If you are new to the world of drones and aerial cinematography, one topic you might not be familiar with is ND filters. But what exactly are ND filters, and are they a necessary piece of equipment for a drone videography setup? Should you always fly with an ND Filter?
In this guide, we will teach you the basics of ND filters so you can approach aerial cinematography with confidence.
What Are Drone ND Filters?
A neutral-density filter, or ND filter, is a small piece of darkened glass that is placed over the drone camera’s lens. ND filters have long been utilized by photographers, but with the advent of the camera drone, ND filters have naturally come into vogue in the drone community.
Any given set of ND filters will consist of multiple filters that range in darkness or light filtering capability. ND filters are rated by the amount of light they filter. A typical set of ND filters might range between ND4 and ND64, with the higher rating filtering more light. There are even filters as high as ND1000 that are ideal for long-exposure photography.
How Do ND Filters Work?
In layman’s terms, ND filters act as sunglasses for the drone’s camera. They restrict the amount of light that enters the drone’s camera. The darker the filter, the less light that makes it through. The amount of light that ND filters restrict correlates with something the photography and videography community calls f-stops.
From a technical standpoint, an f-stop is the ratio of a camera lens’s focal length to the diameter of the entrance pupil. Confusing, right? In plain English, a more potent ND filter means more f-stop reduction which means less light enters the camera. For example, an ND16 filter results in 16 f-stops, whereas an ND 32 results in 5 f-stops.
Why Do You Need ND Filters For Your Drone?
At this point, you might be wondering if ND filters are actually necessary. Technically, ND filters are not necessarily required per se; the drone’s camera will indeed function without them, and in many cases, great video footage can be captured without an ND filter. The average drone pilot that just wants to capture amateur aerial footage can get by without ND filters.
However, the quality of the video captured will be limited without the use of an ND filter, and we will explain why shortly. But first, we first need to understand what a camera’s frame rate is and how a camera’s shutter speed works.
A camera’s frame rate is the number of images it captures per second. The frame rate is expressed in frames per second. So, when a drone camera is set to 30 frames per second, the camera will capture 30 still frames each second.
When a camera is recording video, the shutter is closing and opening rapidly to capture information. After all, a video is composed of a series of photos. Shutter speed is measured in fractions of a second. For example, when the shutter speed is set to 1/800, the shutter opens and closes more rapidly than when the shutter speed is set to 1/250. The lower the shutter speed, the longer the camera’s sensor is exposed to light.
A slower shutter speed typically results in video footage that better mimics how we see things in the real world, whereas faster shutter speeds typically result in video footage that looks choppy and unnatural.
The 180 Degree Rule
Now that we have established what frame rate and shutter speed are, we need to understand how these two variables intersect and impact the video footage, and, most importantly, where ND filters come into play.
When setting shutter speed and frame rate, videographers use the “180-degree rule.” Without getting into the technical weeds of this topic, according to this general rule of thumb, the shutter speed should be twice the frame rate in order to achieve the most natural motion blur.
Basically, after a lot of experimentation with different frame rates and shutter speeds, videographers that know much more than we do conclude that this combination yielded the best, most realistic video.
So, if you want to shoot video at 30 FPS, the shutter speed should be set to 1/60 for the most cinematic footage. Likewise, if you want to shoot in 60 FPS, the shutter speed should be set to 1/120.
The problem, however, is that manually adjusting the frame rate and shutter speed in accordance with the 180-degree rule almost always results in video footage that looks washed out and overexposed. This is especially true on bright sunny days.
To see what we mean, go outside, and power your drone on. Change the exposure setting to automatic. The camera will automatically adjust the shutter speed to reduce the amount of light entering the camera. If it is bright and sunny outside, the shutter speed will be extremely high, which, as mentioned, is less than ideal for cinematic video.
Now, manually adjust the frame rate and shutter speed to 30 FPS and 1/60, common drone videography settings. The display got super bright and washed out, right? That is because the shutter speed is set to a relatively slow speed, so a lot of light enters the camera.
This is where ND filters come in. An ND filter will filter the excess light entering the camera, allowing you to shoot with the desired frame rate and shutter speed without the footage being overexposed.
How Do You Use ND Filters?
To use an ND filter, you first need to determine which filter is needed for the specific conditions. PolarPro used to have a great app that could determine the best ND filter to use, but the app has been discontinued. With that said, PolarPro’s website has a helpful table that will give you an idea of which filter is best for different conditions.
After you select the filter you think you need, secure it to the drone’s camera. This process will vary slightly depending on the particular drone.
Once the filter is attached, power the drone on, change the exposure setting to manual, and select the desired frame rate and shutter speed. If the display looks good, proceed with your preflight checklist, get the drone in the air, and capture some incredible footage.
If the display is underexposed, power the drone off, and swap the filter out for a lighter one. If the display is overexposed, swap the filter out for a more potent one.
Should You Always Fly With An ND Filter?
So, where does this leave us? Is it advisable to always fly with an ND filter? Well, it really depends on personal preference. There is certainly an argument to be made either way.
In short, the only time you should fly with an ND filter is if you are going to be taking aerial videos and if you want to capture realistic motion blur, or if you want to take long-exposure daytime photos. If you just want to take some normal photos and fly for fun without taking the process too seriously, an ND filter is not necessary.
However, even if you do not plan on capturing cinematic video footage or taking long-exposure photos, it still might be a good idea to fly your drone with an ND filter on just in case you happen to see an epic sight that you just have to record. If installing an ND filter prior to each flight is no inconvenience to you, you might as well always fly with one.
But for other people, it is simply too big of a hassle to install an ND filter before each flight, especially for drone pilots that want to get the drone up in the air as quickly as possible. After all, putting an ND filter on takes time, time that your drone is on the ground and not in the air.
It is also worth noting that, if you are using a drone for commercial purposes that do not involve capturing video (like mapping or visual inspection), an ND filter is not needed.