A Guide to Neutral Density Filters

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The neutral density (ND) filter is one of those tools that should be in almost every photographer’s camera bag. However, ND filters are mysterious to some, and many folks just don’t understand how, where, and when to use them. Beyond that, manufacturers seem to vary in their preferences as far as how they name ND filters—adding to the confusion. In this article, let us navigate the world of the ND filter together and see if we can make sense of the nomenclature and also name some appropriate times for their use.

What is a Neutral Density Filter?

The ND filter is basically a filter that, placed before the lens (or dropped into a filter slot) reduces the amount of light making its way into the camera. Think of the ND filter as sunglasses for your camera—albeit sunglasses that do not change the color of the light being captured by the camera and lens—hence the “neutral” nomenclature.

Photographs ©Todd Vorenkamp

Morning in downtown Brooklyn after a snow fall. Captured with a Formatt-Hitech Firecrest Ultra 16-stop ND filter.

What do Neutral Density Filters do or allow you to do?

There are a couple of real-world uses for the ND filter—one involving aperture and one involving shutter speed.

1. Aperture — Shallow depth of field in brightly lit environments

In the world of photography, generally speaking, more light is better. But, if you have ever been outside with an older analog or digital camera and tried to shoot your 50mm f/1.8 lens in broad daylight at wide-open apertures, you might recall seeing your exposure needle seemingly glued to the top of the light meter, or your digital light meter screaming “OVEREXPOSURE!” because the camera’s shutter could not cycle fast enough for the amount of light present.

The ND filter allows photographers to shoot their wide-aperture lenses in bright light without overexposing. This allows shallow depth of field and selective focus effects while under lighting conditions that exceed the shutter speed capabilities of the camera.

Even with the blazing-fast shutter speeds of today’s professional cameras and the previously unattainable shutter speeds introduced by electronic shutters, there is still a place in photography for the ND filter here.

I could shoot cloud abstracts all day with a Formatt-Hitech Firecrest Ultra 16-stop ND filter.

2. Shutter Speed — Slowing your shutter

The more “classic” use of the ND filter regards its effect on shutter speed. With less light entering the camera, you will need to slow the shutter for a given aperture setting. The slower shutter speed will allow anything moving in your frame to become blurred.

In general, camera blur is not desired, but if you work with a tripod or alternative support with an ND filter and a slow shutter, that which is static in the frame stays static and that which moves becomes blurry.

Where can you use this? Basically in any photograph with which you want to emphasize movement. Popular subjects include waterfalls, vehicular traffic, people (not usually portraits), seascapes, rivers, streams, clouds, and smoke.

The Brooklyn-Queens Expressway during early weekday rush hour. If it only looked like this in real life. This is a 2-minute exposure using a Formatt-Hitech Firecrest Ultra 16-stop ND filter.

What do the numbers on ND filters mean?

ND filters come in different strengths or darkness levels. For the photographer, the easiest thing would be to have ND filters that tell you how many stops of light they will darken your exposure. Designed by optical engineers, most brands of ND filters label their products with either an ND filter factor number or optical density number. Unfortunately, for the photographer, neither the filter factor nor the optical density number are equal to the number of stops by which the light is reduced.

So, here is a handy chart to reference when shopping for an ND filter or employing a filter you already own.

