| Photography
Introduction
ES 555 Small Format
Aerial Photography
James S. Aber |
Cameras
Photography is one of the oldest and most versatile forms of
remote sensing of the Earth's surface. All photographic cameras have
certain basic components: lens, diaphragm, shutter, viewfinder, and
image plane. Geometry of the lens and film format determine the
scene area focused onto the image plane. The diaphragm and shutter
control the amount of light to expose each photograph. Conventional
film cameras are designed to accept film of certain format or
width--for example 35 mm, 70 mm, 5 inches, or 9 inches. Most popular
cameras are for 35-mm film, whereas professional photographers tend to
employ 70-mm format cameras. The larger formats are utilized mainly
for scientific and engineering applications.
Compact, point-and-shoot, 35-mm (film) camera--see Olympus Stylus Epic.
Compact, point-and-shoot, digital camera--see Canon Digital Elph.
Single-lens reflex, high-resolution digital camera--see Canon Digital Rebel.
Complete, 70-mm format, film & digital camera systems--see
Hasselblad.
Most people are familiar with 35-mm film cameras and compact digital cameras. There is a geometric relationship between the lens focal length, the
area (angle) of view, and the amount of light that enters the camera. Some common lens focal lengths are given in the following table for 35-mm format film and typical cameras. Many cameras are now equiped with zoom lenses that allow the user to vary the focal length.
Focal lengths for 35-mm film and digital cameras.
Focal length
35-mm film | Focal length
Digital CCD* | Field of view | Light |
| 15 mm | 9 mm | Fish eye | High |
| 19 mm | 12 mm | Super wide | High |
| 28 mm | 18 mm | Wide angle | High |
| 50 mm | 31 mm | Standard | Moderate |
| 200 mm | 125 mm | Telephoto | Low |
*Approximate ratio of 1.6 to 35-mm film format.
Photographic film and digital
Photochemical imagery is based on the reaction to light of silver
halide crystals, which undergo a chemical change when exposed to
ultraviolet, visible or near-infrared radiation. This photochemical
change can be "developed" into a visible picture. The spectral sensitivity
of chemical photography ranges from about 0.3 µm to 0.9 µm.
The lower limit is based on available ultraviolet energy and strong atmospheric scattering; film sensitivity determines the upper limit. Most digital cameras are limited to visible light, although a few special digital cameras extend into near-infrared radiation.
Different parts of the spectrum may be photographed by using
various combinations of films and filters. Photographs are
routinely taken in b/w panchromatic, b/w minus blue, b/w infrared,
color-visible, color-infrared, and multiband types. For example,
color-infrared film is exposed to green, red, and near-infrared wavelengths,
which are depicted as blue, green and red in the photograph. This shifting
of bands to visible colors is called false-color.
Ultraviolet photography is also possible for special applications.
| Color photograph in visible light of ghost town at La Veta Pass, Colorado with Mount Maestas in the background. Taken in late May with active grass and conifer trees (pine & spruce). |
| Tetracam ADC color-infrared photograph of approximately same scene as above. Grass is more reflective in near-infrared (pink) compared with conifer trees (red) in this standard false-color format. |
During the 1990s, digital still cameras were developed for popular and scientific use. These cameras employ a charged couple device (CCD) to sense light. A CCD is essentially a microscopic array of semiconductors that measure light intensity in a raster grid. The picture is made up of many small dots--also called cells or pixels (picture elements). Early digital cameras could not deliver the high resolution possible with conventional film. However, within the past few years, digital cameras with megapixel CCDs have come on the market at moderate cost. These cameras produce images that rival the sharp detail of 35-mm and 70-mm film. Digital cameras are quickly becoming more popular, and their capabilities are increasing dramatically. Digital technology rapidly is replacing film for most applications, except for the low-end disposable-camera market and special high-end applications.
Aerial photography
Conventional aerial photography is done from specially equiped airplanes nowadays.
Large-format cameras take photographs on 9-inch (23 cm) wide film or large-format digital cameras. In either case, the camera, lens,
film spools, and accessory equipment weigh several 100 kg and require considerable
cargo space in the plane. Normally a twin-engine airplane is necessary. Small-format aerial photography (SFAP), in contrast, is typically
done with 35-mm or 70-mm film or with small video or digital cameras. This allows use of
light-weight equipment that can be lifted in a variety of manned or unmanned platforms.
Aerial photographs are routinely employed for all manner of mapping and evaluation of natural and cultural resources, including agriculture
(crops and soils), archaeology, biology (habitat, wildlife census), forestry, geology, geomorphology, engineering, hydrology, industrial development, military (camouflage detection, espionage, terrain models), mineral and oil prospecting, pollution (air, land, water), property management, reclamation, transportation, urban planning, zoning and tax appraisal, etc.
Related site
Return to SFAP schedule.
ES 555 © J.S. Aber (2008).
