Auto Mode Options

When you tap on the Camera Mode icon at lower right in the screen to activate Auto mode, your choices are limited to those shown in the three screen variations illustrated in Figure 5.2. In all three, Air 2S owners (only) can tap on the AF icon seen at upper right of each variation to switch from AF (autofocus) to MF (manual focus). I’ll explain focus options later.

At the bottom-right corner of the screen are the three exposure controls available in Auto mode, labeled Storage, Format, and EV. When you tap on each of those labels, you gain access to the following adjustments:

• ■Storage. One or two icons shaped like an inverted microSDXC memory card appear. (See Figure 5.2, left, which shows the Air 2S screen.) If you don’t have a memory card inserted in the aircraft, a single icon representing the drone’s internal storage will appear. The icon has the same upward-pointing icon DJI uses to represent the drone itself in the map display. If a microSDXC card is installed, a second icon (lacking the “drone” symbol) will be shown.
  • The amount of total storage and available storage for each is shown next to the icon. The currently active storage location is highlighted in orange. You can tap the icon you want to use as the drone’s storage to switch between them. The total number of photos you can expect to take with the selected storage using the current format (in this case 153) is displayed under the Storage label.
  • The Mavic Air 2/2S each have 8GB of internal storage, which is generally enough for an average still photography session. Video clips, especially 4K movies, consume considerably more space, of course. If you expect to capture files totaling more than 8GB, or want to be able to swap cards or transfer to your computer with a card reader, you’ll definitely need to use a microSDXC card.
• ■Format. Click on the Format label, and the options shown in Figure 5.2, center, pop up. You can choose JPEG alone, RAW, or both JPEG and RAW. As I’ll explain shortly, JPEG files are the most compact, while RAW files allow more flexibility in post-processing. While saving both types simultaneously gives you the best of both worlds, storage is slower.
• ■Exposure Value. When you click the EV label, you’re given the opportunity to add or subtract up to three stops of exposure, in one-third stop increments, as seen in Figure 5.2, right. In effect, you’re telling the drone to use slower or faster shutter speeds to increase or decrease the amount of light reaching the sensor. I’ll explain exposure compensation shortly, too.

Pro Mode Options

When you tap on the Camera Mode icon at lower right in the screen to activate Pro mode, a much wider range of adjustments become available, giving you a greater degree of creative control. I’ve drawn a green box around the relevant labels in Figure 5.3 (the boxes do not appear on your screen). At center right is an AF label (available on the Air 2S only), which indicates that the camera is set for autofocus; tap it to switch to manual focus, in which case a “mountain” icon replaces the AF label. I’ll explain the Air 2S’ AF options later; for now I’m going to concentrate on the other camera settings within the three green boxes at the bottom of the screen in the figure. Here are the parameters available in the left-most box, shown in Figure 5.4, left:

• ■White Balance. You can trust your drone’s electronics to select an appropriate white balance using the Auto setting. You’ll find that works most of the time, particularly if you do most of your shooting outdoors in daylight. Alternatively, you can choose a color temperature manually (in Kelvin) from 2,400K to 10,000K. I’ll explain white balance in more detail later.
• ■Format. This entry functions like its Auto mode counterpart. Choose JPEG alone, RAW, or both JPEG and RAW.
• ■Size. Choose 16:9 and your still images will be cropped and saved in the same aspect ratio (proportions) as your video clips. That can be useful if you intend to mix video and still photos in a presentation and want to retain the same look. Or, you can choose to put the full image area of your sensor to work for your stills and select the 4:3 aspect ratio (Air 2) or 3:2 aspect ratio (Air 2S). This setting is not available in Video mode.
• ■Storage. In Pro mode, this option is used exactly as it is in Auto mode. One or two “SD card” icons appear, along with capacity information. Tap either the internal storage icon or the external storage icon (if installed) to specify where files should be saved.

