What are ND filters and why you need them on your drone

Recently I purchased a DJI Phantom 3 Advanced, and there are SO many possibilities when shooting with its camera that I soon discovered the need for ND filters to be able to get the full potential of the video capabilities.

ND filter for drone
Demonstration of the effect of a neutral-density filter - Photo courtesy Robert Emperley

First of all, WHAT are ND filters and WHY do you need them?

ND filters are dark glass or plastic layers you mount in front of the lens of your camera. ND stands for Neutral Density meaning that they do not change the colours perceived by the camera sensor, but they only reduce the brightness of the scene. A common way of explaining their role is:

ND filters are like sunglasses for your camera.

The reasons you might want to put sunglasses on your camera are basically two:

- The scene you are filming is very bright AND
- You are shooting video

If were talking about photography, when the scene is very bright you generally increase your shutter speed (unless you are trying to obtain particular effects, such as motion blur like in the following picture in which case there is nothing new for you here!).

ND filter blurred effect
Neutral density filters are used to achieve a "motion blur" effect with slower shutter speed - Photo courtesy JJ Harrison

If you are shooting video though, there is a certain recommendation you should keep in mind to obtain smooth video:


This means that if you are shooting at 25 fps your shutter speed should not exceed 1/50th of a second.

Now try that on a bright summer afternoon or a snowy landscape: everything will be burnt and you have wasted your time.
Then why not increase your shutter speed to bring back your exposure to 0? 
Well, the reason is that increasing the shutter speed produces a choppy video, and the results is very unpleasant to watch. 
To guarantee a more cinematic look, one of the "trick of the trade" is to keep a low shutter speed, which in returns produces smoother video, thanks to the MOTION BLUR effect.

I hope it appears now clear that if you cannot increase your shutter speed you have to reduce the light that enters the camera, and you do that with ND filters.

ND filters are referred to as "ND2" or "ND4", "ND8", and so on.
What does this mean? Basically the number indicates the lens area opening as a fraction of the complete lens...huh? Check the table below for better clarity:

ND Number notation Fraction of full lens f-stop reduction
ND2 1/2 1
ND4 1/4 2
ND8 1/8 3
ND16 1/16 4
ND32 1/32 5
ND64 1/64 6
ND100 1/100 6 2/3
ND128 1/128 7
ND256 1/256 8
ND400 1/400 8 2/3

After doing some research on the web, it appears that there are numerous companies offering ND filters specifically for the Phantom 3 here are some of them:

Snake River Prototyping, very good comments, screw on glass filters, $$$$
Rainbowers Pro set, probably the best out there, screw on glass filters, $$$
Polar Pro, good quality, screw on glass filters, $$$
Taco RC, good quality, push on glass filters, $$
DJI, good quality, screw on glass filters, $$$
Phantomfilters.com, good quality, film only, $

I was really intrigued by the "deluxe" Phantomfilters set, because it comes with four different filters, plus a circular polarised filter, all for $30 shipped.

Phantomfilters Nd filter review
The Phantomfilters.com set comes in an envelope with all the four ND and the CP filters inserted in transparent bags, glued to the envelope itself.

The installation, even it is very easy, it is the most complex of all the three types (screw on, push or insert the film) because you have to unscrew the UV filter, insert the selected ND filter and screw the UV filter back on.

DJI Phantom camera
The Phantom 3 camera, before removing the UV filter

DJI Phantom ND filter
The UV filter removed, and the ND16 filter about to be inserted

DJI Phantom ND filter mount
The UV filter reassembled with the ND16 filter installed

 Versatility is certainly one of the winning point, because the whole set is incredibly easy to transport and to store, and even if the filters are relatively easy to scratch, with a little care they will last you a long time.

I hope to have been able to shed some light to what an ND filter does, for us drone pilots, and if you have any question...you know where to find me!

In the meantime, happy landings!
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