Camera Exposure Settings

Settings for image exposure are best understood using the concept of stops. A stop is defined as doubling or halving of any factor that affects the exposure (Shaw, 1994). Notice that values for each variable—shutter speed, f-stop, ISO rating, or film speed—are arranged by doubling intervals. Among these variables there is a relationship called reciprocity. In other words, changing one variable by one stop can be matched exactly by changing another variable in the opposite direction by one stop.

For photographs under bright sun, which applies to most SFAP, the sunny f/16 rule can be employed (Caulfield, 1987). For a given film or sensor under full-sun conditions, the shutter speed should be the approximate inverse of the ISO rating at f/16. For example, ISO 200 film—or a digital sensor at ISO 200 setting—should be exposed at f/16 and 1/250 shutter speed under bright sun conditions.

The exposure factors have other influences on the resulting photograph. In general faster shutter speeds are desirable for SFAP in order to "freeze" the motion between the airborne camera and the ground. This necessitates using high ISO ratings (fast film) and/or lower f-stops. High ISO ratings (>400) tend to result in lower image quality, due to grainy appearance, in comparison to lower ISO ratings. Lower f-stops reduce the depth of field, which refers to the range of distance over which the image is in good focus. For ideal SFAP, a fast shutter speed should be combined with low ISO settings or high-quality (slow) film and a medium to high /-stop setting. However, this combination simply does not work in practice. SFAP, thus, represents a trade-off involving these factors.

Most modern cameras employed for SFAP utilize built-in light meters and automatic adjustment of shutter speed and /-stop. Such automated photography introduces certain artifacts in the picture-taking process. Inexpensive point-and-shoot cameras normally have a light meter that is separate from the lens or viewfinder; whereas the light meter in SLR cameras and recent mirror-free system cameras operates through the lens. The latter is clearly preferable, as the meter registers the light actually entering the camera through the lens.

Advanced point-and-shoot and most SLR cameras allow the user to set a priority for shutter speed or /-stop. For example, cameras with a high-speed (sports) mode select the fastest possible shutter speed in order to minimize blurring effects of camera or object motion. Faster shutter speed is highly desirable for effective SFAP, but represents a compromise with lower /-stops. Assuming the ground target is at infinite focal distance, depth of field and thus /-stop should be negligible factors. However, image sharpness usually is better at medium /-stops due to higher lens aberrations and diffraction at low and high /-stops (Langford and Bilissi, 2007).

Cameras normally determine exposure settings based on a medium gray tone—equivalent to the light reflected from an 18% gray card—to represent the average value sensed by the light meter. This works well for a scene in which most objects are uniformly lighted, for example blue sky and green trees or grass. However, the results may be unsatisfactory for a scene comprised of bright highlights and dark shadows, such as sunlit hill tops and valleys in shadows or for scenes that are generally brighter or darker than average. Some objects are intrinsically bright—snow, ice, concrete, chalk, deciduous trees, and grass; other objects are naturally dark-burned ground, basalt, asphalt, conifer trees, etc.

For scenes with mixed illumination, the bright features would be overexposed and washed out, and the dark objects remain nearly black and lacking in details (see Chapter 5.4). In the case of a generally bright scene, the image would be underexposed because the camera aims at a medium gray tone, while a generally dark scene could be overexposed for the same reason. These effects can be avoided with most cameras by adjusting the exposure correction factor accordingly.

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