Exposure adjustments are available from the center box (highlighted in green in Figure 5.3), shown in Figure 5.4, right:

• ■ISO Setting . Your Mavic can calculate an appropriate ISO sensitivity given your selected or automatically chosen shutter speed when this is set to Auto. You can also specify a fixed ISO in the range ISO 100 to ISO 12800. There are some drawbacks to using higher ISO settings, including increased noise, as I’ll explain later.
• ■Shutter Speed. You can choose shutter speeds from 1/8000th second to 8 full seconds, or allow the camera to select a speed for you by setting this to Auto. Believe it or not, your drone’s stabilization features are effective at shutter speeds much longer than you, a mere human, would be able to hand-hold a conventional digital camera. Because the Mavic’s camera has no adjustable aperture, you’ll be using shutter speed, ISO, and, at times, neutral-density filters to achieve optimal exposure. As you’ll learn, in video mode shutter speed changes are used less and ND filters more because faster shutter speeds produce unwanted jerkiness in movies.
• ■EV. When either Shutter Speed or ISO are set to Auto, or when both are in Auto mode, you can override their settings by applying exposure compensation, expressed as EV (Exposure Value) changes. I’ll explain EV in more detail later in this chapter.

Basics of Exposure

In the most basic sense, exposure is all about light. Exposure can make or break your photo. Correct exposure brings out the detail in the areas you want to picture, whether you’re trying to capture details of a construction site for aerial progress shots or detailing the range of colors in an overhead view of vibrant fall foliage, providing the range of tones and colors you need to create the desired image.

Poor exposure can cloak important details in shadow or wash them out in glare-filled featureless expanses of white. However, getting the perfect exposure requires some intelligence—either that built into the drone’s camera electronics or the smarts in your head—because digital sensors can’t capture all the tones we can see. If the range of tones in an image is extensive, embracing both inky black shadows and bright highlights, we often must settle for an exposure that renders most of those tones—but not all—in a way that best suits the photo we want to produce.

If you’re a photo enthusiast, you’re probably aware of the traditional “exposure triangle” of aperture (quantity of light and light passed by the lens), shutter speed (the amount of time the shutter is open), and the ISO sensitivity of the sensor—all working proportionately and reciprocally to produce an exposure. The trio is itself affected by the amount of illumination that is available. So, if you double the amount of light, increase the aperture by one stop, make the shutter speed twice as long, or boost the ISO setting 2X, you’ll get twice as much exposure. Similarly, you can increase any of these factors while decreasing one of the others by a similar amount to keep the same exposure. Of course, with your Mavic drone, only two of the three legs can be adjusted directly, as the aperture is fixed at f/2.8.

The traditional exposure triangle is shown in Figure 5.7. Working with any of the three legs (aperture, shutter speed, or ISO sensitivity), involves trade-offs:

• ■Aperture. Larger f/stops (the smaller numbers, like f/2.8) provide less depth-of-field, while smaller f/stops (larger numbers, like f/11 or f/16) increase depth-of-field. As I noted, you don’t have to worry about the effects of this leg of the triangle, since the Mavic Air 2 and 2S do not have an adjustable physical aperture. Yes, you can simulate its effect on the exposure with neutral-density filters, but filters don’t change depth-of-field.

• ■Shutter speeds. Shorter shutter speeds (like 1/250th or 1/500th second) do a better job of reducing the effects of camera/subject motion, while longer shutter speeds (such as 1/30th second) make that motion blur more likely.
• ■ISO sensitivity. Higher ISO settings increase the amount of visual noise and artifacts in your image, masking detail. Lower ISO settings reduce the effects of noise.

If you’re a photo enthusiast who has experience with conventional still cameras, you might mourn the loss of the options when an adjustable aperture is available. Since you are always shooting at f/2.8, reducing the intensity of the light reaching the sensor with a neutral-density filter is inconvenient, and can’t be done in mid-flight. You’ll need to plan ahead, or bring your drone back to home and mount the filter when needed. The fixed f/stop is a limitation, but at the same time it offers some advantages:

