Commercial drones equipped with digital cameras utilise software to control default image capture. AUTO exposure settings examine the light conditions through the lens, then arrive at an image capture solution. But the AUTO setting quite often fails to deliver, particularly if the drone is moving during capture.

Motion blur is a common outcome in photogrammetric imagery captured with AUTO exposure settings. Simply put motion blur results in degraded imagery. It is a consequence of the drone capturing data whilst moving through the sky. Severity will be dependent upon:

• the height of the drone above the surface,
• the velocity of the drone,
• the shutter speed of the camera.

Motion blur in photogrammetric surveying is analogous to being on a high-speed train and looking out at the passing vista. Objects close up are blurred as you speed past, with only features at a distance being in focus. What solutions can you apply?

• Simply fly higher (increasing the distance between camera and object). Yes, but there is a downside with this approach. Increasing flight height will push up Ground Sample Distance (GSD). Image capture will generate comparably lower quality output than capture at lower heights.
• Set the drone to be stationary when taking images. A stationary drone will typically take sharper images than one moving. Depending upon the type of drone and the flight controller stopping the drone could be impractical, or not possible.
• Decrease the flight velocity. When preparing flight plans the flight controller reports a suggested maximum velocity based on the flight height. At lower heights the drone needs to travel slower to capture clear imagery. Remember that with decreased velocity there is a trade-off with increased flight time, resulting in increased power consumption.

When undertaking a photogrammetric survey, it’s important to understand how the exposure triangle works and its influence on data capture. Capturing data within certain regions of the exposure triangle will result in producing high quality images.

The exposure triangle comprises a combination of 3 camera setting variables (select any variable for more details):

Shutter speed

It is possible to compensate for motion blur through switching off AUTO exposure.

Setting image capture with “Shutter Priority” ensures a minimum shutter speed for each photograph captured. Typically, a speed of 1/1000 second will eliminate forward motion blur, yet even this may require increasing to a faster rate when flying at a very low elevation. Additionally, some cameras switch from a mechanical shutter to an electronic shutter at a certain speed. Knowing the threshold for the switch between the two is important as electronic shutters comparably produce poorer results. For example, on the DJI P4 RTK a shutter speed above 1/2000 sec would see capture electronically controlled rather than mechanically.

Another potential downside with pushing the shutter speed too high is under exposed imagery when lighting conditions are so poor that the aperture can’t compensate.

If the recommended shutter speed can’t be achieved, then reduction of flight velocity is one way to compensate for motion blur and allow slower shutter speeds to be utilised.

Reducing the maximum suggested flight velocity by around 10% of the suggested limit will pay dividends.

ISO

Film based cameras utilise high speed film to deal with low light conditions. Digital cameras offer the same solution. ISO can be utilised to compensate for light conditions.

Increasing the ISO setting enables higher shutter speeds to be maintained in lower light conditions. The downside of increasing ISO is the same as in high-speed film photography with, the result showing increased granularity.

Using the native ISO setting is preferable and maintaining the ISO below 200 avoids degraded image quality.

Aperture (F-stop)

The aperture or F-stop setting defines how much light enters the camera through the lens opening. Changing the aperture impacts not only the exposure but also the depth of field (or focal length).

At lower F-stop settings the aperture will be at its widest allowing more light to enter the cameras lens. With this wider opening is a decrease in the focal length (or distance) at which objects are clear. It is possible for some objects to be out of focus whilst others are in focus.

Conversely increasing the F-stop closes the aperture of the lens. This results in less light entering, and a larger focal length. The camera maintains focus on objects at different distances within the field of view.

Operating aperture in the mid-range of available F-stop settings provides the greatest opportunity for higher quality imagery.

Another setting of value is Exposure Value (EV). This is calculated from shutter speed and aperture. The value can be increased or decreased incrementally, depending on the requirements to either brighten or darken an image. EV is a useful setting that operators can adjust without degrading image quality.

Typically, the sharpest image capture is the result of a higher shutter speed with the lowest ISO settings. The F-stop balance should fall in the range of a more open aperture. Remember that in low light conditions decrease the drone’s velocity. This will allow slower shutter speeds than 1/1000 second and still keep the ISO within a tolerable range.

Take the time to test your camera before surveying, determining optimal exposure settings.  Adjust the EV setting if required. 

Record your settings for comparison and later review to hone capture skills in the future.

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