What is an ND filter? Quick Guide to Neutral Density Filters

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An ND (Neutral Density) filter reduces the amount of light entering the lens without altering the image’s color. This allows photographers and videographers to use wider apertures or slower shutter speeds for creative effects, such as motion blur or shallow depth-of-field in bright conditions.

This guide teaches you how, why, and when to use ND filters and discusses the pros and cons of ND filters, focusing on videography and filmmaking.

What is an ND filter?

ND is an abbreviation of “Neutral Density”. ND filters filter out a certain amount of light to reduce the exposure. That way, ND filters are like sunglasses for your camera lens and sensor.

Because of this, ND filters are a handy tool for video – especially if you’re filming in bright sunlight – because they make it possible to capture footage even at fast apertures and shallow depth-of-field without overexposing the image.

Why photographers use ND filters

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An ND filter is a great tool for taking long-exposure shots in bright daylight, e.g., to achieve the silky smooth look when capturing a waterfall.

If you’re a photographer, ND-filters is an incredibly creative tool to capture long-exposure photographs.

So, if you’re looking to capture that silky look of waterfalls, the glass-like look of lakes, or blurry streaks of clouds, an ND filter is the way to go.

How does an ND filter work?

ND filters stop a certain amount of light from passing through, which can be written differently depending on the manufacturer.

Before I get into the semantics, let’s talk about the general way to know how much light the ND filter stops – which is called “stops.”

The amount of light passing through photographic lenses is described in f-stops.

Here, you can see a typical range of f-stops for a lens:

How aperture is related to the depth of field

The wider the aperture, the more light is let through the lens.

Wider apertures are described with lower f-stop numbers.

Lower f-stop numbers also equal a shallower depth of field.

If you’re new to apertures and want to learn more about exposure, apertures, f-stops, and t-stops, I would recommend you start by reading my guide, How to Choose the Right Camera Settings for Video Production, which will guide you through all the basics.

Each lens f-stop halves or doubles the amount of light reaching the sensor.

For example:

  • A lens with an f-stop of f/1.4 will allow twice the amount of light through compared to a lens with an f-stop of f/2.0.
  • A lens with an f-stop of f/2.8 will allow half the amount of light compared to a lens with an aperture of f/2.0.

Each ND filter stops half or doubles the light reaching the sensor.

For example:

  • An ND filter of three stops will allow only half the light through compared to an ND filter of two stops.
  • An ND filter of three stops will allow double the amount compared to an ND filter of four stops.

If you want to compare the amount of light that is let through a filter (fx two stops) to a filter with another number of stops (fx 7 stops), you must add each halving of the light sequentially.

ND filter in stopsAmount of light let through compared to no filter
0 (no filter)All light
1½
2½*½=1/4
3½*½*½=1/8
4½*½*½*½=1/16
5½*½*½*½*½=1/32
6½*½*½*½*½*½=1/64
7 ½*½*½*½*½*½*½=1/128
8 ½*½*½*½*½*½*½*½=1/256
9 ½*½*½*½*½*½*½*½*½=1/512
10 ½*½*½*½*½*½*½*½*½*½=1/1024

So, an ND filter of 10 stops will only allow 1/1024 of the light through compared to using no ND filter.

An ND filter of 10 stops will only allow 1/512 of the light through compared to an ND filter of 1 stop.

An ND filter of 10 stops will allow 1/256 light through compared to an ND filter of 2.

You can find ND filters that filter above 30 stops of light. You can also find regular ND filters that are measured to a fraction of a stop (like 13 1/3).

Variable ND filters without hard stops let you continuously adjust the light reduction between full stops.

With me so far? Great! Now, it gets a bit confusing.

What do the numbers on ND filters mean, and how do you read them

Okay, so let’s say you change your aperture from f/2.8 to f/1.4 but want to maintain the same exposure.

That’s two f-stops, which equals four times as much light passing through the lens to the sensor.

Okay, now all you have to do is grab the ND-filter in your bag, which says “ND-filter reduction of 2 stops of light”. Easy right?

Well, if only it were so easy!

Unfortunately, manufacturers have decided it would be awesome to use the terms ND filter factor or optical density instead, each corresponding to a specific number of stops.

You know, to separate the wheat from the chaff or something.

So here’s a chart for you, which compares the different numbers to each other.

Reduction of Light in StopsOptical DensityFilter FactorAmount of light let through compared to no filter
0 (no filter)000
1ND 0.3ND21/2
2ND 0.6ND41/4
3ND 0.9ND81/8
4ND 1.2ND161/16
5ND 1.5ND321/32
6ND 1.8ND641/64
6 2/3ND 2ND1001/100
7ND 2.1ND1281/128
8ND 2.4ND2561/256
9ND 2.7ND5121/512
10ND 3.0ND10241/1024

Optical Density and Filter Factor numbers are sometimes written with an “ND” before the number itself and sometimes not. For example, it might say ND2 – or it might just say 2.

So before you buy any ND filter, you need to read the fine print from the manufacturer carefully.

As you can see from the table above if the ND filter only says ND2, it could mean both 6 2/3 stops of light (optical density) or one stop of light (filter factor).

That’s quite a significant difference!

And to make matters a bit more confusing, an ND filter of 10 stops is also known as an ND1000.

P1177723
My Hoya ND1000 Neutral Density Filter. Here, I’ve blown out the sky to show how dark an ND10 is.