• ■Faster shutter speeds. Because the aperture is fixed at f/2.8, you’ll be using motion-freezing faster shutter speeds for still photography, eliminating blur even beyond what the drone’s image stabilization features can provide. At the lowest ISO setting, ISO 100, you’ll probably be shooting at speeds in the 1/1250th second range or faster, which will largely eliminate most blur from subject motion and from camera (drone) shake.
  • Even so, note that brisk shutter speeds are not an advantage when shooting video. As I’ll explain in Chapter 7, viewers are conditioned to see a small amount of blur as objects move from frame to frame, resulting in what is called a “cinematic” look. When individual frames are razor-sharp, however, the lack of transitional blur produces a hyper-realistic, jittery look that viewers find unsettling. For the best video quality, you’ll want to use neutral-density filters to reduce the amount of light reaching the sensor and allowing shutter speeds no faster than twice the frame rate (60, 30, or 24 frames per second).
• ■Depth-of-field. The range of sharpness of an image (depth-of-field) changes as the aperture is adjusted. Relatively larger f/stops, such as f/2.8, produce less depth-of-field than smaller stops, such as f/8 or f/11. Because your drone’s aperture never changes, the depth-of-field remains exactly the same shot to shot and video clip to video clip. In practice, that’s more of an advantage for movie shooting, where unwanted changes in focus plane and depth-of-field may be distracting. I’ll explain depth-of-field in more detail later.
• ■Less diffraction. Small f/stops cause some minimal sharpness lost due to a phenomenon called diffraction. “Small” is relative and is affected by the size of the sensor. I won’t bore you with diffraction limitation formulas, but the bottom line is that with a sensor the size of the one found in the Air 2S, diffraction starts to affect the image at about f/5.6 and smaller (larger numbered) f/stops. The effect is gradual as the aperture is reduced. The good news is that at f/2.8, there should be no image loss from diffraction at all. In this sense, your drone’s fixed f/stop is a very good thing.

Dynamic Range

Getting the perfect exposure can be tricky because your Mavic’s sensor, like all image-capture devices, can’t capture all the tones we can see. If the range of tones in an image is extensive, embracing both inky black shadows and bright highlights, the sensor may not be able to capture them all. Sometimes, we must settle for an exposure that renders most of those tones—but not all—in a way that best suits the image we want to produce. You’ll often need to make choices about which details are important, and which are not, so that you can grab the tones that truly matter in your image. That’s part of the creativity you bring to bear in realizing your photographic vision.

For example, look at three bracketed exposures presented at left in Figure 5.8. For the image at upper left, the darker areas and shadows are well exposed, but the brightest highlights—chiefly the sky—are seriously overexposed. The version at upper center does a better job on the clouds, sea, and land, but the three are still unbalanced. At lower left, the sky looks great, but the land portions are much too dark. The camera’s sensor simply can’t capture detail in both dark areas and bright areas in a single shot.

With digital camera sensors, it’s tricky to capture detail in both highlights and shadows in a single image, because the number of tones, the dynamic range of the sensor, is limited. That’s one reason why it’s a good idea to master using RAW format. However, the solution, in this case, was to resort to a technique called High Dynamic Range (HDR) photography. With HDR still images, multiple shots at different exposure settings are used to extract the best detail from each and combine them, resulting in an image like the one seen at right in Figure 5.8.

Your drone can accomplish much the same thing automatically using the Smart Photo setting in still photography mode, and when working with any of the HDR Panorama modes (Vertical, Wide, 180 Degree, and Sphere.) You can also capture a bracketed set of images taken at exposure increments you specify, and combine them using an image editor such as Photoshop, or a specialized HDR tool like Photomatix. I’ll explain exposure bracketing shortly.

Choosing an Exposure Mode

To calculate exposure automatically, you need to tell the camera what to measure (this is called the metering mode) and what adjustments should be used (ISO sensitivity, shutter speed, or both) to set the exposure. Together they provide your exposure mode.

If you’re a conventional camera veteran, you know about metering modes. Digital cameras typically have three or more metering modes. The most sophisticated is Matrix/Evaluative mode, in which various parts of the scene are compared to determine what kind of scene it is and measure exposure from the most important subject. That’s distinctly different from Averaging mode, in which the illumination of the entire frame is examined and exposure is based on the average brightness. Additional modes include Partial or Spot metering, in which all or most of the exposure is based on giving the most weight to measurements from a relatively small portion of the frame.

Your Mavic generally uses the Averaging mode by default. That’s because aerial scenes often include important areas located throughout the frame rather than concentrated in one spot, which is typical with, say, portrait or macro close-up photo subjects that are common fodder for conventional digital cameras. So, Averaging works quite well most of the time.

