What is a Neutral Density Filter?
A neutral density (ND) filter reduces the amount of light coming into the camera’s sensor.
The ND filter allows photographers to choose a variety of combinations of aperture, exposure, and ISO sensitivity that, without the ND filter, would result in overexposure. With this flexibility, a photographer can achieve a more limited depth of field or motion blur in a number it situations and conditions with no color shift or loss of image quality.
In this article, we’ll discuss what ND filters do, how photographers use these filters, the construction of a quality filter, and choosing the right one for the light conditions.
What Does a Neutral Density Filter Do?
A neutral density filter is darkened glass. The cheaper ones are made of resin. At the same time, reducing the amount of light entering the camera lens results in no changes in hue or color casts. That is, all the light wavelengths, or colors, are reduced equally. In addition, the polarization of light is unaffected. So the filter’s influence on hue and polarization is neutral.
The capacity of ND filters to impact light reduction is measured in stops. A one-stop filter lets in half as much light as no filter. A two-stop filter allows one-quarter the amount of light as no filter.
ND filters come with densities of up to 32 stops of light reduction, exceeding practical application for most photographers.
How are Neutral Density Filters Labeled?
This gets a little confusing.
A one-stop filter, which reduces the amount of light by 50 percent, has an ND factor of ND2 and an optical density number of ND0.3.
A two-stop filter reduces the amount of light to 25 percent. Its ND factor is ND4, and its density is ND0.6.
We simplify matters by concentrating on how many f-stops of light reduction we want. Retailers such as Adorama, Amazon, and B&H Photo list their products as “two-stop” or “one-stop,” along with the other designations.
In practice, selecting the number of stops of light reduction desired clarifies which filter to use in a specific setting.
How to Calculate Density Options
As an example of how ND filters work, suppose you are shooting a scene, and the camera tells you that f/8 at 1/125 second is good exposure. You’d like to maintain the depth of field that comes with f/8, but you want a slower shutter speed, something around 1/15 second. In other words, slow the shutter speed by three stops. Attach a 3-stop ND filter, and you’re there.
Unless you’re good at doing exposure time calculations in your head, you might want to look into the ND calculator app – especially for those longer exposures. This app calculates shutter speed based on aperture, ISO, and filter stop number. That process works in reverse as well. Enter the shutter speed, aperture, and ISO, and the app tells you what strength of filter to choose. Check the App Store on your phone for ND calculator.
Can ND Filters be Stacked?
ND filters can be stacked, and the effect is cumulative. That is, if you attach a 2-stop ND filter to your camera lens and then add a 6-stop filter on top, the ND filter performance equals eight stops.
But stacking filters can create issues. It can lead to vignetting. Also, stacking may cause other optical problems, such as internal reflection, color shift, loss of sharpness, and chromatic aberration.
If you choose to stack multiple filters, go with no more than two.
The Shapes of ND Filters
The majority of ND filters are round and attached to the front of the lens with screw threads. If the lens lacks filter threads, probably because the front element is so large, opt for drop-in filters near the lens mount.
Round filters come with a clear specification of their size. Common sizes are 67 mm, 77 mm, and 82 mm. Match that to the filter diameter noted at the front of the lens.
If you go with round filters, you should buy a filter or a set of filters to fit your largest lens. Then purchase step-up rings to adapt to the smaller lenses.
ND filters are also available in rectangular or square shapes. These fit into a slot of a special holder that goes on the front of the lens. These are bulkier but also more versatile. And a single filter fits all your lenses with the proper adapter ring.
The Types of ND Filters
A graduated neutral density filter (GND) is dark at the top and gradually becomes clear toward the bottom. This evens out the exposure between a bright sky and a dark foreground; however, this can also be achieved in post-processing. The GND comes in a number of f-stops and in several variations.
A hard GND filter has a sudden transition between the dark and clear sections. A soft GND transition is slower and more subtle. A medium GND takes the middle ground.
The center graduated neutral density filter (CGND) is darkest at the center and becomes progressively clearer toward the edges. Not widely used, the CGND can offset the vignetting that happens with wide-angle lenses.
Variable ND filters (VND) change density according to how far it rotates. It works much like a circular polarizing filter. It is convenient. Simply turn the rotating piece. However, some photographers recommend against it. A variable ND is thicker and may result in vignetting with wide angle lenses. Also, variable ND filters have a reputation for creating odd crosshatch-appearing artifacts in the sky during a long exposure.
