In photography, exposure bracketing is a technique of taking several shots of the same scene at once using different exposure settings. Exposure bracketing is especially useful in difficult lighting situations when it’s hard to be sure of the correct exposure.
In aerial photography this can become even more useful since you typically have no means to make adjustments to the camera settings once in the air. As well your dealing with shooting in a number of directions in a single flight, so the sun and other light sources become an ever changing target to adjust for. By bracketing the shot you are able to capture a range of images automatically, giving you great flexibility in the post processing to select or create the best final image.
Automatic exposure bracketing (AEB) is available most DSLR cameras, and generally have settings to take 3-5 photos with a trigger of the shutter, each in different exposures. Basically you are taking a series of images, some slightly brighter and some slightly darker than a single normal image. For aerials, 3 brackets generally work best.
Check you specific camera’s manual of details on how to enable this mode. There is usually a setting to allow you to set the amount of exposure change between steps. Also make sure to set the in Aperture Priority mode when you using AEB, which will lock the aperture value.
Do hand held tests so you get familiar with the setup and resulting images that are created. It is much easier to work with the camera in hand than trying to configure an test once mounted to the gimbal and aircraft.
The resulting images can be used a number of ways. You might use one of the resulting images as is, as out of the bracketed set there may be one that is “just right” exposure wise, and you can then use it as you would a single shot image.
You can also blend multiple images into an HDR (High Dynamic Range) image, which takes images where there are extreme lights and extreme darks and blends them two together to enhance the overall dynamic range that wouldn’t be possible in a single shot.
For UAV based bracketing, a key factor is keeping the aircraft as still as possible when taking the burst of bracketed shots, any movement will make aligning the images for HDR type usage more difficult. Get into a solid stable hover before triggering.
Also keep an eye on shutter speed. Generally making sure the shutter speed does not drop below 1/125 of a second. Below that speed you may get a motion blur in images. Handheld/tripod based is not a problem but in the air there is always some slight movement.
Exposure bracketing is a powerful tool for aerial photographers, giving you some automatic control over your camera when beyond arms reach and allowing you to maximize the time in the air and reduce the possible need to re-shoot shots that are under or over exposed.
The following articles provide more insight to bracketing: