Nothing screams “action cam footage” more than the stuttery, stroboscope-like motion caused by the high shutter speeds used to compensate for high exposure on sunny days, making it particularly difficult to match to other cameras. Read on to see how you can fundamentally change the viewer’s perception of your action cam footage with the use of some GoPro ND!
One of the first things you learn as a filmmaker is to use the 180° rule for your shutter speed, a techinque based on the shutter mechanism of old film cameras which effectively means that you should always use a shutter speed equal to double the reciprocal numerical value of your frame rate in order to achieve a smooth motion sequence (e.g. a shutter speed of 1/48s when shooting at 24 frames per second). The resulting motion blur of moving objects in the frame creates a smoothness that is an intrinsic part of the “cinematic look”, and it has a huge impact on the viewer’s experience.
This, however, poses a problem when using small action cams like GoPros: their fixed aperture (around f/2.8 in the Hero5 Black, for example) means that the only way to correctly expose the image is by adjusting shutter speed and ISO. F/2.8 on a bright sunny day in snow-covered mountains means the shutter speed can easily drop down to the neighbourhood of 1/2000 – 1/3000s, which translates to zero motion blur even for the fastest action scenes.
To bring the shutter speed down, you need to use heavy neutral density (ND) filtration. In our example above, this would be a 5-stop or ND32 filter (32 = 2^5). This reduces the shutter speed from ~1/3000s to ~1/94s, which is close to ideal for a typical action sports frame rate of 50fps.
Now, a quick online search for GoPro ND reveals that ND8 and sometimes ND16 filters are what’s mostly available, which is clearly not enough for our example above.
Here’s where the PolarPro GoPro Hero5 Black cinema series filters come into play. The set includes an ND8, ND16 and ND32 filter as a replacement for the front lens element (the HB5 lens cover), using multi coated glass in an anodized aluminium frame. You basically unscrew the original front lens cover on your GoPro Hero 5 Black, and replace it with the PolarPro GoPro ND filter.
The result is a nice, flare-free concept that works very well! I did have a little trouble, however, when fitting the GoPro ND as the tolerances are very tight, and you could almost break your fingers while attaching them. I have contacted PolarPro with regards to this problem, and they replied that all orders would be replaced with new filters automatically and free of charge, as there was indeed a tolerance issue. That is some really good customer service!
For my skiing video above shot in Flachau, Austria, I mostly used the ND32 filter, and the ND8 on the cloudy shots, which was not enough to produce motion blur. For these scenes, I should have used an ND16 – see if you can spot it!
As you can see, the filtration creates a nice motion blur, which in my opinion completely changes the character of the action cam sequences. The GoPro ND makes the footage look organic, helps the subjects stand out from a more blurred background, and on top of that the H.264 encoder has an easier job of distributing the limited bandwidth of ~60Mbit/s in the image to reduce macro-blocking. It certainly does not scream “action cam footage” any longer!
Since there are no exposure aids on the GoPro, it can a bit difficult to decide which ND filter to use. I used the manual shutter option and set it at 1/100s in order to match my 50fps, and tried different filters until I got a nicely-exposed image, judging only from the LCD back screen. I then swapped the shutter option back to AUTO, and set the max. ISO to 800 to allow for some automatic exposure compensation by the cam.
Using the linear field of view and the Karma Grip (which I reviewed in an earlier post here) offers the additional advantage of spending almost no time in post with image stabilization or optical correction issues. I did use the Protune flat settings, however, so I did add some saturation and contrast.
All in all, I can highly recommend the use of ND filters on your action cam as it truly changes the viewing experience and makes it much easier to match action cam footage with other cams. The PolarPro ND filters are just one option, with others including filter sets from Freewell, Holaca, Sandmark and others. In any case, make sure you choose filters that use high-level multicoated anti-flare glass!
GoPro settings for the video above: 2.7K 50p, linear field of view (no fisheye!), max ISO 800, Protune flat, lowest sharpness. The cam was used in conjunction with the GoPro Karma Grip Gimbal, and the time lapse sequences were shot using my Sony A7S II.