If you follow any of the drone forums, you’ll often see discussions around using ND, or Neutral Density filters on aerial cameras. TDC tester Pawel Dwulit recently purchased a set of Polar Pro filters, and we thought this would give us a chance both to discuss why such filters are useful – and also to produce a video so you can clearly see the difference.
In a nutshell, ND filters reduce the amount of light that reaches your lens (and your sensor). They are often used in still photography when a photographer wants to dramatically reduce shutter speed in bright conditions. So, for example, when you see those daytime shots of waterfalls where the water appears to be a continuous stream of froth, you can bet an ND filter was on that camera. Basically, the higher the ND number, the greater the equivalent aperture stop reductions:
- ND 4 = two stop reduction
- ND 8 = three stops reduction
- ND 16 = four stops
- ND 32 = five stops
The same filters can also be tremendously useful in video productions, and most people who are serious about aerial photography or videography eventually wind up purchasing a set. Pawel is a pro, so it was only natural – especially since the Mavic Pro has a fixed f/2.2 aperture. Your only options for changing the amount of light are ISO settings (which you’ll want as low as possible on a bright day) and your shutter speed.
“The ND filters allow you to maintain more of a cinematic motion blur to the footage,” says the TDCs Dwulit.
The general rule of thumb is to try to shoot at 24 frames per second, or fps. This gives more of a film-like look, while 30 fps produces more of a “video” style of footage. But the shutter rate you select with that 24 fps plays a huge role in the overall look of the final product. If it’s too high, it can give the footage almost a brittle look.
On sunny days, that’s a problem. In the absence of an ND filter, the shutter rate will jump into high gear in auto (or you’ll be forced to crank it manually), in order to obtain the correct exposure.
THERE’S A FORMULA
Really, there is: The ideal shutter rate is half the inverse of the shooting rate. So if you’re shooting at 24 fps, the inverse is 1/24. Half of that inverse is 1/48.
“Most cameras don’t have a 48 shutter setting, so most people go with 50 or 60 so that the footage has a distinctive cinematic motion blur,” says Dwulit. “Otherwise the motion seems to have a staccato effect, or a very sharp, crisp look.”
When we flew this test on the weekend, it was a bright sunny day, and there was snow on the ground reflecting back up at us. You could not have asked for a prettier day – nor a more demanding one for the Mavic Pro to cope with on its own.
Pawel took the machine up a total of four times:
- With no filter and just the stock lens
- With a circular polarizing filter (though we flew it just once and in a single position)
- With an ND 8 filter
- With an ND 16 filter
The results? Well, the day was so bright we likely could have used an ND 32 filter (which would have allowed in half the light of the ND 16), but Dwulit says the results were clear.
SEEING THE DIFFERENCE
“The ND 8 just automatically has almost a smoother feel in the footage,” he says – despite the fact the shutter was still at a very high rate.
“I think we were still at 2400; it wasn’t that much lower, but it looks way better. Only with the ND 16 did we get to 1/125, which still isn’t ideal, but really the 32 would bring you down to 60 which would be ideal.”
The Polar Pro sets come in a few variations:
- The three-piece set Pawel used, for $49.99
- A six-piece kit for $99.99
- A “Pro Vivid” 3-piece for $79.99
- A “Cinema Series” 6-pack, $149.99
DJI has finally got into the game and is producing its own three-piece set. It’s available for special order, with 3-5 weeks delivery as of publication date.
Finally, if check around some of the forums people also have positive things to say about the offerings from the Taco-RC store.
They offer a four-piece set, which includes the ND 32 that Pawel could have used the other day, for $64.99. You can find that set here.
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