If you do want to base your exposure on a certain subject within the frame, your drone offers a metering mode that DJI calls Spot, although it actually is more of a Partial metering scheme because the area measured is not the typical 3 to 5 percent of the frame used for traditional Spot metering. Because DJI feels you won’t need Spot metering very often, it’s activated on a per-capture basis:

• ■Activate touch focus/spot metering. Touch the controller screen with your finger on the area you want to meter. An icon like the one shown at left in Figure 5.9 appears as you hold your finger down on the screen.
• ■Press to focus/meter. With the Mavic Air 2S, the icon will appear as a square with the cross-hairs that represent focus. (The Air 2 is fixed-focus only.) As you press down, the Mavic will calculate the correct exposure and, with the Air 2S, will in addition focus on the area within the square.
• ■Adjust brightness. A vertical line with the sun icon representing brightness will appear to the right of the focus/metering square. The “sun” will initially be in the center of the line. You can drag it upward or downward to adjust the exposure, producing a brighter image (drag upward) or darker image (drag downward). You are, in effect, changing the Exposure Value (EV) and applying exposure compensation.
• ■Lock exposure. Keep your finger in place until the Auto Exposure Locked indicator appears (see Figure 5.9, right).

As you might expect, because exposure is adjusted using only shutter speed and ISO sensitivity (plus ND filters), your Mavic doesn’t have an exact counterpart to the Program Auto, Shutter-priority, and Aperture-priority modes you may be familiar with when using conventional still cameras. Next is a rundown on the choices you do have in Pro mode.

Adjusting EV

Making EV adjustments is a way of applying exposure compensation to override the shutter speed and ISO settings calculated by the camera. You can add or subtract exposure in 1/3-stop increments, thereby making your image brighter or darker than if you’d allowed the Mavic to choose its settings automatically.

I showed you how to apply EV changes when Auto camera mode is active earlier in this chapter. In Pro mode, there are two ways to override the drone’s exposure settings, using Touch Focus/Metering, as described above, and manually applying plus or minus values using the exposure controls shown in Figure 5.10.

As I mentioned earlier, when either Shutter Speed or ISO are set to Auto, or when both are in Auto mode, you can override their settings by applying exposure compensation, usually expressed in terms of EV (Exposure Value) changes. When you click the EV label, you’re given the opportunity to add or subtract up to three stops of exposure, in one-third stop increments.

• ■Shutter Speed: Auto. If Shutter Speed is set to Auto, then the shutter speed will increase or decrease while the ISO remains fixed as you add or subtract EV.
• ■ISO: Auto. If ISO is set to Auto, the ISO setting will change instead with the shutter speed remaining constant.
• ■Shutter Speed/ISO: Auto. If both are in Auto mode, then both will be adjusted.
• ■Shutter Speed/ISO: Manual. If you’ve disabled Auto adjustments for both shutter speed and ISO, then you are in Manual Metering mode, indicated by the M.M. label applied above to this EV setting, and EV changes are not possible.

Is Visual Noise a Problem?

With conventional cameras, the graininess we call “noise” can be an important concern. Visual image noise is that random grainy effect that some like to use as a special effect, but which, most of the time, is objectionable because it robs an image of detail even as it adds that “interesting” texture. This noise is caused by three different phenomena:

• ■Sensor area. You’ll often see assertions that larger sensors are subject to less visual noise because the individual photosites (pixels) are larger. That’s not correct. Some types of noise are actually a function of the total image area of the sensor; it makes no difference how the sensor is divided up into individual pixels. Larger sensors do allow more pixels and higher effective resolutions, but the actual noise levels of a given sensor size are the same whether it’s a 12MP sensor or 24MP sensor. It’s the increase in total area that lowers noise levels. Figure 5.11 shows the relative sensor sizes of a variety of digital camera digitizers, from a 36 × 24mm full-frame sensor to the 6.4 × 4.8mm sensor of the Mavic Air 2.
  • As you might guess, the Mavic Air 2 and 2S have more visual noise resulting from sensor size than cameras with much larger sensors. In practice, you’re more likely to see noise caused by sensor size than from length of exposure or high ISO.