Stops of Light Reduction
(There are filters that are measured to a fraction of a stop, but, for simplicity, we are using whole numbers here with the exception of a few filters.)
Optical Density Number
(Sometimes prefaced with an "ND" before the number)
ND 1 Number Filter Factor Number
(Sometimes prefaced with an "ND" before the number)
Amount Light is Reduced
0 0 0 (a.k.a. Clear Filter) 0
1 ND 0.3 or "ND 0.3" ND 101 2 or "ND2" 1/2
2 ND 0.6 ND 102 4 1/4
3 ND 0.9 ND 103 8 1/8
4 ND 1.2 ND 104 16 1/16
5 ND 1.5 ND 105 32 1/32
6 ND 1.8 ND 106 64 1/64
6 2/3 ND 2   100 1/100
7 ND 2.1 ND 107 128 1/128
8 ND 2.4 ND 108 256 1/256
9 ND 2.7 ND 109 512 1/512
10 ND 3.0 ND 110 1024 (a.k.a. ND1000) 1/1024
11 ND 3.3 ND 111 2048 1/2048
12 ND 3.6 ND 112 4096 1/4096
13 ND 3.9 ND 113 8192 1/8192
13 1/3 ND 4.0   10000 1/10000
14 ND 4.2 ND 114 16384 1/16384
15 ND 4.5 ND 115 32768 1/32768
16 ND 4.8 ND 116 65536 1/65536
16 2/3 ND 5.0   100000 1/100000
17 ND 5.1 ND 117 131072 1/131072
18 ND 5.4 ND 118 262144 1/262144
19 ND 5.7 ND 119 524288 1/524288
20 ND 6 ND 120 1048576 1/1048576
22 ND 6.6 ND 122 4194304 1/4194304
24 ND 7.2 ND 124 16777216 1/16777216

So, for every stop of ND filter, you halve the amount of light entering the camera. When the light is halved, to maintain the same exposure, you need to double your shutter speed. Add another ND stop; double the shutter speed again.

Sunrise in Brooklyn. The ND filter not only smooths the clouds a bit, but, if you look closely, the steam from building heating systems shows the slow shutter speed. Captured with a Formatt-Hitech Firecrest Ultra 16-stop ND filter.

Let’s see, in graphical form, how an ND filter effects exposure time:

Original Shutter Speed ND Filter Stops New Shutter Speed
(Rounded to standard camera shutter speeds when applicable)
1s 0 1s
1s 1 2s
1s 2 4s
1s 3 8s
1s 4 15s
1s 5 30s
1s 6 1m
1s 7 2m
1s 8 4m
1s 9 8m
1s 10 16m
1s 11 30m
1s 12 1hr
1s 13 2hr
1s 14 4hr
1s 15 8hr
1s 16 16hr
1s 17 32hr
1s 18 64hr
1s 19 128hr
1s 20 256hr
1s 21 512hr
1s 22 1024hr
1s 23 2048hr
1s 24 4096hr (170 days 16 hours)
Sunlight captured in the windows of a skyscraper. The ND filter permits the shutter speed to be slow enough to allow the clouds to streak even with reflected direct sunlight in the frame. Captured with a Formatt-Hitech Firecrest Ultra 16-stop ND filter.

Practical Examples

Here is an example of the change in exposure affecting shutter speed when using an ND filter where your goal is to shoot at a slower shutter speed to blur a waterfall. Because of the bright daylight, the original shutter speed, even with the lens stepped down to f/16, is a fast 1/800th and freezes the water. You have a 6-stop ND filter in your bag and you screw it onto your lens. Here is the result:

Original exposure: ISO 200, f/16.0, 1/800.

Exposure with 6-stop ND filter: ISO 200, f/16.0, 1/13.

Here is an example of an exposure adjustment for trying to maintain a specific aperture when using an ND filter. You are shooting in broad daylight and want to take a photo of a flower with a soft background. You open your lens to f/1.4 and your exposure meter is pegged because the camera cannot fire the shutter faster than 1/4000 to get a proper exposure. Add an ND filter and see what happens:

Original exposure: ISO 200, f/1.4, 1/4000 overexposed.

Exposure with 6-stop ND filter: ISO 200, f/1.4, 1/60... still overexposed, but the shutter speed is easily achievable by the camera. So, now you can shoot the same scene at, say, 1/500 and get your shallow depth of field in direct daylight.

A building at sunrise. Again, the Formatt-Hitech Firecrest Ultra 16-stop ND filter allows the clouds to blur and the steam to streak.

