How Neutral Density Filters Work and How to Use Them For Better Photography
Taking good photographs isn’t just about framing your subject and learning composition. Learning how to control how much light enters your camera and for how long can help you take photos that elude the average photographer. Neutral density filters are a powerful tool towards that end. Here’s what they are and how to use them.
Neutral density filters (or ND filters) reduce the overall intensity of the light that passes through them, without affecting the color of that light. When you place an ND filter like these over the lens of a DSLR camera, it allows less light to pass through, which gives you the flexibility to open the aperture wider or expose a photo for longer than you would be able to otherwise.
To understand why this is helpful, we need to look a bit at how a camera works. When you take a picture with your camera, the aperture opens to allow light to hit your camera’s sensor. On more advanced cameras like DSLRs, you can adjust two key elements of that process: the size of the aperture (measured in f-stops), and the shutter speed, which determines how long the aperture is opened. These two numbers combined determine how much light hits your camera’s sensor.
This information is important for a ton of common photography tasks. If you want to photograph a sports game, for example, you’ll need to use a fast shutter speed in order to reduce motion blur. If you take pictures at night, you’ll either need a wider aperture in order to let in more light, or a slower shutter speed (and a tripod) to catch enough light for a well-lit picture. You can even do cool things like use an extra-long exposure to take pictures of fireworks.
Neutral density filters give you more flexibility to play with these settings. For example, a long outdoor exposure with a wide aperture at night may look good, but if you used those same settings during the day, your photos will be a blown out, overexposed mess. With an ND filter, however, you can filter all the daylight coming into your camera and still use a long exposure to get the effect you want without ruining your photo.
For example, consider the above image, comprised of two similar photos from Wikimedia. The left side of this photo was taken with an exposure of 1/30th of a second and no filter. The right side, however, was shot with an ND1000 filter, which allows only .1% of light through it. That photo was taken with a 57 second exposure. Yes, that means the shutter was open for nearly a full minute. Despite the much longer exposure time, the filtered photograph on the right still looks relatively normal. The only change is that the surface of the water (which would’ve been in constant motion while the shutter was open) now has a silky smooth look. You can frequently see this effect used to create dazzling images of waterfalls, oceans, and other scenery where one element of a scene moves but the rest of the shot stays stationary.
You can also use ND filters to selectively control the light in a scene. Graduated ND filters feature clear glass on one side of the filter, a full ND filter on the other side, and a small gradient in between. This is helpful if you want to take a picture of a scene where one half of the image (say, the sky) is very bright, but the other half (say, the ground) is darker. Place the darker half of the graduated ND filter over the sky, and the light entering your camera will be more even. You can expose for the ground without blowing out the sky.
Tricks like this are common in the world of film, as well. Whenever you see a scene with people in front of a window, chances are good that the window is layered with an ND filter gel that reduces the light shining through. If you were on set, the windows would look dim, since your eyes do a better job of distinguishing between the darker room and brighter outdoors. However, when you shoot the room through a camera, that ND gel makes the window look a lot better. Most of the time, photographers won’t need to ND gel a window, but it’s important to be aware of the light sources in your set up and pay attention to how to properly filter them to make the lighting in your scene even.
Above: Fountain shot at f/6.3 and 1/200 second exposure. Below: f/6.3 and 0.4 seconds (or 80x longer) with ND16 filter. Note: some color correction was done to the second photo to fix a color cast from the ND filter.
To get started with ND filters, you’ll need to know the notation used to rate them. While ND filters use several confusing notations, the most common is the ND number, often written as ND2, ND4, ND8, and so on. You can think of the number in this notation as the bottom of a fraction. An ND2 filter allows 1/2 of all light through it. An ND4 filter allows 1/4th of all light through it. An ND8 filter allows 1/8th of all light to pass through, and so on.
You might notice the numbers for these ND filter ratings double with each new filter. With a few exceptions, most ND filters you find will be a successive power of two. The reason for this is every time you halve the amount of light that passes through a filter, you effectively reduce the light entering your camera by one whole f-stop. So, an ND2 filter reduces the light by one f-stop. An ND4 filter reduces it by two f-stops, and on and on.
This is a handy shorthand when you’re calculating your light needs for a shot. Say you’re taking a photo of a waterfall and you want that silky smooth look for the water. You take a normal shot with an f-stop of f/22—this very small aperture ensures the entire scene is in focus—using Aperture Priority mode on your camera. At this setting, say your shutter speed for a properly exposed photo would be one second (to make the math easy for now).
Anything longer than one second and your photo would be blown out and too bright. So, you place an ND16 filter and an ND4 filter over your camera. This effectively reduces the light by six total stops. So, to compensate with your shutter speed, you’ll need to double the length of your exposure six times. A one second exposure, doubled six times (1 * 2 * 2 * 2 * 2 * 2 * 2) comes out to 64 seconds. You’re gonna need Bulb Mode for this one, but now you’ve calculated your proper exposure time.
You can also use this shorthand for taking basic landscape photography. Often, the sky is brighter than the ground, so to take a good picture of both, you want the sky and the ground to be within about one f-stop of each other. So, for example, if your sky is properly exposed at f/16, while the ground is properly exposed at f/5.6, then they are three full stops apart from each other. But you now know you can position a graduated ND4 filter (which reduces light by two stops) over the sky to bring the light closer together. Now you can take a picture of the landscape without blowing out or under exposing half the frame.
ND filters give you a whole new degree of flexibility that you can’t always get by adjusting your aperture or shutter speed alone. They’re also relatively cheap. This kit is the one I used for the photos of the fountain above. It costs less than $30, comes with ND2-ND16, and a variety of adapter rings for your DSLR camera. While the glass isn’t perfect (you may need to do some color correction later), it’s a handy tool to have in your bag.
Eric Ravenscraft covers smarthome tech for How-To Geek. He’s a problem solver who never learned to say no to a project. When he’s not fixing things, he’s cosplaying at cons, playing video games, and watching too many comic book movies. You can follow him on Twitter or Instagram.