- What is a ND Filter?
- Reflective and Absorbtive ND Filters
- Optical Density
- Variable ND Filters
- Avoiding Cross Polarization
- Situations Where I’ll Choose a Fixed Density ND Filter over a Variable ND Filter
- Why Use a ND Filter?
- IR Pollution and Color Shift
- Correcting ND Filter Color Shift in Post Production
- ND Filter FAQ
- What do the filter ratings mean?
- How do I know if a ND filter is good?
- How do I attach a filter to my external lens?
- I don’t use any external lenses, can I still use ND filters?
- How do I know which density ND filter to use?
- ND Filters are Essential Accessories
- iPhone ND Filter Reviews
- Further Reading
- Stay in Touch
ND filters give you control over exposure and shutter speed in bright light. A good set of iPhone ND filters are essential in any smartphone filmmakers kit.
If you’re new to video completely you may never have heard of an ND filter before. Maybe you’ve heard and read about ND filters, but never used them. You might be a seasoned pro but are unsure how to mount ND filters on a phone camera. Hopefully, this article will answer all those questions, and more.
If you’re looking to create cinematic video with your iPhone, you’ll need a solution to mount ND filters in front of your camera lenses in order to take control of your shutter speed in bright light. ND filters evenly reduce the light entering your camera, allowing you to set a slow shutter speed even in bright conditions. A slow shutter speed is what gives you the kind of motion blur you see in movies, that you don’t ever see in smartphone video… unless using an ND filter.
What is a ND Filter?
ND stands for Neutral Density. An ND filter uniformly reduces the amount of light passing through it in order to control exposure and shutter speed. A slow shutter speed is what gives you a film like smooth motion blur to objects in motion through the frame. It’s important that the filter reduces all visible wavelengths (colors) of light equally to prevent any color shifts in the recorded image. Some filters do this better than others, and none are perfect.
Reflective and Absorbtive ND Filters
There are two ways to reduce the light passing through a filter. You can either reflect the unwanted light away, or absorb it. ND filters can employ either of these methods. Reflective ND filters use metallic coatings to reflect a specific range of wavelengths. Absorbtive ND filters use the glass itself to absorb unwanted wavelengths of light.
ND filters come in different strengths, this is referred to as optical density. Optical density defines the amount of light allowed to pass through the filter. The higher the value, the less light is transmitted through the filter.
Good ND filters are made with the highest quality optical glass and coatings, so they tend to be expensive. It’s worth spending a bit more on filters that you can trust, and many come with a lifetime warranty. If you take care of them, they’ll take care of you for a long time.
Variable ND Filters
Most ND filters come in fixed densities, and so you need a set of filters to control exposure in different lighting conditions. This means physically changing the ND filter every time the light changes. However, there is another solution.
A variable ND filter allows you to change the amount of light that passes through the filter just by rotating it. They work differently to fixed density ND filters. A variable ND filter is made up of two stacked polarizing filters, one of them is fixed, the other is able to rotate. As light is polarized passing through the first filter, some of it is blocked from passing through the second depending on the angle of rotation. When both polarizing filters are aligned, light is allowed to pass through. When the front polarizing filter is rotated 90 degrees to the rear filter, no light will pass through. Between 0 degrees and 90 degrees the effective density can be changed.
The advantages of using a variable ND filter are not having to swap out filters all the time, and being able to adjust the density to any value needed. However, there are some important limitations.
Because of the way variable ND filters work, they are not suitable for lenses with a very wide field of view. They also introduce what is called cross polarization when the rotation gets close to 90 degrees. This creates a visible “X” shape in the image.
Avoiding Cross Polarization
Some manufacturers solve the cross polarization issue by splitting the density range into two separate filters. Each filter will have a smaller range of density, and no cross polarization. A typical split is a 2-5 stop filter and a 6-9 stop filter. This does increase cost because you need to buy two filters to cover the full range instead of one. But, you should consider that the highest densities of a single wide-range variable ND filter will likely be useless anyway because of this issue.
Situations Where I’ll Choose a Fixed Density ND Filter over a Variable ND Filter
Most of the time I use a variable ND filter, but there are situations where I’ll break out my set of fixed density ND filters instead.
- Whenever using a very wide angle lens attached to the phone camera (variable ND’s cause problems with very wide angle lenses).
- When using a lightweight smartphone gimbal. (I’ll use my Moondog Labs ND filters… only 2mm thick and super lightweight).
- When using a circular polarizer filter.
Variable ND filters are more expensive than a single fixed density ND filter, but you only need to buy one, or two instead of a set. I use the pair of PolarPro Peter McKinnon Edition Variable ND Filters and highly recommend them.
Why Use a ND Filter?
One of the factors that influences the look and feel of your video in a subtle but important way is how motion is recorded. A natural looking blur of objects in motion is important when you want your video to look more like motion picture film, or like it was shot with a cinema camera, and less like smartphone video. I go into this, and other factors in more detail in my guide to shooting cinematic video with your iPhone and FiLMiC Pro.
Motion blur is directly connected to shutter speed, and shutter speed is directly connected to exposure. A slow shutter speed gives you a longer exposure and more motion blur, but the longer exposure can mean an over exposed image.
In order to maintain correct exposure with a slow shutter speed in bright light, you have to reduce the amount of light entering your lens. This is exactly what an ND filter does.