Stacking Filters

One technique photographers use is filter “stacking.” If you have more than one ND filter, you may combine the two (or more filters) to get more ND stops for different photographic needs. The stacking math is easy: If you combine a 6-stop ND filter and a 10-stop ND filter, you now have a 16-stop ND filter.

The downside to stacking filters is that, for each filter you add, you are forcing light to pass through more and more glass (or resin) elements. The more things that the light has to traverse, the more it is likely to get slightly refracted in some way that causes softness or chromatic aberrations in an image.

Urban landscapes with a Formatt-Hitech Firecrest Ultra 16-stop ND filter

Filter Shapes

Most “solid” ND filters are round and screw onto the front of the lens. Larger lenses may have circular drop-in filters. However, some ND filters are rectangular or square-shaped and are inserted into special holders that affix to the front of the lens. The filter ratings for round and rectangular filters are identical.

That is the sun, not the moon, in the frame. The clouds were not especially thick, but the shot required a Formatt-Hitech Firecrest Ultra 24-stop ND filter.

Other Types of ND Filters

Graduated Neutral Density Filter (GND) — The GND filter is an ND filter that transitions from light to dark. The rectangular GND filters are more popular than circular because they allow the photographer to adjust the position of the transition area from light to dark. The main purpose of the GND filter is to balance exposure in an image that contains a bright sky and relatively darker foreground. Landscape photographers are big consumers of GND filters and they perform especially well when capturing sunset images.

Variable Neutral Density Filter (VND) — The VND filter gives the photographer the ability to “dial in” the amount of filtration by turning the outer ring of a dual-ring filter. The maximum and minimum ND rating differ with different filters, but the 2-stop to 8-stop variety are most popular. The advantage of the VND filter is that you only need to carry one ND filter with you to get a variety of darkness levels. The disadvantage of the VND filter is that, due to the design of the filters, as you approach the maximum ND setting, you can get a cross pattern across the image. This is remedied by dialing the ND setting back a bit.

Center Neutral Density Filter (CND) — The smallest category of ND filter, the CND filter has a darkened center and lighter edges. It serves to balance exposure across the frame when using extreme wide angle lenses.

Polarizing Filter — Yep, your polarizing filter is an ND filter that you may already own. Most polarizers give a 2-stop ND filter effect while providing the cannot-achieve-it-in-post-processing polarizing effects of cutting down glare, darkening the blue skies, and seeing farther into water.

Yes, an object 93 million miles away can have motion blur if your shutter speed is slow enough and the Earth rotates during your exposure. This 15-minute exposure was captured with a Formatt-Hitech Firecrest Ultra 24-stop ND filter. The varying diameters of the sun is caused by refraction through passing clouds.

Solar Photography

This is one more thing you can do with your ND filter(s). Many ND filter manufactures state that filters with a density of 16-stops or greater (shaded in the above table) are suitable for solar photography and solar eclipse photography. WARNING: If using an ND filter (or stack of ND filters) for solar photography, do NOT use an optical viewfinder. Specialized solar imaging and viewing filters not only filter visible light, but harmful UV and IR radiation as well. ND filters do NOT provide this protection. Use them only with electronic viewfinders and/or Live View mode.

The sun captured through a Leica APO-Televid 77 spotting scope with a Formatt-Hitech Firecrest Ultra 18-stop ND filter. The orange color was added in post-processing.

Recommended ND Filter Factors

Many landscape photographers recommend that you head out into the field with a 6-stop ND filter that should be perfect for slowing your shutter speeds enough to show smooth motion in mountain streams and waterfalls. Add your polarizer to make it an 8-stop ND stack.

Clouds passing overhead. Image shot with a Formatt-Hitech Firecrest Ultra 16-stop ND filter.

Some wedding and portrait photographers prefer the 3-stop ND filter to give them a wide-open aperture option while shooting in sunlight. Combine this with a 6-stop for a 9-stop combo when needed.

