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Filters and Capturing Images with Drones

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In these days of digital post production, filters are often overlooked as an image-control tool. But there are times when filters can be extremely useful, especially when capturing images from a drone. Common filters available for done cameras include UV, which are typically used as “protective filters,” neutral density filters (ND), which reduce light levels, and circular polarizer filters (CP), which reduce glare and have other properties. Unlike typical screw-on camera filters, filters for drones, in many cases, need to be made for the specific camera with which they will be used to avoid throwing off the balance of the gimbal on which they're mounted.

UV Filters

Technically, UV stands for “ultraviolet,” meaning UV filters block ultraviolet-wavelength light headed for the lens. Though not part of the visible light spectrum, camera sensors and film are sensitive to UV, with excess UV exposure reducing image contrast and creating haziness. In this respect, however, UV filters are obsolete because modern lens coatings typically filter most harmful UV out. Apart from UV, UV filters are transparent to visible light—they are a type of “clear” filter. This makes them popular as a “protection filter”—an extra barrier of glass between your expensive-to-replace lens and the world at large.

DJI UV Filter for Phantom 3 Professional / Advanced

In consumer applications, it makes sense to leave the UV filter on all the time. If image quality is your main concern, though, you should only use a UV filter when needed. In the real world, no filter is perfectly transparent. The more layers you put between the image sensor and what it is capturing, the more potential there is for quality reduction. Inexpensive filters that don’t have good coatings can also produce unwanted effects, such as glare and ghosting. Therefore, you have to weigh the risks of lens damage against any negative impact on image quality.

ND Filter

The ND, or neutral density filter, is probably the most useful filter in the aerial imagist’s kit. An ND is pretty much a sunglass lens for your camera, reducing light levels without altering color or having any other side effects.

Snake River Prototyping i1 Series Graduated ND16-8 Filter for DJI Zenmuse X3

NDs provide an optical means of controlling exposure. To refresh, a typical camera has three ways of controlling exposure:

  1. f/stop (aperture)
  2. Shutter speed (“shuttle angle” on cinema cameras)
  3. ISO (“gain” on video cameras)

Each of these has side effects. Changing the aperture changes depth of field. Changing shutter speed affects motion blur. Setting ISO far from the camera’s “native” ISO can increase noise levels. In addition, many aerial cameras have fixed apertures and limited or non-existent gain adjustment, leaving you with shutter speed as your only option.

Since more light requires an increase in shutter speed, you might think too much light wouldn’t be a problem. The faster the shutter speed, the less blurry the image. While you might think we don’t want blurry images, in reality, when shooting video, you do want some motion blur. One of the major breakthroughs in stop-motion animation was the invention of artificial motion blur. Images that lack blur have a “video-y”, uncinematic quality. As a rule of thumb, you want the denominator of your shutter speed to be 2x the frame rate. So, for a 30 fps (30p/60i) frame rate, you want a 1/60-second shutter speed. For 24 fps (24p/48i) you want a 1/48-second, or as close as your camera gets.

Using ND filters allows you to retain a natural motion blur in bright conditions. Also, image sensors may not like excessive light levels, so lowering the amount of incoming light to a nominal level can also improve overall image quality.

NDs come in a range of strengths that, confusingly, are measured in one of several ways.

  • ND number notation (e.g., ND 1.2)
  • ND number notation (e.g., ND16)
  • Light reduction in f/stops (e.g., 4-stop)
  • Lens-area opening, as fraction of the complete lens (e.g., 1/16)

This can make it hard to decide what strength to use. With a normal terrestrial camera, you can often “stack” ND filters on top of each other to increase the strength, but stacking may be ill-advised with a drone camera (see “Why Stacking is a Bad Idea” section below). Therefore, you need to pick out the right filter or make sure you have a collection on hand. Drone filter maker Polar Pro provides a helpful chart for picking NDs and other types of filters. Regarding NDs, Polar Pro suggests the following.

ND number

F/stop reduction

Use Case

ND4

2-stop

At dusk or dawn or where the shutter speed would otherwise be at 1/250 second

ND8

3-stop

Cloudy conditions or where the shutter speed would otherwise be at 1/500 second

ND16

4-stop

Conditions where the shutter speed would otherwise be at 1/1000 second

ND32

5-stop

Conditions where the shutter speed would otherwise be at 1/1250 second

ND64

6-stop

Under extremely bright conditions

Image taken with the Polar Pro Cinema Series, 4-Stop Neutral Density Filter – (ND16)

Polarizing Filters

Polarizing filters are mainly considered “glare-reduction” filters. Since reflections tend to be inherently polarized, they can be eliminated only by passing light with the opposite polarization of the reflection. The catch is, the filter has to be rotated to the correct angle to achieve the desired effect. Otherwise, it may enhance the reflection! This fact has tended to limit the polarizer’s usefulness in video applications, where both the camera and, potentially, the glare-producing subject can be moving. However, with careful composition, polarizers can be used fruitfully in video.  

Image taken with the Polar Pro Filter, Circular Polarizing Filter– (CP)

They also offer another benefit when shooting outdoors. Polarizers can help boost vividness where you have clouds against a blue sky. Additionally, polarizers act like ND filters in that they reduce overall light levels slightly. However, for bright settings, this may not be nearly enough, so you may need a hybrid filter (discussed below).

A common confusion is that “circular polarizer” (CP) means “rotating” polarizer. Polarizers can be circular or linear, but these terms have nothing to do with the shape of the filter or whether it rotates. It has to do with the physics of the polarization, though the end result is the same. Linear polarizers (LP) are incompatible with certain SLR autofocus technologies, which is why filters used for video and photography virtually always use circular polarization.

Why Stacking is a Bad Idea

Drone cameras are typically mounted on a gimbal, which needs to be precisely balanced. Some hacks involve taping coins to the back of the gimbal to counteract the added weight of multiple filters. I would not recommend this. While the gimbal may appear to operate normally, the incorrect balance can cause the servo motors to wear out faster. Therefore, unless the filter maker explicitly sanctions doing so, I would only use one filter at a time on a drone camera or any gimbal-mounted camera where the gimbal cannot be rebalanced and recalibrated by the end user.

The other problem with stacking is that drone cameras tend to use wide-angle lenses. Stacking filters can lead to vignetting or even subtle but annoying optical falloff. 

Hybrid Filters

Since stacking filters isn’t always possible or desirable, what does one do when more than one filter type is needed? This is where hybrids come in. For drone filters, the most common hybrid is the ND/PL. This combines an ND filter of a given strength with a polarizer.

Polar Pro ND32/PL Filter for Zenmuse X3 Gimbal Camera

Conclusion

In the age of digital image manipulation, it can be easy to overlook the enduring usefulness of filters in many situations. In the case of aerial imaging, this applies especially to ND and polarizing filters—ND filters because they allow you to record video with natural-looking motion blur and, by lowering the shutter speed, reducing the appearance of the rolling shutter “jello” effect. Polarizers reduce glare and can enhance daylight scenes. The UV, though not needed so much for its filtration properties, is a protective barrier between your expensive-to-replace lens and the world. Because aerial cameras frequently employ carefully balanced gimbals, any old screw-on filter probably won’t work. Instead, you will need to look for custom filters made specifically for your aircraft or gimbal camera.

Which filters do you prefer for shooting with your drone? Share with us in the Comments section, below.

3 Comments

any filters for astrophotography usage on drone? i.e. the UHC or the LPR filter

Are there any filters available for Solar Eclipse vieweing?  Specifically that fit the mavic?

Unfortunately there are no filters specifically designed for solar eclipse viewing that are compatible with the Mavic. 

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