There is a lot of misunderstanding regarding ND filters, and what they are actually used for on drones. Here’s a brief 101 regarding ND filters and their use on drones. I hope my experience can help you answer your questions.
Why ND Filters are Necessary
Video is a series of still images played back to back in quick succession. Each image has its own exposure. Brighter conditions require shorter exposures. At noon, your exposure will likely hang around or be shorter than 1/1,250th of a second long.
If you’re recording 30fps video, and each frame’s exposure lasts only 1/1,250th of a second, then there is a huge gap between when the picture was taken (beginning of the frame), and the following frame of video (1/30th of a second later). Things in the scene have moved. YOU have moved. That motion is not captured, and your video will appear to stutter. Think of a strobelight effect. This is where an ND filter comes into play.
An ND Filter will lengthen the exposure of each video frame by darkening the light striking the camera sensor. Going back to our 1/1,250th exposure example here, placing an ND32 filter will lengthen the exposure to about 1/60th of a second. This is the ideal shutter speed for each frame of a 30fps clip. Natural motion blur is introduced to each frame since each frame is exposed for more time, and each frame will now blend together smoothly instead of appearing jittery or with frame stutter.
The 180° Shutter Rule
This method of lengthening the exposure to (framerate x 2), is called the “180° shutter rule”. It is the industry standard for exposing video. If you’re shooting 30fps, expose at 1/60th sec. If you’re shooting 60fps, expose at 1/120 sec. If you’re shooting 120fps, expose at 1/240sec, etc. Following this rule will improve your cinematic footage dramatically.
There are also 90 degree rules (commonly used in action scenes, 30fps at 1/90 sec I believe), and 360 degree rules (dreamy effect, 1/30 sec, very blurry), but in general, you see the 180 degree rule implemented. The math is easy, just double your frame rate.
This isn’t an exact science though. At elevations beyond about 100 feet, perceived motion is lessened due to the distance from your subject and your ultra-wide drone lens, and likewise if you’re within 20 feet of a subject, motion and frame stutter is much more apparent. If your shot is beginning and ending further than 100 feet then a shorter shutter speed can be acceptable. I believe you should get as close as you can to the 180 shutter rule, but don’t panic over an exact number – especially at extended distances.
A Use for Still Images?
If you’re shooting still images on a drone, an ND filter is likely not necessary.
They will not improve image quality – in fact it will likely degrade image quality if you do not have high-quality filters.
They can smooth out motion, but likely only down to (about) 1/30 sec.
Still images can appear blurry, because that’s the purpose of these filters – to introduce motion blur. Despite the stabilization, it is never 100% tripod status rock-hard sturdy.
Motion blur can still be a great asset to an image of course. The upper left image was taken at 1/4 second. Note how smooth the water appears! This effect CAN be performed on still, calm days using filters like ND64, ND128, and ND256. However, the longer your exposure on a moving, floating machine, the more likely your image will not come out clear and sharp. There’s a trade off here, that can only be determined by your current light and weather patterns during the shoot.
Extremely long exposure photography is generally performed with ND1,000 filters, not ND64 or even ND128 filters. If you want dreamy images with lots of blur to demonstrate a long passage of time, forget it. Your drone isn’t stable enough even on the clearest and calmest of days to do so.
What about CPLs (Circular Polarizer filters)?
If CPLs weren’t included in most drone filter kits, I wouldn’t even bring them up. However since they are, you should know that I don’t recommend using a circular polarizer on any drone, at any time, whatsoever. By nature, CPLs don’t pair well with wide angle lenses and either introduce uneven banding into the sky, or will polarize only portions of a surface (like a lake, stream, or wet leaves). This leads to odd polarized/non-polarized patterns in the image that are very unnatural and unpleasing to the eye. Drone lenses are ridiculously wide and suffer these effects immensely as a result.
It’s not a question of the quality of the filter, rather this is a light physics problem. In general though, if a lens is wider than a 90° view of the sky, then the effect will worsen as the view gets wider from there. One edge of your frame may face the sun and be polarized, with the opposite edge being far from the sun. The effect of the polarizer will be seen as a transition from one side to the other and this looks awful in drone video footage and still images.
If you asked my advice, I would tell you to leave your polarizer in the box, or possibly even throw it in the trash. Using one on a drone is exceptionally difficult. While possible, it requires a massive amount of careful planning before your shot for what is in my opinion, an often negligible result. It is better to correct or compensate a lack of polarization during post production with a clean capture, than to repair a poor capture resulting from bad polarizer usage.
Many people disagree with my advice and love CPLs. I’m not part of that community (obviously). As with everything, experiment and practice to see what you like and what works for you.
ND Filters are a great creative tool, but on drones their overall intended use is to introduce the appropriate amount of blur during video capture. They make playback smooth, pleasant to watch, and stutter free.