• ■Exposure time. Longer exposures allow more photons to reach the sensor, increasing your ability to capture a picture under low-light conditions. However, the longer exposures also increase the likelihood that some pixels will register random phantom photons, often because the longer an imager is “hot,” the warmer it gets, and that heat can be mistaken for photons. CMOS imagers, like the ones found in the Air 2/2S contain millions of individual amplifiers and analog/ digital converters, all working in unison. Because all these circuits don’t necessarily process in precisely the same way all the time, they can introduce something called fixed-pattern noise into the image data.
  • The good news is that it’s unlikely you’ll be capturing many images using long exposure times. Video shutter speeds are 1/30th second or shorter, and the vast majority of your still photos will be taken at even briefer shutter speed settings—frequently 1/1250th or faster. In any case, the longest exposure time available with your Mavic is 8 seconds, a duration that does not produce much long exposure noise.
• ■High ISOs. This kind of noise commonly first appears when you raise your camera’s sensitivity setting above ISO 1600. This kind of noise appears as a result of the amplification needed to increase the sensitivity of the sensor. Because your sensor has twice as many green pixels as red and blue pixels, such noise is typically worse in areas that have red, blue, and magenta tones, because the green signals don’t have to be amplified as much to produce detail. While higher ISOs do pull details out of dark areas, they also amplify non-signal information randomly, creating noise.
  • Your done is capable of using ISO settings that venture into the “noisy” zone. The Mavic Air 2 can use up to ISO 6400 for video and 12MP still photos (ISO 3200 in 48MP mode), while the Air 2S can use up to ISO 6400 for video and ISO 12800 for still photos (both in Manual exposure mode). You’ll rarely need those lofty settings, however, thanks to the fixed f/2.8 aperture. Under dim sunlight, you’d need an exposure of 1/8000th second at f/2.8 to push ISO to 3200 or an exposure of 1/4000th second at f/2.8 to require ISO 6400 at dusk.

If you do encounter objectionable noise in any of your still photos, most image-editing software includes noise-reduction features that will reduce the graininess, while masking a little detail.

Histograms and Contrast

Your camera’s histograms are a simplified display of the numbers of pixels at each of 256 brightness levels, producing an interesting “mountain range” shape in the graph. As I mentioned earlier, each vertical line in the graph represents the number of pixels in the image for each brightness value, from 0 (black) on the left to 255 (white) on the right. The vertical axis represents the number of pixels at each level on a scale that is compressed or expanded to more or less fill the available vertical area of the graph.

Although histograms are most often used to fine-tune exposure, you can glean other information from them, such as the relative contrast of the image. Figure 5.23, top, shows a generic histogram of an aerial image having normal contrast. In such an image, most of the pixels are spread across the image, with a healthy distribution of tones throughout the midtone section of the graph. That large peak at the right side of the graph represents all those light tones in the sky. A normal-contrast image you shoot may have less sky area, and less of a peak at the right side, but notice that very few pixels hug the right edge of the histogram, indicating that the lightest tones are not being clipped because they are off the chart.

With a lower-contrast image, like the one shown in Figure 5.23, center, the basic shape of the previous histogram will remain recognizable, but gradually will be compressed together to cover a smaller area of the gray spectrum. The squished shape of the histogram is caused by all the grays in the original image being represented by a limited number of gray tones in a smaller range of the scale.

Instead of the darkest tones of the image reaching into the black end of the spectrum and the whitest tones extending to the lightest end, the blackest areas of the scene are now represented by a light gray, and the whites by a somewhat lighter gray. The overall contrast of the image is reduced. Because all the darker tones are actually a middle gray or lighter, the scene in this version of the photo appears lighter as well.

Going in the other direction, increasing the contrast of an image produces a histogram like the one shown in Figure 5.23, bottom. In this case, the tonal range is now spread over the entire width of the chart, but, except for the bright sky, there is not much variation in the middle tones; the mountain “peaks” in the midtones are not very high at all. When you stretch the grayscale in both directions like this, the darkest tones become darker (that may not be possible) and the lightest tones become lighter (ditto). In fact, shades that might have been gray before can change to black or white as they are moved toward either end of the scale.

The effect of increasing contrast may be to move some tones off either end of the scale altogether, while spreading the remaining grays over a smaller number of locations on the spectrum. That’s exactly the case in the example shown. The number of possible tones is smaller, and the image appears harsher.

With drone photography there is very little you can do to adjust contrast. In daylight, the sunlight you have is the sunlight you have to use to capture your images. If lighting is too harsh, the best you can hope for are some clouds that will soften the illumination and fill in inky shadows. However, understanding contrast as displayed by the histogram can be useful when you need to match the shots you take today with those you capture on some other day. When I’m shooting construction progress images, I like to have light with good contrast to better show the details. I try to shoot at the same time of day for consistency, and perhaps wait, if necessary, for the same kind of light contrast when taking each set of images.

