By Eric Andelin, Photogrammetrist/UAS Program Manager
Finding the optimal settings for photographs of your family vacation at the beach can be tricky. Fortunately cameras these days have a lot of built-in intelligence, and with a little help, you can usually get a pretty good photo.
A good UAS pilot needs to know his camera settings given specific conditions to produce optimal imagery for mapping applications. Sure you could go with the automatic settings, but at the speeds, drones operate even a setting such as automatic highlight balance can go awry when little puffy clouds cross over during your mission.
Most information on aerial photography and camera settings for mapping are related to large format cameras. These operate at higher altitudes and under very different conditions, but they do offer some cross over in basic principles of photography.
Why is this the quality of your aerial imagery so important? Given the shift to Structure from Motion (SFM) where multiple images with overlap are automatically matched together via algorithms to produce an automated deliverable…The image exposure, quality, and resolution become paramount in the software’s ability to perform these matches with a high degree of accuracy. For example, if imagery is over exposed to the point of blowing out your highlights on rooftops, there is nothing for the software to match and as a result you’ll either get no data or bad data. Conversely the same is true of areas that appear to dark, such as roads. If you’re doing conventional photogrammetry with stereo models that crispness in the image really allows you to “find ground”, if the imagery is blurry that ground you seek becomes “soft” meaning difficult to interpret. In this example the image is overexposed and also shows has image blur from a shutter speed that was to slow.
The UAS pilot needs to understand not only his aircraft but his camera, lens, and the lighting conditions in order to produce the best images possible. A typical mission with our UAS will cover 400 acres and take about 40 minutes of flying time. In that 40 minutes we’ll capture up to 800 exposures. Optimal conditions would be sunny, calm, and cloud free. Unfortunately, we do not always have this luxury.
Good mission planning includes diligent review of the local weather conditions. If there will be clouds in the area, check their direction of travel and wait for breaks in the conditions that might allow you to complete a mission. Scrubbing cloud shadows from 100’s of exposures is not very fun or practical, and it will likely consume your budget. Knowing you have the correct settings from prior testing and experience will save you a lot of time in the field and minimize the possibility of re flight due to poorly collected imagery.
Beginning with the basics, the 3 most important settings in any camera are ISO, Aperture, and Shutter Speed. When set correctly these all work together to create a photograph with optimal color, detail, and exposure. ISO stands for International Standards Organization, and is a standardized industry scale measuring sensitivity to light. If you remember film, it was sold in different ISO values used to show how sensitive the film was to light. With digital cameras ISO increases the gain of the signal sent from the sensor to the processor sensor can be adjusted, without changing any hardware to be more or less sensitive to light. Adjusting the ISO is a major factor in controlling exposure of the photograph. Lower ISO settings like 100 or 200 are usually used in brighter settings such as outdoor sunny days and ISO settings of 400 and above are helpful when the lighting isn’t as bright. Raising the ISO to high can produce noise in the image that may be undesirable.
Aperture describes the diaphragm in the lens itself and how open or closed this diaphragm is. The different number settings of aperture are also referred to as F-Stops. Aperture or F-Stop controls the amount of light that is passing through the lens. A lower aperture setting (f/1.4, f/2.0, f/2.8) means a larger diaphragm opening which allows more light to pass through the lens. A higher aperture setting (f/8, f/11, f/22) reduces the diaphragm opening allowing less light to pass through the lens.
Shutter speed controls how long the camera shutter remains open. The shutter speed is directly related to how much light is allowed to hit the camera sensor. For example, a shutter speed of 1/60th of a second will let in twice as much light as a shutter speed of 1/120th of a second.