Perfect long exposure photography with ND filters

Perfect long exposure photography with 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:

  1. Waterfalls and sea flowing over rocks to give ‘lines’ of water: 1/5s
  2. Heavy seas on rocks to give the ‘mist’ effect: 1-2 minutes
  3. 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 to reduce the amount of light from the scene, so that a correctly exposed long shutter speed can be used.

Working with ND filters

Using ND filters reverses the normal photography exposure process. Instead of finding the exposure time you need for a given amount of light on the scene, instead you have to find the ND filters you need for a given exposure time, of say 2 minutes.

This is a non-intuitive process. Starting from the shutter speed without filters, you have to calculate the strength of ND needed to get you the required time with filters. The way you typically do it is to use an app like Photopills on your phone. 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 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. This shows the ND filters needed for a given LE exposure time and non-filter shutter speed. It is set out for combinations 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

Perfect long exposure photography with ND filters

After a while, I realized that the same numbers kept coming up under normal conditions. For a 2-minute exposure, the combination needed was: 1/640s (without filters) and 16 stops of ND for a bright day, 1/80s and 13 stops for a medium day, and 1/10 and 10 stops for a dull day. This is easy to remember, or at least to remind yourself of before starting the session. The green rectangle shows where in the table these values are. For a 4 minute LE, halve the shutter speed. For a 1 minute LE, double the shutter speed.

Practical ND implementation for “long” LE

“Long” LE shots must always be on a tripod, so they are fairly constrained. You usually cannot meter the scene in-camera with upwards of 10 stop NDs attached as there is not enough light. So you have to meter without NDs in place, calculate the ND needed (with a phone app or my table), then attach the filters.

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.

Another downside is that if you have a range of lenses with different threads (as I do, at 52, 58, 72, 77 and 82mm) you need a full set of filters for each major thread size.

Perfect long exposure photography with ND filters

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

Perfect long exposure photography with ND filters

For these “short” LE times, a much less strong ND filter is required. If you can get a 1/32s or 1/320s shutter speed without filters, the common 3 stop or 6 stop ND filters 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, 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.

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