We should mention here that a circular polarizing filter provides approximately a 2-stop ND effect.
How Photographers Use a Neutral Density Filter
Neutral density filters allow photographers to produce a variety of effects in two different ways:
- Shoot with a wider aperture for a shallow depth of field, creating a soft background.
- Use slower shutter speeds to create long exposure effects, such as blurred moving objects.
Slower shutter speeds add an ethereal feeling to water movement. Clouds photographed at long shutter speed take on a surreal effect.
In portrait photography, a wide aperture makes the primary subject stand out against the background.
For landscape photographers in bright sunshine and harsh lighting, ND filters come into their own. Overexposure can be avoided while enhancing the image quality and avoiding color shifts.
Using the ND Filter to Shoot in Very Bright Light
Here’s an example: shooting on a clear sunny day, the light is bright, and the shadows are harsh. But you’d like to create a shallow depth of field, so you want to use a wide aperture. However, the bright light poses the risk of overexposing the scene, even with a fast shutter speed. But a neutral density filter of about six stops allows you to use a wide aperture without overexposure.
Also, the ND filter tends to create better image contrast, capturing detail in both shadows and highlights.
ND Filters Enable Long Exposure Times in Daylight
Let’s take another example: shooting at the beach. Again, it’s bright and sunny, but you’d like to capture a misty, silky effect of waves splashing on the shore. This calls for longer shutter speeds, maybe a 20-30 second shot, without completely washing out the image. A 10-stop ND filter delivers the desired effect.
Yet another application is shooting a waterfall in a wooded setting. With trees blocking the sun, the light reaching the camera sensor is more subdued. You might achieve the effect of water motion blur with 10-15 second exposure using a 2-stop or 4-stop ND filter.
Creative Uses for ND Filters
Shooting into the light, such as a setting sun, is an opportunity to create a silhouetted foreground against the brilliant colors of a sunset. A neutral density filter can help you avoid overexposing the sun while retaining detail in the deep shadows.
ND filters enhance some of the more subtle colors. An early morning shot, just before the sun rises, brings out purple and gold shades that you won’t get with just a normal lens.
What Keep in Mind When Buying ND Filters
Photographic optics are expensive, and neutral density filters are no exception. With that said, invest in a filter made of multi-coated glass, preferably tempered mineral glass. This ensures that there is no color cast or other optical defects that reduce image sharpness.
Make sure the filters precisely match their optical density number. So you’ll know what to expect from each one. Several fixed ND filters of different density options will give you greater flexibility in the field.
Square Filters vs. Screw-In Filters
The first step in buying a neutral density filter is to determine which filter type you want to invest in. Both have advantages and disadvantages.
A square filter system, which may be square or rectangular, offers flexibility. In the field, it’s quicker to change ND filters. Stacking is easier, and stacking square format filters is less likely to cause vignetting.
The downside is the expense. Square filters generally cost more. You need a filter holder in which to place the hard-edge filters; however, that holder can be adapted to fit multiple lens sizes.
Screw-in filters are more compact, an advantage for both carrying and storing. The outer ring protects the glass, making them more durable and drop resistant. Also, a screw-in filter features a tight seal to the lens, so light leaks are less of an issue.
Fixed ND filters are less costly; however, they come in specific thread sizes. So you’ll need multiple sizes of the same strength filter or invest in adapters.
Lee Filters produces a line of high-quality filters, and Big Stopper Lee Filters rank high in favorability ratings by photographers. They’re also some of the more expensive.
Hoya, NiSi, and Tiffen are also quality brands and a little less costly.
Other Considerations When Buying
Having the right gear for your niche is critical to your success as a photographer.
If portraits or weddings are your specialties, 3-stop ND filters excel at reducing the depth of field with a larger aperture and low ISO setting.
The ND 6-stop filter is a good first buy. With it, you can take long exposures in daylight in the 5-10 second range. And at sunset or sunrise, you may capture long exposure effects of a minute or more.
A neutral density 10-stop filter creates the opportunity for even longer exposure, particularly in bright conditions. With creativity and control of how much light enters the camera, a landscape photographer can produce striking results in ordinary settings.
If you plan to shoot long exposures, you need a sturdy tripod. In addition, a sandbag, available in weights up to 50 pounds, provides extra stability in windy conditions.
I hope this article on the neutral density filter helps you enhance your photography skills. ND filters are very useful items to have in your camera bag. Understanding how and when to use them can enhance your creativity and help you develop as a photographer.