The 10-stop and darker ND filters are becoming popular with many photographers as they allow extremely slow shutter speed shooting and extremely wide aperture shooting under bright sunlight. If you have the time to crank out night photography-like shutter speeds, you can get some pretty cool effects with these super-dark filters in urban and natural settings. At the extreme end, the 24-stop ND filter is great for images with the sun directly in the frame.

Sunset behind Manhattan through a Formatt-Hitech Firecrest Ultra 18-stop ND filter. Not the smooth waters and sky.

Formatt-Hitech Firecrest Ultra Neutral Density Filters

The images used to illustrate this article were captured using Formatt-Hitech Ultra Neutral Density Filters. Firecrest filters feature extremely neutral optical coatings in between two pieces of optical glass—protecting the coatings from wear and tear and delivering enhanced durability and lifespan over normally coated filters. The new Firecrest Ultra filters are the only photographic filters that undergo an additional finishing process referred to as “lapping & polishing” that brings the filters up to cinema-grade standards of clarity, sharpness, and optical flatness.

The moment of urban sunrise. The sun is directly peeking into this frame, but the Formatt-Hitech Firecrest Ultra 24-stop ND filter allows some blurry steam action regardless.

Do you have any questions about neutral density filters or ND filter photography? Do you have some creative uses for ND filters? Feel free to ask questions or leave comments below!

22 Comments

I'm having trouble with the dark X that shows when using a 10+ variable Neutral Density filter. What I've read here and other places is that by opening the f-stop too wide it will generate that miserable black X and closing the f-stop to f4 or F5.6 will eliminate the problem. I'm using a Nikon D3200

and a Tamron 18-400 mm lens.  Is opening the f-stop the best and/or only way to prevent the blackX?    Thanks.  Paul G

Hey Paul,

Thanks for your question!

There are a two ways to combat the X-pattern on your VND filters is to simply dial back the density a bit. Changing aperture may help—my guess is that a larger aperture would reduce the effect. My other thought is that, like vignetting, changing aperture reduces, but doesn't really eliminate the effect. Your best bet is dialing in less density. Focal length may be a factor as well—zooming towards telephoto may push some of the X-pattern to the edges of the frame. You won't be eliminating the X, but simply "cropping" it a bit.

The up-sell: There are some brands, like NiSi, that claim they have eliminated the X-pattern in their newest VND filters. And, of course, non-variable filters will not create this pattern. As you use your VND filter, pay attention to how many stops you usually dial in—your favorite setting. It might be beneficial to get a non-variable filter around that density to avoid the X-pattern.

Please let me know if you have follow-up questions and thanks for stopping by Explora!

In the second practical example when using the ND to shoot the flower the settings are iso200, f1.4, 1/4000 and it's OVEREXPOSED, and after using a 6-stop ND filter the settings are iso200, f/1.4, 1/60 which they should not be like this in this situation bcus it's still OVEREXPOSED, you cut the light by 6 stop with the ND filter but you again gained the same amount of light back by slowing the shutter 6 stops which brings you back to OVEREXPOSED. correct me if I'm wrong or if  missing something 

Hey c m.

Yikes! You are correct! Thanks for catching that. I am going to have the editor make a change to the text.

I am glad B&H has eagle-eyed, fact-checking customers like you to keep me honest. Thanks for taking the time to point this out!

An essential ND filter use occurs when making 4K and other videos in daylight at the standard 24 FPS which equates to 1/48 second shutter speed. There are very few options re changing shutter speeds without degrading the motion smoothness. Increasing FPS forces dropping resolution from 4K to 1080P or lower.

Thanks, Jon! Video makes my hair hurt, but I will take your word for it. I know that video shooters are big on ND filters!

Another great use for ND filters, particular on a person or item (some what close-up) to achieve a more dramatic shot is to use the NDF to darken a bright day, then a flash on the subject.  POP!

james d

Nice, james! Thanks for the tip. I had not thought of that one!