IR Pollution and Color Shift
Camera image sensors are sensitive to more than just visible light. They can also pick up invisible light, especially infra red. Infra red light is emitted by many light sources, including the sun. Most camera manufacturers build IR filtration into their cameras, but some don’t.
If an ND filter only reduces visible light, but lets IR pass though unaffected, you may see an obvious color imbalance if your camera image sensor doesn’t have built in IR filtration. This is called IR pollution.
An IR pollution issue is easy to see. Shadows and dark areas of the image will look brown and muddy. Other colors will be affected as well. IR pollution is very difficult to correct in post production.
I haven’t seen a problem yet with IR pollution when shooting with a smartphone, so I assume most, if not all smartphone cameras have built in IR filtration.
Many ND filters are full spectrum filters in any case and are designed to reduce IR wavelengths along with visible light. Look out for the terms IRND, or full spectrum when choosing ND filters.
Correcting ND Filter Color Shift in Post Production
As long as an ND filter doesn’t introduce too much color imbalance, and you’ve set and locked white balance correctly in your camera app, it’s possible to correct color imbalances manually in post production. The only way to do this perfectly and consistently is to shoot a color chart, and use it to color correct in post. You can learn how to do this in DaVinci Resolve with my tutorial Shoot and Color Grade FiLMiC LogV2 with the X-Rite Colorchecker Passport Video.
It’s safe to assume you’ll always need to color correct when using any ND filter, which is a good reason to use a color chart such as the X-Rite ColorChecker Passport Video when shooting. It takes all the guesswork out of color correction in post.
ND Filter FAQ
I’ve tried to answer some common ND filter questions below. If you have a question which isn’t covered, please contact me and I may add it.
What do the filter ratings mean?
There are two common ways that manufacturers use to describe the densities of an ND filter.
One is with a ND number, such as ND2, ND4, ND8 etc. and this refers simply to the ratio of light it will allow to pass. A ND 4 allows 1/4 of the light to pass. This is common with photography filters.
The second is by optical density, this will be 0.3, 0.6, 0.9, 1.2 etc. which is most common for ND filters used in cinematography.
How do I know if a ND filter is good?
Unfortunately, you will rarely find actual transmittance data given by manufacturers for specific products. The two most important things that should be mentioned is minimal (or no) color shift, and an IR coating.
Typically the more expensive the filter, the better it will perform, although price isn’t always a determining factor. It is also worth looking for independent user reviews.
How do I attach a filter to my external lens?
Many popular iPhone lenses also have filter adaptors available. A filter adaptor may not come with the lens, and may have to be bought separately. Look for the filter thread diameter that matches the lens filter adaptor.
The links in the table below are affiliate links.
I don’t use any external lenses, can I still use ND filters?
There are now a few good ND filter systems available for mobile filmmakers that fit directly over the phone camera lens. I have been using the PolarPro Iris filter system for some time and love it. It even fits over the wide and telephoto cameras on the iPhone 11 Pro and Pro Max. Sadly, it seems that PolarPro are no longer making the Iris system.
However, they have recently launched the PolarPro LiteChaser Pro. The LiteChaser Pro is a case mounted filter system that accommodates a set of proprietary filters. These filters range from fixed ND filters to a variable ND filter, and a circular polarizer. They are available in various kits that include some filters, and the filters are available separately also.
If you have a metal cage or a case with a threaded lens mount, then you can use one or more step up rings to mount standard circular photographic filters over one or more cameras depending on the cage. A common size threaded lens mount is 37mm.
My favorite lightweight filter solution for the iPhone 11 Pro Max is the Moondog Labs Multi Camera Filter Mount. It provides a standard 52mm filter thread, and is compatible with all iPhone 11 models. It will cover all three cameras on the iPhone 11 Pro and iPhone 11 Pro Max.
How do I know which density ND filter to use?
When shooting video in bright conditions you will usually aim to keep your shutter speed value at double your frame rate. For example, if your frame rate is set to 24fps, you’ll aim for a shutter speed of 1/48th second.
You can calculate which ND will reduce your shutter speed easily by dividing the auto shutter speed value by two until you reach close to your desired shutter speed. Count how many times you had to divide, and you’ll know how many stops you need to reduce.
In bright direct sunlight, you will usually need a 6-7 stop reduction, which is a ND64 (Optical Density 1.8) or ND128 (Optical Density 2.1).
ND Filters are Essential Accessories
A set of ND filters are an essential part of your filmmaking kit. Being able to control shutter speed and achieve cinematic motion blur in your shots will bring a big camera quality to your smartphone videos.
If you’re interested in learning about other key technical and creative factors that will improve the cinematic quality of your videos, I recommend reading my guide to shooting cinematic video with your iPhone and FiLMiC Pro next.
iPhone ND Filter Reviews
I’ve started testing fixed ND and variable ND filters from various manufacturers. So far I’ve begun with the Beastgrip Pro Series ND, VND and CPL filters. I will also be putting my other filters through similar tests and writing up full reviews for each set. These reviews will eventually cover the Moment VND filters, the PolarPro Peter McKinnon Edition VND filters, Freewell VND filters, and the Moondog Labs (Nisi) ND filters. They will also all be added to my gear page. So bookmark this page if you want to come back later to check or better yet subscribe to my very occasional email updates.
- Beastgrip Pro Series ND, VND and CLP filter review
- Learn my method to nailing perfect video exposure using FiLMiC Pro and a variable ND filter.
Stay in Touch
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Please don’t hesitate to comment with your questions either here, on Youtube, or hit me up on twitter, I will always reply.