For a filmmaker, crafting an image is far more than pointing a camera and hitting record. Making deliberate technical and creative choices is what makes an image yours. You are the author of the world you’re creating shot by shot. Deliberate choices require control, and that’s where a smartphone camera often needs some help.
If you’re new to video you may never have heard of an ND filter before. Maybe you’ve heard and read about ND filters, but never used them before. You might be a seasoned pro but are unsure how to mount ND filters on a phone camera.
There are many different ND filters available in different sizes, densities, types and prices. Some are specifically made for phone cameras, and some are standard round photographic filters that will need an adapter. It’s easy to be overwhelmed by choice, so I’ve put together this information to make it easier.
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.
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.
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.
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.
ND Filter FAQ
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.
|Optical Density||ND Number||Transmission||f-stop Reduction|
|ND 0.3||ND 2||1/2||1|
|ND 0.6||ND 4||1/4||2|
|ND 0.9||ND 8||1/8||3|
|ND 1.2||ND 16||1/16||4|
|ND 1.5||ND 32||1/32||5|
|ND 1.8||ND 64||1/64||6|
|ND 2.1||ND 128||1/128||7|
|ND 2.4||ND 256||1/256||8|
|ND 2.7||ND 512||1/512||9|
|ND 3.0||ND 1024||1/1024||10|
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.
|Lens||Separate Filter Adaptor Required||Filter size|
|Moondog Labs Anamorphic (All Versions)||Yes||52mm|
|Moment Tele Portrait||Yes||62mm|
|Zeiss ExoLens Pro Wide||No||40.5mm|
|Zeiss ExoLens Tele||No||40.5mm|
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 Polar Pro 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.
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.
For ND filter systems that work directly on the phone without a cage or case, take a look at the solutions below.
How do I know which 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.
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