Thanks for the response Doing some research found 2 apps for calculating shutter speed with specific ND filters. One is ‘slow shutter’ the other is ‘ND timer’ will continue to experiment Excellent article very inspiring I was thinking the only use was for water.

Hey Greg,

No worries!

Yep, there are a bunch of apps out there that do ND calculations. Some are dedicated ND filter apps and others are comprehensive photo apps that have an ND function.

With the magic of digital photography, you can always check your exposure after capture, or, if you are using a heavy ND filter, do a High ISO test shot before dialing back the ISO for the "real" shot.

Does anyone have an idea of exposure times when using ND filters?

thanks

greg

Hey Greg,

Of course it all depends on the scene, your metering, your aperture, your ISO, etc. Check the charts above. The second one shows how much longer your 1-second exposure will be with different ND filters.

I hope this helps. Thanks for reading!

Great intro article on ND filters.  It looks as though the images used for illustrative purposes uses the Firecrest Ultra models recently released.  For those new to ND filters, it might be helpful to note that most photographers can achieve desired results using a 3, 6 or 10 stop filter - for the solid NDs.  These are likely the top 3 densities used.  Also, there are a  number of popular brands including Lee, Cokin, Hoya and B+W to name a few aside from the tested brand in the article.  Some will give a color cast that can be corrected in post but like everything gear, read reviews and comments from others before committing.  Thanks as always Todd for the shared knowledge.

Hey Sean,

Yep, I used only the Firecrest filters for the illustrations, but there are, indeed other brands. In my limited experience working with ND filters, I have become a fan of the "black glass"...the lighter ND filters seem to be "weak" for my needs as I try to resist stopping down further than f/8 to maintain sharpness.

You are welcome for the shared knowledge and thank you, Sean, for the kind words! I am glad you enjoyed the article!

I'll have to give ND filters a try the next time I do a panorama photo series of Lake Murray, SC at the dam. I shot at a low ISO using Kodak Ektar 100 with a 28mm f2.8 lens and it was manual exposure with aperture and shutter speed on my Canon A-1. But I couldn't stich the three photos together seamlessly even though they were done in fast succession. The wind was moving the clouds and there were the waves. I was more successful doing panoramic photos at a quiet cove near home.

Hey Ralph,

I hate to say it, friend, but you might want to try digital for those panoramic shots! :)

Hi Todd,

I think this photo was successful. The local camera club's Show & Tell theme for one meeting was panoramic photos. I was going to leave it to the Photoshop guys, but I thought "Why not?" I found that Corel Paint Shop Pro could stitch photos together.

https://www.flickr.com/photos/ralphhightower/8436105383/in/album-7215763...

More than successful! Great shot, Ralph! Thanks for sharing it.

You win! :)

Thank you Todd. With film, one doesn't know what they got until they get the film back. I was pleased with the sun coming over the trees with the rays and the mid level fog.

I now have both the Canon FD Lens Work book and the EF Lens Work III book. The FD Lens Work covers the various focal length coverages in horizontal, vertical, and diagonal angles of coverage, where as the EF Lens Work just covers the diagonal angle of coverage for the lenses. I wish that Canon would publish horizontal and vertical degrees of converage in their EF book. But the FD book can be used for EF lenses since an FD 28mm lens has the same coverage as an EF 28mm lens. I had maybe 1/3 overlap of coverage planned for the three photos.

Hey Ralph,

Do you have gridlines in your film viewfinders? I find those are super helpful when shooting panos...I look what is on the right 1/3 gridline when I take a shot and then pan so that same object is now on the left 1/3 gridline.

My A-1 just has the standard split-image/microprism; I think changing that may be a bear. When I bought the New F-1, I bought the split-image/microprism in center-weighted, partial, and spot metering focusing screens; the others, such as the grid, are harder to find on the used market.

What I did was set the tripod head on the zero mark and then advance X number of degrees for the next two shots using the horizontal angle of coverage (Canon FD Lens Work) minus some overlap.

Gotcha. Nice thinking with the tripod markings!

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