Histograms and Exposure

The important thing to remember when working with the histogram display in your camera is that changing the exposure does not change the contrast of an image. The curves illustrated in the previous three examples remain exactly the same shape when you increase or decrease exposure. I repeat: The proportional distribution of grays shown in the histogram doesn’t change when exposure changes; it is neither stretched nor compressed. However, the tones as a whole are moved toward one end of the scale or the other, depending on whether you’re increasing or decreasing exposure. You’ll be able to see that in some illustrations that follow.

So, as you reduce exposure, tones gradually move to the black end (and off the scale), while the reverse is true when you increase exposure. The contrast within the image is changed only to the extent that some of the tones can no longer be represented when they are moved off the scale.

What you can do is adjust the exposure so that the tones that are already present in the scene are captured correctly. Figure 5.24, top, shows the histogram for an image that is badly underexposed. You can guess from the shape of the histogram that many of the dark tones to the left of the graph have been clipped off. There’s plenty of room on the right side for additional pixels to reside without having them become overexposed. So, you can increase the exposure (either by changing the shutter speed, ISO, or by adding an EV value) to produce the corrected histogram shown in Figure 5.24, center.

Conversely, if your histogram looks like the one shown in Figure 5.24, bottom, with bright tones pushed off the right edge of the chart, you have an overexposed image, and you can correct it by reducing exposure. In working with histograms, your goal should be to have all the tones in an image spread out between the edges, with none clipped off at the left and right sides. Underexposing (to preserve highlights) should be done only as a last resort, because retrieving the underexposed shadows in your image editor will frequently increase the noise, even if you’re working with RAW files. A better course of action is to expose for the highlights, but, when the subject matter makes it practical, fill in the shadows with additional light, using reflectors, fill flash, or other techniques rather than allowing them to be seriously underexposed.

A traditional technique for optimizing exposure is called “expose to the right” (ETTR), which involves adding exposure to push the histogram’s curve toward the right side but not far enough to clip off highlights. The rationale for this method is that extra shadow detail will be produced with a minimum increase in noise, especially in the shadow areas. As I noted earlier, half of a digital sensor’s response lies in the brightest areas of an image, and so require the least amount of amplification (which is one way to increase digital noise). ETTR can work, as long as you’re able to capture a satisfactory amount of information in the shadows.

Autofocus and Manual Focus (Air 2S Only)

While the Mavic Air 2 is fixed focus, the Mavic Air 2S will automatically focus when in AF mode. An icon located to the immediate left of the shutter release icon on the controller screen will display either AF or an outline of a mountain range (which represents manual focus). Tap the icon to toggle between the two.

In AF mode, the Air 2S will select a focus plane automatically. You can choose a particular subject to focus on by pressing a fingertip on that subject. The aircraft will focus on the subject within the yellow target shown in Figure 5.28, while calculating exposure at the same time. As I described earlier in this chapter, you can slide the “sun” icon upward or downward to add or subtract exposure compensation, and hold your finger down to lock exposure.

When you’ve toggled to MF, you can drag a yellow marker up and down a scale that has a mountain icon at its top (for distance subjects) and an icon of a person at the bottom (for close focusing). (See Figure 5.29.) You can visually evaluate manual focus on the screen of your controller, or take advantage of the drone’s Focus Peaking feature. You can activate Peaking Level in the System Settings menu’s Camera tab (shown earlier in Figure 5.20). Choose Low, Normal, or High levels.

When activated, the drone will look for sharp edges for you, and outline them in red. The higher the peaking level, the thicker the outline will be, as shown in Figure 5.30. I use the highest level under bright, ambient-light conditions, because the red highlighting is much easier to view. However, the glowing red edges can make viewing your image difficult once focus has been achieved. DJI does not give you a choice of accent color (some platforms let you choose from blue, yellow, and red), so the contrast between edges can be difficult to discern if your subject has a great deal of red detail, which is the case in the autumn example used for the figure.

Neutral Density

There are times when you can have too much light. When reducing ISO isn’t enough to get the desired shutter speed, you need a neutral-density filter to block additional light. ND filters, as they’re known, come in a variety of “strengths,” and vendors use somewhat confusing nomenclature to define how much of the light is blocked. You’ll see neutral-density filters from one manufacturer labeled according to something called a filter factor, while others, from different vendors, are labeled according to their optical density. There are actually no less than four variations of naming schemes, all equally cryptic; and even if you understand the difference, you still probably won’t know which is which, or why.

