Image Degradation

Photographs do not record the energy reflected off their motifs flawlessly. Chemical and physical characteristics of films and image sensors may introduce unwanted effects and artifacts into an image. Also, the lens is crucial for image quality, and some degree of distortion always is introduced into the optical paths by deviations from a perfect central perspective. The most frequent types of distortions and artifacts are briefly described in the following (see Chapter 11

for more details).

• Radial distortion slightly changes the image scale with increasing distance from the image center. Short focal lengths (wide image angles) typically show barrel distortion, which causes straight lines to bend outwards, while long focal lengths (telephotos) tend to exhibit pincushion distortion, which causes straight lines to bend inwards (Fig. 6-7). These effects tend to be stronger in zoom lenses than fixed-focus lenses. Radial distortion can be corrected with image processing.

• Chromatic aberration is caused by the inability of a lens to focus all wavelengths onto the same plane (longitudinal CA) and/or by varying magnification of different wavelengths (latitudinal CA). With increasing distance from the image center, the three primary colors become slightly offset, causing a color-fringe effect around contrasting edges (Fig. 6-8). Very wide-angle lenses and the extreme focal lengths of super-zoom lenses are most prone to chromatic aberration. The color offset in the image bands can be corrected with some image-processing packages.

• Similar-looking colored fringes also can be caused around overexposed image areas by overflowing electrical charge (blooming) or near high-contrast boundaries by aberration effects associated with microlens arrays placed onto some sensors (purple fringing; Langford and Bilissi, 2007). Blooming and purple fringing are not easy to correct, and the first is best prevented by avoiding overexposure.

• The term vignetting describes the gradual darkening of an image toward the edges and corners (Fig. 6-9) and may have different causes, all of which are related to the lens design (Ray, 2002; Kessler, 2007). Optical vignetting results from the reduction of the effective lens opening for oblique light rays, because the lens diaphragm is set back from the front rim of the lens tube. Thus, it can be reduced or cured by stopping down the lens to smaller apertures. Similarly, mechanical

FIGURE 6-7 Barrel distortion (left) and pincushion distortion (right) are typical optical lens aberrations leading to the bending of straight lines.
FIGURE 6-8 Chromatic aberration causes colored fringes around the edges of long afternoon shadows cast by soil clods in a dry river bed in South Morocco. Subset of a kite aerial photograph taken by IM and JBR; aberration effect is strongly exaggerated for illustration purpose.

vignetting is caused by too long extensions of the lens (filters, lens hoods) that narrow the effective lens opening; in this case, the image corners will be blackened out. Finally, natural vignetting or natural light falloff, which is described by the cos4 law, occurs with all lenses but is more prominent for wide angles. The darkening effect by vignetting is usually quite small, but it can become more obvious when several images are stitched together in a mosaic. As wide-angle lenses are frequently used for SFAP and vignetting generally is at its worst when the lens is focused at infinity, some contrast and brightness adjustment may be necessary for correcting vignetting effects in aerial photographs. Other than the distortions listed above, image noise is independent of focal length and aperture. It is defined as the random variation of pixel values caused by fluctuations of the signal transmitted by the sensor. While there are several sources of noise (Langford and Bilissi, 2007), they all result in high-frequency brightness and color variations ("image speckles''), which are more pronounced atlow signal-to-noise ratios. Thus, noise is mostprevalent in dark image areas (e.g., hard shadows in SFAP images), for high ISO speeds (where the sensor signal is amplified to provide for poor light conditions), and for small sensor cells (compact cameras with high megapixel numbers; see Fig. 6-3). Image noise may be smoothed and made less conspicuous with image processing, but choosing a good-quality sensor with large pixels is probably the most important remedy against noise.

FIGURE 6-9 Vignetting of an aerial photograph showing experimental reforestation plots near Guadix, Province of Granada, Spain. Hot-air blimp photograph taken by IM and JBR, March 2002; vignetting effect (originally inconspicuous) introduced artificially for illustration purpose.
Digital Camera and Digital Photography

Digital Camera and Digital Photography

Compared to film cameras, digital cameras are easy to use, fun and extremely versatile. Every day there’s more features being designed. Whether you have the cheapest model or a high end model, digital cameras can do an endless number of things. Let’s look at how to get the most out of your digital camera.

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