In today’s digital photography world many photographers are eschewing in-field filter use for digital processing techniques. In some cases, like using a graduated ND filters to deal with extreme dynamic range, or using a polarizing filter to increase color saturation*, the equivalent post processing techniques can actually produce superior results. But when it comes to a specific kind of filter, the strong neutral density (ND) filter, there is nothing in Photoshop’s arsenal that can quite re-create its unique effects.
In essence a strong ND filter serves one purpose: to allow for increasing shutter speeds while maintaining proper exposures. Some strong ND filters allow for 30-second (or longer) exposures even in broad daylight, creating long streaks of motion in clouds, smoothing out rivers and streams, rendering ocean waves as misty fog, and causing moving objects in a scene (like people walking across the street) to effectively disappear. But because of the unique qualities of these filters having an understanding of their behavior will help you use them to their highest potential.
Most major filter manufacturers have a strong ND in their line-up. LEE, Formatt-Hitech, B+W, and Singh-Ray all make high-quality filters (personally I find the drop-in filters to be much more useful and practical than the screw-in filters). The most popular strength is 10-stops, and personally I believe a 6-stop to be very useful as well, as a 10-stop is often overkill, especially around sunrise or sunset when light levels are already low. This article will discuss the use of those two densities specifically. In particular there are three areas in which strong ND filters require special consideration.
1) Composition and Focus
Because these strong NDs are so dark (a 6-stop filter blocks 98% of incoming light, and a 10-stop blocks a whopping 99.9% of incoming light) cameras often have a hard time seeing through them**. And not just your camera: your eye itself won’t be able to see through a dark ND filter, so forget about composing or focusing with one on your camera. The best practice is to compose your image and focus without the ND filter on. Then switch your lens to manual focus and attach the ND filter. This will ensure that your composition is what you want and that your focus won’t accidentally “hunt” on you.
The next step is to determine the proper exposure. Strong ND filters often fool a camera’s light meter, so forget about shooting in aperture priority mode. Even in full manual you shouldn’t rely on the light meter to give you accurate results. Many photographers will simply use trial and error to determine their exposure times, but if you understand some basic principles of photography figuring out what shutter speed to use will be a piece of cake.
Here again the best practice is to start without the ND filter on your camera. Set your aperture to your desired f-number and take note of what shutter speed gives you a proper exposure. Now put the ND filter on your camera and get ready to do a little math. Since an ND filter’s strength is measured in stops, when you add a filter to your camera you simply need to compensate for the filter by increasing your shutter speed by the same number of stops as the filter. In other words, if you add a 10-stop filter you need to increase your shutter speed by 10 stops. Sounds obvious but it’s important to remember nonetheless.
So the question becomes “how do I know how much 10 stops is???” Well, there are two ways to figure it out: 1) the empirical way, or 2) the shortcut way (warning: this way uses math!). Let’s start with the empirical way. By default, most cameras are set up so that every three clicks of the shutter speed dial (or the aperture dial too for that matter) is equal to one stop. For example, if your shutter speed is set to 1 second and you turn the shutter speed dial three clicks you will all of a sudden find that your shutter speed is now 2 seconds, which is exactly one stop more. Turn your dial three clicks again and you will be at 4 seconds, again a jump of exactly one stop. So for every stop of shutter speed you need, turn the dial three clicks. Meaning that if you add a 10-stop filter you need to increase your shutter speed three clicks for each of those 10 stops, or 30 clicks in total. A six-stop filter means you will need to increase your shutter speed by 18 clicks.
Ok, if that seems simple enough let’s look at the slightly mathier, slightly quicker way of calculating shutter speeds with ND filters. We already know that we need to increase our shutter speed by the same number of stops as the filter we use, and basic photography principles tell us that increasing shutter speed by one stop is the same as doubling it. In other words, if you double your shutter speed 10 times that’s exactly equivalent to increasing it by 10 stops. While the math of doubling your shutter speed 10 times might sound daunting it actually turns out to be really simple. Doubling shutter speed 10 times is the same as multiplying it by 2^10, and 2^10 equals 1024. This is pretty darn close to 1000, meaning that increasing your shutter speed by 10 stops is roughly the same as multiplying it by 1000. For example, take a shutter speed of 1″; to increase 10 stops multiply it by 1000 and you get 1000″. A shutter speed of 1/2″ becomes 500″; 1/100″ becomes 10″ and so on.
