Drone lens filters. How good are yours and can you trust them?

For almost as long as there have been cameras with lenses, there have been lens filters. Today, polarized filters are a favorite for their ability to cut down glare and improve the color saturation of pictures and video. Another modern favorite, neutral density (ND) filters, are used to cut down the light reaching the camera’s sensor. Reduced light allows for long exposure photographs and smoother, more cinematic, video. Despite the popularity of filters, trusted filter manufacturers like Tiffen, B+W, and Hoya have been slow to enter the drone market. As a result, start-ups like Polar Pro, Skyreat, and a host of Chinese accessory companies have flooded the market with drone lens filters. So what is a buyer to do? Can you trust these startups?

Maybe you can trust filters from the official DJI store. At the time of publication, no polarized filters were available on DJI.com. Here is a link to the Mavic 2 Zoom filters and the Mavic 2 Pro filters that I used in this test.

Don’t trust your drone filters

Filters of mediocre quality are not unique to drones, but they are certainly affecting drone pilots. Probably the most common complaint about drone lens filters is that they introduce aberrations that affect image quality. There are also reports of poorly-designed drone lens filters that don’t even fit the camera correctly. Often the orientation of polarized filters is not identified, or worse, the orientation is marked incorrectly.

Drones have another consideration that does not affect handheld cameras – drone cameras are on balanced gimbals. What does that mean? It means that if your filter is too heavy (or light) your drone may send a gimbal error and not respond correctly. Some drones, like the original DJI Mavic Pro ($999), DJI Spark ($399), and Parrot Anafi ($649) are more likely to have this problem. These drones were not designed with filters in mind.

The Parrot Anafi has no filter you can screw off. Instead, aftermarket drone lens filters are slipped on with a foam pad on the filter helping to secure it to the body of the camera. 

Drones like the Mavic Air ($799), Mavic 2 Zoom ($1,299), and Mavic 2 Pro ($1,499) are far less likely to have this problem. These three drones come with stock clear glass that can be removed. With these drones, you are removing the weight of the included glass before you add the weight of your drone lens filter.

Test your drone lens filters

There are a number of simple tests you can perform to evaluate the quality of your drone lens filters. First and foremost is the image quality test. This test is pretty simple. Find a well-lit scene with a lot of detail. Take photos at full resolution with and without the filters on. Crop the images and compare them closely to see if your filter is degrading image quality. The errors may be different across the filter, so make sure you check the middle and at least one corner when you compare. I did a 10X crop of the image below to compare the different drone lens filters for my Mavic 2 Pro and Mavic 2 Zoom.

As you can see, for the Mavic 2 Pro, there is some very minor blur that is introduced with the Mavic 2 Pro polarized filter. The ND filters for the Mavic 2 Pro all looked fine, as did the ND filters and the circular polarizers (CPLs) for the Mavic 2 Zoom.

Testing polarized drone lens filters

Polarized drone lens filters are often used by photographers to cut down on haze and glare. A polarized filter will increase the saturation of landscapes and often they help to add visual interest to shots over water. So how do you know if your polarizer is doing its job? Try rotating it when there is glare coming off of a puddle of water.

You can also try rotating a polarized drone filter in front of an LCD TV screen. LCD TVs almost always are polarized 90 degrees compared to how reflections come off of water. The TV should go black when the filter is rotated 90 degrees from “normal.”

The filters for my Mavic Air came without markings. If your filter comes with no orientation marking then you will need to use these tricks to mark them yourself. Use a Sharpie or a paint pen to mark the filter once you are sure the orientation is correct.

I marked the correct orientation on the Mavic 2 Zoom filters. Only the filter at top center is marked correctly by the manufacturer.

Even if your drone lens filters come marked, you should check them. Only one of the three polarized filters that came for my Mavic 2 Zoom was marked with the correct orientation.

Testing ND drone lens filters

Imagine this scenario: You line up the perfect shot for a video with you Mavic 2 Pro and find the shutter speed is 1/480 of a second. You want to drop it to 1/60. You hesitate to close the aperture by three stops to f/5.6 because you have discovered, like I have, that the Mavic 2 Pro is actually sharpest at f/2.8. So you decide to go for an ND8 filter to slow things down. You land your bird, change the filter, and take off again knowing you are ready for the perfect cinematic shot. You line up the shot again only to find that the shutter speed is at 100, not 60 like you expected.

What happened? Did you mess up the math? Did the sun just get brighter on this clear-sky day? No, the ND rating is incorrect.

Here is a table that relates ND filter number to the expected increase in exposure time. The math is really simple. Just multiply by the ND number. Calculating the equivalent change in aperture stops is a slightly more complicated matter. Not many of us remember how to do base 2 logarithms.

I measured the darkening power of a set of Freewell ND filters for the Mavic 2 Zoom and the Mavic 2 Pro by seeing how the cameras on these drones respond to the filters. I fixed the ISO at 200 and the f/number on the Mavic 2 Pro to f/2.8. What I discovered was surprising.

It turns out that the darkening of the Mavic 2 Zoom filters from Freewell is generally spot on. They all ring true, even the filters that are combined ND and polarized filters. Unlike the Mavic 2 Zoom, the Mavic 2 Pro filters did not fare so well. The ND 8 and 16 were both not as dark as they should have been. The ND16 is more like an ND10. Does this mean that the ND filters from Freewell are bad? Not really, it’s just that they are mislabeled. The moral of the story is that you need to measure your filters. Trust no one.

Drone lens filters inspection

In addition to testing polarization and ND rating, it is also worth taking a close look at the construction of your filters. My Mavic 2 filters from Freewell came with a nice clean glass that I can easily clean without damaging the coatings. I did notice that the threads on the Mavic 2 Zoom filters are not as smooth and shiny as those on the original cover glass from DJI. That said, the filters thread on and off without a problem. I have no issues using them on my Mavic.

The threads on the stock filter (left) look better, but the performance of the aftermarket filter is equivalent.

Like the Mavic 2 Zoom filters, the Mavic 2 Pro filters from Freewell assemble on and off camera really nicely. There have been some complaints that I have heard about other filters being difficult to get on and off. This was not an issue with the Freewell filters. The manufacture of the retention features looks to be identical to the stock parts. Freewell left off the steel inserts on the back side of the filters. This means the little ball plungers in the Mavic 2 Pro camera will rub directly on the anodized aluminum. The lack of a steel insert is probably a minor point, but it could become an issue if you change your filters in and out several times a day.

The metal frame of the aftermarket Mavic 2 Pro filter (left) is machined with excellent quality.

What can you do?

At the end of the day, you just need to check the tools you are using. My Freewell filters are far from perfect. Will I still use them? Yes, I will. I have confidence using these filters because I have marked my polarized filters and I now know the true ND value of all of my filters. I look forward to the day that Hoya or B+W enters the drone market. Until then, my Freewell filters get the job done.

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