For example, one of the ND filters I own of a particular value is labeled, by different manufacturers, with the following names: ND8, 8X, ND 0.9, and “Three Stop.” All four monikers mean that the filter reduces the amount of illumination by three full f/stops. If the correct exposure were 1/500th second at f/8 without any filter at all, you’d need an exposure of 1/60th second at f/8 with the filter in place.

Because fast shutter speeds produce an undesirably hyper-realistic look in video, neutral-density filters are especially important when shooting movies. I’ll cover their application in more detail in Chapter 7. The Mavic Air 2 and Mavic Air 2S Fly More combos include sets of ND filters. My Air 2 combo came with three: labeled ND16, ND64, and ND256 (4, 6, and 8 stops, respectively). The Air 2S combo included ND4, ND8, ND16, and ND32 filters (reducing the light by 2, 3, 4, and 5 stops, respectively). DJI also sells an expanded set for the Air 2S with ND64, ND128, ND256, and ND512 filters (6, 7, 8, and 9 stops).

Third-party vendors sell their own ND filters alone or in combinations. Several companies, such as Freewell and PGYTECH, offer variable neutral-density filters, which allow you to dial in 2- to 5- or 6- to 9-stop density by rotating a ring around the filter’s circumference. Some ND filters are sold that also serve as polarizers (which I’ll discuss shortly).

A special kind of neutral-density filter—the split ND or graduated ND filter—is a unique tool for landscape stills and video. In some scenes the sky can be significantly brighter than the foreground and well beyond the ability of the camera’s sensor to capture detail in both. An ND filter that is dark at the top and clear at the bottom can even out the exposure, restoring the puffy white clouds and clear blue sky to your images. These filters can be tricky to use.

Here are some factors to keep in mind:

• ■Don’t tilt. The transition in the filter should match the transition between foreground and background, so you’ll want to avoid FPV (first-person view), in which the image rotates as the drone rolls during turns.
• ■Watch the location of your horizons. The ND effect is more sharply defined with a split ND filter than with a graduated version, but you need to watch your horizons in both cases if you want to avoid darkening some of the foreground. That may mean that you have to place the “boundary” in the middle of the image to properly separate the sky and foreground.
• ■Watch the shape of your horizons. A horizon that’s not broken by trees, mountains, buildings, or other non-sky shapes will allow darkening the upper half of the image more smoothly. That makes seascapes a perfect application for this kind of neutral-density filter. However, you can use these filters with many other types of scenes as long as the darkening effect isn’t too obvious. That makes a graduated ND filter a more versatile choice, because the neutral-density effect diminishes at the middle of the image.

Polarization

Polarizing filters are useful in many ways. Light becomes polarized (and therefore susceptible to the effects of a polarizing filter) when it is scattered by passing through a translucent medium. So, light from a clear-blue sky becomes polarized when it passes through the dust-laden atmosphere. Light striking, passing through, and reflecting off water also becomes polarized. The same is true of many other types of objects, including foliage, and the partially transparent paint on an automobile body (think about it: that’s why cars need several coats of paint). Nontransparent or translucent objects, like the chrome trim on the automobile, aren’t transparent, and don’t polarize the light. However, if the light reflecting from the metal has already been partially polarized (that is, it is reflected skylight), you still might be able to see a reduced amount of glare reduction with a polarizing filter.

How does it work? A polarizer contains what you can think of as a tiny set of parallel louvers that filter out all the light waves except for those vibrating in a direction parallel to the louvers. The polarizer itself consists of two rings; one is attached to the lens, while the outer ring rotates the polarizing glass. This lets you change the angle of the louvers and selectively filter out different light waves.

A polarizing filter can be mounted on the camera to replace the empty frame that surrounds the lens. Rotate the frame to remove it, then attach the polarizer to the same mount. A ring around the outside of the filter allows adjusting the amount of polarization. (See Figure 5.31.)

You can rotate the ring until the effect you want is visible, but polarizers designed for use with drones will have a pair of marks on the rim of the filter that can be rotated to produce the best effect. Keep in mind that the amount of glare reduction depends heavily on the angle from which you take the photograph, and the amounts of scattered light in the reflections (which is determined by the composition of the subject). Polarizers work best when the sun is low in the sky and at a 90-degree angle from the camera and subject (that is, off to your left or right shoulder). Blue sky and water, which can contain high amounts of scattered light, can be made darker and more vibrant as glare is reduced. You can also reduce or eliminate reflections from windows and other nonmetallic surfaces.

