ND FILTERS GUIDE
How to use ND Filters with your Drone
In aerial photography and especially videography ND (Neutral Density) filters reduce the amount of light entering the lens without affecting the colors. ND filters can be clear, grey or almost black.
ND filters allow the operator to freely change important camera settings: the ISO, aperture and, in drone terminology most importantly, the shutter speed.
During a bright day the shutter speed of our drones camera has a very high value. This leads to a look that is being used in sports- and action films, but in almost no case in feature films and documentaries.
The footage will look very crisp, oversharpened and somehow stuttering. We know the look from blood splatters or racing cars who drive through splashing up dirt.
It is our goal, as aerial filmmakers, to add some motion blur to the recorded footage.
How do we do that? We need to enter a lower shutter speed value. And that is the moment when ND filters get interesting.
They let less light enter the camera, which means that we can bring the shutter speed down without having a super bright, almost white, image.
The footage will look properly illuminated, while we can change the shutter speed of our drones camera.
FINDING THE CORRECT SHUTTER SPEED
Choosing a shutter speed is critical - in the eyes of people who don't know the maybe easiest calculation in the world.
There is a rule you can keep in mind that almost always applies (except when filming action scenes, extreme sports, et cetera).
2x Framerate = Shutter Speed
24 frames per second (fps) x 2 = 48 Shutter Speed.
But: 48 might not be available. What do we do? We choose the closest amount, in this case 50.
30 fps x 2 = 60 Shutter Speed
120 fps x 2 = 240 Shutter Speed
If you follow this super simple rule, your shots will look natural and a good amount of motion blur, neither to less nor too much, will be visible.
But we're not done yet. There is one more thing to consider: there are plenty different ND filters available (f.e. ND4, ND8, ND16, et cetera).
WHICH ND FILTER STRENGTH SHALL I USE?
Answering this question is a little more tricky. There are no exact ND filter values that are always correct in certain situations.
It often is about trying on location, it depends on the actual scenery and strength of the light.
We could say: the filter that allows you to set the correct Shutter Speed while achieving a nicely illuminated image is perfect.
But before you get stressed out, let me share this little graphic with you that is based on my experiences (and I do have quite some experience).