ND Filter Tips - How to use ND filters
Tips for using ND filters (howto):
Using an ND filter can lead to some really nice, natural and smooth movement. To get the most out of an ND filter, we need to be thoughtful and careful about how the camera is set up.
Other than protect the lens, ND Filters help us do 2 things:
- Reduce the jello effect caused by vibrations and a rolling shutter
- Give the video a more cinematic and natural look that is similar to how our eyes see while in motion.
(You can read a more in -depth description of what ND filters do and how they work right here - it's worth a read!).
It does both of these things by introducing a small amount of motion blur. Motion blur is a result of the shutter of the camera being open longer per frame because it is trying to make up for the fact that the ND filter is blocking some of the light. Because the shutter is open longer, the video is “smeared” a small amount, which we call motion blur. The faster something is moving in the frame (usually at the edges), the more blur you will see.
- An ND 4 filter lets through ¼ of the light
- An ND 8 filter lets 1/8th of the light
- An ND 16 lets through 1/16th of the light
- An ND 32 lets through 1/32nd of the light
The Easy way (“Set and forget”)
You can leave your camera in auto mode with very good results. Just put an ND filter (ND4 or ND8 works great for this, or an ND16 only if you always fly in very bright sunlight) on your camera, and leave it. You would only need to remove it if you are flying in low light (e.g. sunset) or dark conditions. This will smooth out your video and reduce jello.
The drawback of this easy way is that the amount of motion blur may change during the flight if the light changes and your camera has to adjust the shutter speed (This happens regardless of having an ND filter or not, it is just less pronounced without).
The “Expert” way
The shutter speed is indicated in 1/seconds. A shutter speed of 60 means that the shutter is open for one 60th of a second. A shutter speed of 120 means that the shutter is open of one 120th of a second. So, because 1/60th of a second is twice as long as 1/120th of a second, twice as much light gets in.
Generally, to get a “cinematic feel”, we want to use a shutter speed of one over 2x the frame rate. So, with a frame rate of 30fps, we want a shutter speed of 1/60th of a second. (The shutter is open for exactly half of the time of a frame).
We can’t just set the fps to 30 and the shutter speed to 1/60 on a sunny day, because the GoPro would let in too much light and the footage will be mush too bright and overexposed. We would use a shutter speed of 1/120 or 1/240 of faster – but that wouldn’t give us the smooth motion blur that we want. Additionally, some GoPros (such as the session) do not allow us to use shutter speeds that are fast enough to properly expose the image. So, we use an ND filter to block some of the light so we can use that slower shutter speed.
To be able to best control the shutter speed, we need to put the camera into Protune mode. Here are some settings and process to start with, but ultimately you will want to experiment and develop your own method:
- Protune: on
- Color: Doesn’t matter for this, your preference
- White balance: Doesn’t matter for this, your preference
- ISO: 400
- ISO Mode: Max (Alternatively, you can set it to “lock” and 100 if you do not want the camera to adjust exposure in your flight or if the light will not be radically changing from going into trees or shade, into buildings, etc.)
- Shutter Speed: Set to whatever exposes the image properly (use your phone app to preview the image). If you are using the right ND filter, then the correct shutter speed should be 60 (assuming you are using 30fps, if you use 60, then set this to 120).
Use the following ND filter based on the light available:
- Cloudy/low light: ND4
- Partially cloudy: ND8
- Sun: ND16
- Very bright sun: ND32
Open up your phone or tablet and connect to your GoPro to preview – does the image look properly exposed? If it’s too bright, you need to use a stronger ND filter. If it’s not bright enough, use a smaller value ND filter. If you don’t want to have to pull off the ND filter and change it, you can always try changing the shutter speed up or down to the next setting (but of course this will also decrease/increase the motion blur - however it is more critical to get the exposure right, though!
Regularly check the exposure with the preview on your phone. If the light level changes (e.g. getting darker at the end of the day), you will need to either change the ND filter or use a slower shutter speed or your footage will be too dark. Eventually, you will get a good feel for the settings and this will become second nature.