What is a Neutral Density Filter?

Published: August 16 2023

Categories: Tech Talk

Neutral Density Filters; Written by Brittany Griffin with Photo Credits to Brianna Pflug, Chris Homer, and Justin Curtis

Trying to achieve that smooth water effect but having a little difficulty balancing your settings due to bright light in the daytime and slower shutter speeds? Neutral Density filters can help you achieve that smooth water effect, and even help you limit light into your lens for other styled shots. In this article, we will be covering what they are, the effect they have, and where you can utilize them.

What Exactly is a Neutral Density Filter?

A Neutral Density Filter, or ND Filter for short, is designed to reduce the intensity of light equally from entering the camera in measured amounts. In a more technical sense, the filter itself reduces the intensity of all wavelengths or colors from entering the lens which in turn not only brings your exposure lower/darker it also allows you to widen your aperture to allow more light into the lens to utilize slower shutter speeds.

ND Filter Types

Within the Neutral Density line there are a few types that you will have the option to choose from. Not only are there different densities, there are also different filter sizes and shapes that can produce a slightly different result based on what you will be shooting. There are also different types of transitional effects within those selections that may be better in some applications.

Shapes

Circular Shape - this will be the standard filter on the market, the circular filter attaches like any other filter by the lens thread. Some brands will have vignetting on wider focal lengths however

Square/Rectangular - in general you will find these as "drop-in" filters that require a mount in tandem with the filter itself. This type allows the filter to go beyond the lens shape to form a more consistent effect across the image and allows for flexibility when using graduated filters to where the gradation lands in the shot

ND Effects

Variable ND - this type of ND ranges in filtration amount by offering an adjustable stop system. Typically 2 to 8-stop filters are most popular as they provide an ideal range for all types of shots

Graduated ND - these NDs provide a transitional light to dark effect and are almost always found in rectangular or square filter shapes. These are ideal for balancing a landscape scene as you can flip the filter to determine where the filtration lands (sky vs grass or seascapes)

Center ND - while these are not the most common type, they have a darkened center and lighter edge (similar to a radial gradient). With extremely wide-angle lenses this nd filter helps balance exposure across the frame without too much vignetting

Polarizing ND - you actually may already own one of these, but they are also considered ND filters! With a 2-stop ND filter effect on most polarizers, it allows a user to cut down glares, darken blue skies, and see better through reflective surfaces. (to learn more about polarizing filters check out our other article here!)

Solar ND - note that NOT ALL ND filters are solar certified for shooting. These are speciality filters that are diesnged to not ruin your camera sensor while shooting solar photography and solar eclipse/solar event photography. This filter not only filters visible light but also UV and IR Radiation as well. These do NOT provide protection to your eye, so do not use these in combination with your optical viewfinders ONLY use these while viewing through an electronic viewfinder or live view mode

Situational Use

There are various situations in which you can find yourself using an ND filter. Whether is be in portraiture, landscape, or achitectual photography these filter types are a common addition in people's gear.

Though portrait photography is possibly the last situation where an ND filter may be consided, it's especially important when looking to shoot with a wide aperture in the middle of the day to avoid unpleasant highlights. Wedding and portrait photographers will typically carry a 3-stop ND filter to give them the ability to shoot in sunlight with a wide-open aperture. Sometimes also carrying a 6-stop or 9-stop combination when needed as well.

For general landscape photography, you will find smoother bodies of moving water through long exposures and on the other hand, your skies will have whispier clouds and your greens will be like dreamlike forests. 6-stop ND filters are common as it is just enough to slow shutter speeds for smooth streams and waterfalls but 10-stops are also a popular choice in combination with a tripod mounted camera. This allows for extremely slow shutter speeds with wide apertures in bright sunlight and even night-photography like shutter speeds to get cool effects in urban and natural settings.

One of the more creative content you will see from ND filters is within architecture or street photography. At the right settings you can almost "erase" people from a scene due to their movement before the shutter opens and closes. If you even time it out or set it up just right - a ghostly scene can easily be made alongside some awesome light work or fog/fog spray.

ND Stops Translated to ND Factor

You'll find when shopping for Neutral Density Filters that manufacturers have a somewhat common naming convention across the board:

ND6, ND18, ND 64 etc or even ND 0.3, ND 1.8, ND 2.1

These naming conventions are an easy way to identify what filteration factor will be on that particular filter when applied to your lens. With that said, when creatives talk about exposures you'll find that we all talk in terms of "stops" which just make the transition between camera settings and filter factor easier in the grand scheme of things. A stop in photography either doubles or cuts the amount of light in half, so for example a "1-stop nd filter" would cut the light by 50%.

Once you understand this and apply it to filters, you can then see how a "10-stop nd filter" will be stopping the light by 10 halves in a row (this happens sequentially). So lets take a look at how ND filter factor relates to Stops in these fitlers as well as the optical density which are all common naming tools used from manufacturers.

ND in Stops	Optical Density	ND Factor
1	0.3	2
2	0.6	4
3	0.9	8
4	1.2	16
5	1.5	32
6	1.8	64
7	2.1	128
8	2.4	256
9	2.7	512
10	3.0	1024

ND Stops Translated to Exposure Times

Another common talking point about Neutral Density filters and long exposures will be how long your cameras shutter is opened for. Long exposures only affect your shutter speed so with a 12-second exposure time without a filter then added a "1-stop nd filter", you have effectively now halved the amount of light as previously mentioned therefore needing a longer exposure time to 24 seconds for your shot to be balanced.

One way to think about this in terms of your settings, once an ND filter is applied for long exposures you are looking at your other settings to create a proper balanced image. A common way to see this is with seascapes or landscapes.

Here are 10-stop examples where with 1 second exposure time, and an applied ND filter how much your exposure time converts into.

ND in Stops	Starting Exposure Time	Converted Exposure Time
1	1 second	2 seconds
2	1 second	4 seconds
3	1 second	8 seconds
4	1 second	16 seconds
5	1 second	32 seconds
6	1 second	64 seconds
7	1 second	128 seconds
8	1 second	256 seconds
9	1 second	512 seconds
10	1 second	1024 seconds (17 minutes)

Conclusion

While both charts above are not a full view of all ND-stop types you will see available, it is a great reference point in the field when looking to calculate what you have in your gear to get the final shot you want.

With a variety of wide-aperture lenses as low as f/1.8, ND filters allow you to keep your shallow depth of field while helping balancing your shot for overexposed scenes. Even with the technology of electronic shutters and the previously unattainable shutter speeds - you will find yourself in a circumstance where an ND filter will help you achieve the image you want.

ND filters are powerful tools, and knowing when and how to use them effectively can expand your creative abilities to take your photography to a new level. We hope this introductory guide was helpful and we would love to provide further information or recommendations to which filters may be best for you!