I’m not exactly sure why that is the case, so my best guess is that it has to do with the fact that ten stops of light correspond to a filter factor of 1024 (or 1/1024th of the unfiltered light). If you know the answer, please share in the comment section below.

Something that seems to support this case is that a six-stop filter is also known as an ND64 because it only lets through 1/64th of the unfiltered light.

Let’s hope the manufacturers will one day sit down in a room together and agree. I’m sure the day is just around the corner. Or maybe not.

Different mounting options for ND filters

When it comes to mounting a neutral density filter, there are five choices:

  • Screw-in filter (you screw in the filter using filter threads at the end of your lens)
  • Matte box (or slot-in in photography)
  • Magnetic ND filters (you mount a magnetic mount on your lens or matte box, and the filters attach magnetically for easy replacement)
  • Drop-in variable ND filters (sits between your camera and lens)
  • Clip-in. (sits in front of your mirror in DSLR cameras in the camera body itself).

Matte boxes are handy for quickly exchanging filters – or when you want to stack several filters (like an IR filter, a polarizer, and an ND).

Even though matte boxes are great in controlled environments like a studio or a film set, they are also big and clunky.

If you’re going handheld, e.g., for a documentary where you don’t want to draw too much attention to yourself, then a matte box is not the best choice as it screams film.

If you want a quick overview of different types of filters for video, I recommend you read our guide, The Five Types of Lens Filters You Need For A Cinematic Look.

Clip-in filters aren’t great either because they take a long time to remove and attach as you need to remove the lens first and then reach into the camera body to pull the filter out and replace it with another.

This leaves you with screw-in ND filters as your best option for run-and-gun scenarios.

If you’re shooting run-and-gun video, use a flexible ND filter mounting option.

I would recommend a Variable ND filter, a Drop-In ND filter, or a magnetic ND filter for such scenarios, and here’s why.

Even though I own several regular threaded screw-in ND filters, I’m not too fond that switching between them takes quite some time when I shoot video.

Regular filters can tend to get stuck in the filter thread and can be hard to remove and exchange, which means I might miss my chance to get the footage I want.

I prefer to work with Variable ND filters for most run-and-gun scenarios.

You can see some of the best Variable ND filters for video here.

Does An ND filter Affect The Image Quality?

The quick answer to this question is “no, but…”.

As the name implies, neutral density filters should be neutral in color and only reduce the exposure.

Unfortunately, that is not always the case. Cheap filters have nasty color shifts, shadows, or vignetting.

That is why it isn’t wise to cheap out on your ND filter.

Don’t get lured in by those cheap filters you find online. If that is all you can afford, it’s better not to use a filter and wait until you’ve saved up the money for a good filter.

Summary

An ND (Neutral Density) filter is a camera accessory that reduces the amount of light entering the lens without affecting the image’s color.

It essentially acts like sunglasses for your camera.

ND filters come in various strengths, which are measured in stops. Each stop reduces the light entering the lens by half.

Use Cases for Filmmakers and Photographers

  1. Achieving Motion Blur: By reducing the light entering the lens, ND filters allow photographers and filmmakers to use slower shutter speeds in bright conditions without overexposing the image. This is particularly useful for creating motion blur in landscapes, such as smoothing water or showing movement in clouds.
  2. Shallow Depth of Field: ND filters enable wide apertures in bright light, allowing filmmakers and photographers to achieve a shallow depth of field. This helps to isolate subjects from the background, a common technique in portraits and cinematography to direct the viewer’s attention.
  3. Time-Lapse Photography: Maintaining consistent exposure levels as lighting conditions change (e.g., during a sunset) can be challenging. ND filters help manage these changes, allowing smoother exposure transitions across frames.
  4. Cinematic Look: In filmmaking, adhering to the 180-degree shutter rule (which suggests the shutter speed should be double the frame rate) creates a motion blur that matches human vision. Bright environments can make following this rule difficult without overexposure. ND filters solve this by reducing light, allowing filmmakers to maintain the desired motion blur and depth of field.

Pros

  1. Versatility: ND filters enable shooting in various lighting conditions, expanding creative possibilities.
  2. Image Quality: They help prevent overexposure without affecting image color, maintaining high quality.
  3. Control Over Exposure and Depth of Field: ND filters offer greater control over exposure settings and depth of field, which is crucial for still photography and video.

Cons

  1. Additional Expense: High-quality ND filters can be expensive, adding to the overall cost of photography gear.
  2. Inconvenience: Using ND filters can be cumbersome, especially when changing lighting conditions requires swapping filters frequently. This is where variable NDs come in handy.
  3. Risk of Degraded Image Quality: Lower-quality ND filters can introduce color casts or reduce the sharpness of the image. It’s essential to invest in high-quality filters to avoid these issues.
  4. Adjustment Time: In dynamic shooting environments, adjusting to the right ND filter strength can take time, potentially causing missed shots.

In summary, ND filters are invaluable tools for filmmakers and photographers, enabling creative flexibility and control over images in ways often impossible using camera settings alone.

However, their benefits come with additional costs and considerations, including the potential need for investment in high-quality filters and the time required to manage their use effectively.

Up Next: 10 Indoor Lighting Secrets for At-Home Video Content

Author

  • Jan Sørup

    Jan Sørup is a indie filmmaker, videographer and photographer from Denmark. He owns filmdaft.com and the Danish company Apertura, which produces video content for big companies in Denmark and Scandinavia. Jan has a background in music, has drawn webcomics, and is a former lecturer at the University of Copenhagen.

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