As you might expect, polarizing filters solve problems and enhance images in several ways. Let’s look at them:

• ■Reduce or eliminate reflections. Polarizing filters can reduce or eliminate reflections in glass, water, lacquer-coated objects, non-conducting surfaces, and plastic. They don’t have any effect on reflections on metallic surfaces.
• ■Increase contrast. A polarizing filter is the only filter that can increase contrast in color imagery by eliminating those pesky reflections.
• ■Darken pale skies. Polarizing filters block a lot of light (as much as two f/stops), and because the effect is strongest when dealing with polarized light (such as light in the sky), this can help darken pale skies considerably. At the same time, it will also make clouds stand out more strongly.

As you gain experience, you’ll learn what types of scenes benefit from polarization, and what types do not. Look at Figure 5.32, which shows two different shots taken with my Air 2S about 10 minutes apart from approximately the same location. (I had to land the drone after capturing the first image and install the filter to take the second shot.)

In the version at left, reflections in the ponds in the center provide a reverse image of the trees that surround them. The image at right, with the polarizer in place, has a darker sky and the colors of the autumn leaves are richer. But the reflections of the trees are gone. That makes the ponds look duller and flat. I would have gotten a better picture by not using the polarizer and, perhaps, increasing saturation of the trees and sky in post-processing.

One thing to watch out for is the use of polarizers with wide-angle lenses, such as the 24/22mm optics installed on the Air 2S and Air 2. Polarizers work best when the camera is pointed 90 degrees away from the light source, and provides the least effect when the camera is directed 180 degrees from the light source, as when the light source is behind you. With longer lenses, the field of view is narrow enough that the difference in angle between the light source and your subject is roughly the same across the image field.

But the wide-angle lenses of your Air 2S and Air 2 have a field of view of 88 and 84 degrees, respectively. It’s possible for subjects at one side of the frame to be oriented exactly perpendicular to the light source, while subject matter at the opposite side of the frame will actually face the light source (at a 0-degree angle). The possibility is even more acute when shooting panoramas, which can extend the field of view to 180 degrees. With such broad perspectives, everything in between will have an intermediate angle. In this extreme case, you’ll get maximum polarization at one side of your image, and a greatly reduced polarization effect at the opposite edge. Use caution when using a polarizer.

Filters for Protection

Several vendors offer UV filters for the Air 2/2S. Do you really need to use a filter to protect your camera’s lens? I have a definitive answer: yes, no, and maybe.

Each of the three answers can be persuasive. No matter how careful you are, if you’re not using a filter, some dust, mist, or other foreign substances can get on your lens during a flight, particularly if you are flying during bad weather or a dust storm kicks up unexpectedly. Your drone may suffer a “soft” crash. (I’ve collided with trees more than once, and my drone tumbled through the branches before hitting the ground without actually suffering any real damage.) In such cases, a filter can shield your lens from dust and debris, and absorb a few light scratches during minor collisions, assuming the gimbal itself is not harmed.

If you’re clumsy, cleaning a filter may be safer than cleaning the lens itself. If you don’t know how to clean a lens, or don’t have the proper tools, the more often you have to clean your lenses, the greater the likelihood of leaving cleaning marks or clumsily rubbing off some of the vital multi-coating that helps prevent lens flare. Again, a filter may be appropriate for those who need to protect their camera from themselves.

As far as UV removal is concerned, it’s known that general-purpose UV and skylight filters have little to no effect in the digital sensor world, so a UV filter offers nothing but a bit of protection, while adding weight to your gimbal. Lower-quality filters can degrade the image due to aberrations and flare. Why reduce the quality of every image you take to guard against an accident that may never happen?

My recommendation is to use a filter, such as an ND or polarizer, when you need to filter something. When conditions merit, the ND or polarizer you already have in place will serve as lens “protection.” If unfavorable weather threatens, a UV filter can be useful.

Do It Yourself

If you like to experiment, you can make your own filters, using the empty frame around the lens as your filter holder. Carefully cut a piece of filter material to fit and mount it on the camera. Colored gelatin filters, pieces of fine window screen (for sunstar/starburst effects), and other experimental filters can be constructed and used to produce new and interesting special effects.