier results.
Typical Long exposure photography exposure times
The exposure time used for LE of moving water depends on the speed of movement. This is my usual set of times:
- Waterfalls and sea flowing over rocks to give ‘lines’ of water: 1/5s
- Heavy seas on rocks to give the ‘mist’ effect: 1-2 minutes
- Urban rivers and lakes and flat seas to give a mirror effect: 4 minutes
Unless it is dark, all three of these exposure times will let in far more light than the camera can normally accommodate even at high apertures, so Neutral Density (ND) filters are needed, acting like sunglasses to reduce the amount of light from the scene, so that a correctly exposed long shutter speed can be used.
The key question is how strong do the sunglasses need to be? That of course depends on how bright it is, and at this point it is worth introducing the concept of exposure values.
Exposure values and typical light conditions
All modern cameras can measure light to produce a correct exposure. With any given scene lighting condition (more accurately, luminance), you can use many combinations of the exposure triangle to get the same meter reading. The Exposure Value (EV) is a number that represents the ambient light in the scene, i.e. all combinations that yield the same exposure have the same EV.
EVs are measured in stops, i.e. an increase in EV of 2 is a two stop increase, or 4x the luminance. It can be expressed as a table as below:
Knowing the likely EV value is very helpful is deciding what ND filter to use as we shall see. Most cameras do not measure the EV direct, but you can calculate it with an app like Photopills, or measure it direct with an app like Lightmeter (which is what I use).
Working with ND filters
The strength of ND filters is described both as EV stops, or ND numbers. The ND number is the actual reduction in luminance. To give an example, a 3 stop ND filter has an ND number of 8 (2 to the power 3 =). A 10 stop ND filter has an ND number of 1024 (2 to the power of 10 and commonly rounded to ND 1000). Other popular filter strengths are ND64=6 stops, and ND8=3 stops.
For strong ND filters, most camera metering systems are unreliable. Therefore, you have to use the camera metering without a filter to calculate the strength of ND needed to get you the required time with filters.
The way you typically do it is to determine the correct shutter speed without ND filters, and then use an app like Photopills on your phone to find the right ND filter(s). The process is very clumsy. You have to set the initial shutter speed, and try increasing ND filters until you get the long exposure time you want. Doing this on a windswept or freezing cold beach is very tedious and also fraught with errors.
For me, at least, this was also a very dumb process. What I wanted was the answer, quickly. For a given shutter speed, what ND filter is needed for a 2 minute long exposure? This is best expressed in a table, as shown below. It shows the ND filters needed for a given LE exposure time and non-filter shutter speed. It is set out for combinations of physical ND filters that are most frequently used, namely 3 stop, 6 stop and 10 stop.
“Long” ND filter tables for urban rivers and lakes and flat seas (stacked single filters)
This table tells you what ND filter you need for “long” LE exposures of 1, 2 or 4 minutes.
From the table it is clear that there are only two shutter speeds to remember for most normal conditions. For a 2-minute exposure, the combination needed (without filters) is: 1/640s and 16 stops of ND for a bright day, and 1/80s and 13 stops for a overcast day. This is easy to remember, or at least to remind yourself of before starting the session. For a 4 minute LE, halve the shutter speed. For a 1 minute LE, double the shutter speed.
Practical ND implementation for “long” LE exposures
Mount the camera on the tripod with a remote shutter release, in Manual mode, and without ND filters. Set the ISO to the base level and the aperture to about F8. Spot meter on the water (which usually will be around 18% grey), and check the highlights and shadows if necessary.
Then adjust the aperture and ISO to get the shutter speed to either 1/80 or 1/640 (for a 2 min shot) for the right exposure. Finally, add the correct ND filters (usually 13 or 16), change to bulb or time mode (without changing ISO or aperture), and expose for two minutes.
Choosing the right type of ND filter
ND filters are available in several formats. Traditionally, 100mm square or oblong filters were used in a slide-in holder that screws or is otherwise mounted onto the lens (often known as the Lee system). I started this way, with 150mm filters on a Nikon 14-24 lens. The setup was embarrassingly giant, and always attracted unwanted attention wherever I was working.
Apart from the size, the problem with these filters, even at 100mm, is light leakage into the image where the filter slides in. This reveals itself (usually too late) as light streaks in the image corners and is disastrous. I have used every make of holder, and they all have the same problem.
Like many photographers, I moved to circular screw-in filters to eliminate light leakage. These filters screw into each other so compound ND levels can be achieved. The issue here is that the filters can get stuck in their threads, and the process of attaching and removing filters for every exposure is maddeningly tedious. This is made much worse in a cold and windy seaside environment. It’s easy to think that you have screwed the filter in and then take your hand away, and it drops into the sea. This has happened to me on very many occasions.
The next development was magnetic ND filters. This makes the process of attaching and removing the filter vastly easier. These filters are much pricier than the screw-in type, so losing them, as well as having multiple different sets at each size, starts to become unaffordable. The common solution is to have a set or two that is oversize, so it will fit all the lens diameters. The problem here is that an oversize magnetic filter is easy to flip off the lens, no matter how strong the magnet.
My recommendation and current practice is to have magnetic ND sets that exactly fit each lens diameter. I have sets at 58, 72, and 82 which match my small, medium, and large travel kits. My preferred vendor is K&F which makes nice filters with no or minimal colour cast and accurate ratings.
“Short” ND filter tables for waterfalls and sea flowing over rocks (variable ND filter)
This table tells you what ND filter you need for “short” LE exposures of 1/5s.
For these “short” LE times, a much less strong ND filter is required. If you can get a 1/320s or 1/640s shutter speed without filters, the common a 6 stop ND filter and a Circular Polariser (CP) will work. This is likely to be the case for waterfalls, which are usually in valleys and shaded from direct sun. If, as is often the case, a high depth of field and therefore a high f-stop is needed for foreground rocks, it will further reduce the ND level required.
Otherwise, you are faced with an awkward set of filter requirements, such as 4, 5 and 7. This is where a variable ND filter from 2-6 and a built-in polarizer, taking you to 7-8 stops in total (such as this one) can be very helpful.
Practical ND implementation for “short” LE
“Short” LE work is much easier to implement than 2-minute exposures. Firstly, at 1/5 of a second and at short focal lengths, no tripod should be needed for modern cameras with in-body stabilisation (IBIS) like the OM1 or OM5. Even my Nikon Z7 can (just about) manage 1/5s handheld at 30mm.
Secondly, in-camera metering will almost always work, so the correct exposure can be found with the ND filter in place. However, you still need to know what ND level is needed, so initial metering without filters will be required. Once the right ND filters have been found from the table above, you can keep shooting for the rest of the session with them in place, unless the light radically changes.
Because you can meter in-camera, variable ND filters, or VNDs are very useful. Magnetic ones are at the top of the expense scale. The problem is that these are also by far the easiest to lose. VNDs are wide and heavy, often too heavy for the system’s standard magnetic ring. So a tiny flip will send your expensive VND into the sea or river. If you don’t have spares, it’s a nightmare. It’s happened to me twice now, and I am exceptionally nervous about using them in rough or slippery conditions – more or less a given on the sea shore or riverbank.
A screw-in VND makes a lot of sense for this kind of work. Screw it in once, and it will not come off, and you can adjust the LE exposure as much as you need and meter in-camera. This is what I now use for waterfalls and fast seas.