Why a drone camera needs one
Exposure comes from three settings: aperture, ISO and shutter speed. Most drones have a fixed aperture, as DJI's New Zealand store points out, and GoPro cameras have no aperture setting at all. That leaves ISO and shutter. ISO is digital gain and adds noise, so you want it at the lowest value. Once ISO is at its minimum (100 on a GoPro, per Oscar Liang) and the picture is still too bright, the camera can only shorten the shutter. If you want to keep the shutter where you set it, the filter has to take the extra light away.
The 180-degree rule
The usual target is a shutter speed of one over twice the frame rate:
- 24 fps: 1/50 s (the closest common setting to 1/48)
- 25 fps: 1/50 s
- 30 fps: 1/60 s
- 60 fps: 1/120 s
- 120 fps: 1/240 s
Shutter angle is a term from cinematography, and DJI's New Zealand store notes that around 180 degrees has traditionally been the most common setting in cinema. That is the motion blur viewers are used to. Oscar mentions that 360 degrees (shutter equal to one over the frame rate, so 1/30 at 30 fps) is the maximum and gives even more blur, which many people find too much.
FPV is where the rule gets bent. Oscar writes that 180 degrees suits slow, relaxed cruising shots but can produce too much blur on fast, shaky FPV footage, and blurry frames do not stabilize well in software. For aggressive flying or when you plan to stabilize in post, he suggests 90 degrees (one over four times the frame rate: 1/120 at 30 fps, 1/240 at 60 fps) or even 45 degrees (1/240 at 30 fps). Design Tune Operate says the same about ReelSteady: stabilizing a frame that was blurred by fast movement makes the blur look wrong, so a 90-degree shutter is a safer start, and many cinelifter pilots stay there. According to the same source, most freestyle pilots stick to 180 degrees and no stabilization at all.
What the numbers mean
The ND number is the fraction of light that gets through. An ND8 passes 1/8 of the light, and every doubling of the number is one more stop:
| Filter | Light passed | Stops |
|---|---|---|
| ND4 | 1/4 | 2 |
| ND8 | 1/8 | 3 |
| ND16 | 1/16 | 4 |
| ND32 | 1/32 | 5 |
| ND64 | 1/64 | 6 |
One stop halves the light, so each stop lets you double the exposure time. That gives a simple way to choose. Set the frame rate, set ISO to its lowest value and let the camera meter the scene without a filter. Then count how many times you have to double its shutter time to reach your target. If the camera picks 1/400 and you want 1/50, that is 1/200, 1/100, 1/50: three doublings, so three stops and an ND8.
Some filters can be stacked, and the factors multiply. Oscar's examples are ND4 + ND4 = ND16 and ND4 + ND8 = ND32.
Starting points, and why charts disagree
Oscar's choices for a GoPro on an FPV quad:
- ND32 on snow
- ND16 on a sunny day
- ND8 on a cloudy day, or early morning and late afternoon in sun
- ND2 and ND4 rarely
- no filter just after sunset
He adds that a set of ND8, ND16 and ND32 covers almost everything.
Other guides publish similar tables, and Drone Companion points out what they leave out: the lens. Its example is the DJI Mini 4 Pro, whose fixed f/1.7 lens lets in roughly two and a half times more light than the f/2.8 lens on a Mini 4K or Air 2S, so it needs a stronger filter in the same light. A Mavic 3 can stop its aperture down to f/11 and often needs a lighter filter or none. A chart written for one camera can be a full stop off on another. Treat any chart as a starting point and check exposure on site before you take off.
The light also changes during a flight. Drone Companion's example is an ND32 that is right in full sun at takeoff and too dark once a cloud comes over. Carry two or three strengths and swap when the conditions change.
Lock shutter and ISO
Avoid auto shutter and auto ISO with a filter on. The camera sees less light and compensates: it lengthens the shutter, and Oscar warns it may also raise ISO, which only adds noise. The filter then does nothing useful. Set the shutter manually to your target and fix ISO at the minimum. In Oscar's GoPro settings that means ISO Min and ISO Max both at 100 and shutter 1/60 for 30 fps or 1/120 for 60 fps. If you prefer auto exposure, he recommends leaving the filter off.
If the image is too dark with the shutter at your target, the filter is too strong, and raising ISO to compensate brings noise back. If it is still too bright at minimum ISO, you need a stronger one.
Jello is a vibration problem
Jello, the wobble that runs through FPV footage, comes from vibration: poor PID tuning, a damaged frame or props, worn motor bearings. Oscar finds an ND filter can reduce it somewhat because the vibration is blurred together instead of showing as clear wobble, but the picture then looks soft. Find and fix the vibration rather than covering it with a filter.
Buying and fitting filters
- Camera drones. DJI recommends its own ND filters for its drones because they are designed for the balance of the gimbal camera.
- Colour and flare. Drone Companion warns that cheap filters can add a green or magenta tint. Oscar suggests checking reviews or test footage for flare when flying towards the sun, and looking at whether water beads off the coating.
- Fit. A filter that is easy to remove is convenient but more likely to fly off in a crash; a tight one stays on but is a pain to swap often. Oscar ties his to the camera mount with dental floss so a filter that pops off does not get lost in the grass.
- Polarizers. A circular polarizer (CPL) is a different tool. It cuts reflections from water and glass and deepens a blue sky, and it also takes away 1-2 stops, so a combined ND/PL filter or a CPL used with an ND needs a lighter ND than usual. Oscar notes that CPLs suit static or slow-moving shots better than fast FPV flying.
On an FPV quad a filter has one more use: Oscar points out it also protects the camera lens in a crash.