By similar math, a 6-stop filter needs a shutter speed increase of 60 times. This makes it easy because all you do is take your shutter speed in seconds and turn it into minutes. For example, take a shutter speed of 1″ and add a 6-stop filter, your new exposure time is 1 minute. Half a second becomes half a minute, two seconds becomes two minutes, a tenth of a second becomes a tenth of a minute, and so on. Once you understand how the conversions work for both of these filters you can just remember them and forget the math. Here’s a table with some select shutter speeds and their new values for 6- and 10-stop filters.
|Original Shutter Speed
|New Shutter Speed w/ 6-stop ND
1/8 sec (1/500 min)
0.6 sec (1/100 min)
1 sec (1/60 min)
6 sec (1/10 min)
30 sec (1/2 min)
60 sec (1 minute)
|New Shutter Speed w/ 10-stop ND
Note that each filter is slightly different, and usually slightly stronger than their designation. For example, I have a Lee 10-stop filter that’s closer to 10.7 stops, so I know to make a further 2/3 stop increase to my exposure to compensate for that.
Most of the time this quick and dirty math will serve you very well. A major exception is if you are shooting long exposures around sunrise or sunset when light levels change rapidly. For example, once the sun goes down the ambient light level can decrease by a full stop in as little as five minutes. Which means that if you fire off a 5-minute exposure during this time your photo will come out under-exposed, because you’ve lost a full stop of light during the shot. So at sunset you will need to compensate for this loss of light by further increasing your exposure time (usually another 2/3 of a stop is a good starting point), whereas at sunrise you will need to compensate for increasing light levels by shortening your expected exposure time (try 2/3 of a stop and see what that gets you to start).
Now that we’ve got our exposure worked out there’s one more critical factor to take into account: color. Most strong ND filters have a color cast to them, simply because it’s extremely difficult to make a dark filter that’s truly color neutral. The Lees and the Hitechs tend to have a blue/cyan tint to the them and the Singh-Rays and B+Ws are known for a magenta/brownish hue. If the color cast is slight enough your camera’s Auto White Balance can often compensate for it. However if the filter’s color cast is too strong you may need to use a custom white balance and a custom tint to remove the cast. Of course, you can always shoot black and white to remove the color issue completely. Compensating for the color cast of your particular ND filter will simply be a matter of trial and error.
Hopefully I haven’t overloaded you with the theory of ND filters. But if so, here’s my actual workflow in the field for shooting with strong NDs (I use Lee and Hitech filters). Follow a similar process and you’ll be good to go:
- With no filter on the lens, compose the scene, set my aperture to get the DOF I want, and focus.
- Ensure lens is in manual focus.
- Determine correct exposure for the scene without a filter.
- Add filter to lens.
- Increase shutter speed by appropriate amount. If I’m using a 10-stop I multiply by 1000 to get my new shutter speed. If it’s a 6-stop I change seconds to minutes.
- Increase shutter speed by 1/3 or 2/3 of a stop because I know my filters are slightly darker than they say.
- Depending on how long a shutter speed I want, I may further adjust my aperture in order to compensate. For example, say I’m at f/11 and 30 seconds. If I want a 60-second exposure I will increase my aperture to f/16.
- Change white balance either to Auto or to Cloudy / Shady to compensate for the blue tint of my filters.
- Take a picture. Tip: be sure to cover your viewfinder during long exposures as light can leak in and mess up the picture.
- Celebrate its awesomeness.
- Bonus: Take over the world.
If you’d like to practice these skills or learn more about long exposures and other kinds of photography, consider joining us on an upcoming workshop:
*Polarizers have two primary uses: 1) Increasing color saturation, which can be simulated with post-processing and 2) Reducing glare from reflected light, which cannot (yet) be accomplished via post-processing.
**Some recent model cameras can see through strong ND filters when in Live View. This can be a great way to focus and compose without having to remove the filter. However, the filters can still fool the camera’s light meter so don’t necessarily trust what the light meter or the exposure simulator / preview histogram are telling you. It’s better technique to calculate the exposure yourself.