Correcting Lens Dependent Aberrations

Lens-dependent aberrations are circular effects centered on the optical center of the image. Therefore, it is important to correct them before cropping or warping the image. Many photo-editing software packages and some modules of professional remote sensing software include tools for correcting vignetting, CA, and sometimes even lens distortion.

The degree of vignetting depends on lens characteristics, aperture, and exposure and is thus not an invariable effect—it is difficult to correct automatically and its correction settings usually need to be judged visually for each image. Using sliders for the amount and progression radius of the lightening, the brightness of the image is modified until the light falloff toward the corners is compensated. Correcting for vignetting, which is most noticeable in homogeneous, light portions like the sky or bare soil, can be sensible not only for aesthetic reasons but also prior to image classification in order to balance the spectral characteristics of the scene.

The correction of CA and lens distortion involves not brightness or color modifications but geometric transformations of the image. Latitudinal CA is rectified by slight adjustments of the image channel sizes, usually by radially scaling the red and blue channels individually to match the size of the green channel. Radial lens distortions (see Figs. 6-7 and 6-14) are lens-specific geometric distortions that are best corrected with lens-specific and focal-length-specific settings using dedicated software like PTLens. It is also possible to correct radial lens distortion visually, but only for scenes where its degree can be judged from the deformation of straight lines or grids—something that is usually not the case for SFAP. Correcting radial distortions prior to quantitative image analysis might be especially useful if the image is to be georectified using simple polynomial equations. Some recent camera models automatically apply CA and lens-distortion correction in-camera to JPG or even RAW files, so the user does not even get to see the original distortions any more.

Gully Erosion Morocco

FIGURE 11-4 Controlled image mosaic of gully erosion near Icht, South Morocco, created with ERDAS Imagine 9.3 from nine orthorectified aerial photographs. (A) Original images mosaicked without color corrections. (B) Original images mosaicked with color dodging corrections. Contrast enhancement (see Fig. 11-8) applied subsequently to both mosaics. Note the perfect color-adaptation of the surrounding desert surface but still different tints of shadowed areas in version B. Kite aerial photographs by IM, JBR, and M. Seeger, March 2006; image processing by IM.

FIGURE 11-4 Controlled image mosaic of gully erosion near Icht, South Morocco, created with ERDAS Imagine 9.3 from nine orthorectified aerial photographs. (A) Original images mosaicked without color corrections. (B) Original images mosaicked with color dodging corrections. Contrast enhancement (see Fig. 11-8) applied subsequently to both mosaics. Note the perfect color-adaptation of the surrounding desert surface but still different tints of shadowed areas in version B. Kite aerial photographs by IM, JBR, and M. Seeger, March 2006; image processing by IM.

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