In this paper, we focus on the monochromatic aberrations that deform the image. We call such aberrations geometric distortions. A geometric distortion is a deviation from the rectilinear projection, a projection in which straight lines in a scene remain straight in their image. While similar distortions can also be seen in display (display distortion, especially in cathode ray tube display), we mainly focus on the geometric distortions caused by geometric optics. Among different types of geometric distortions, radial distortions are the most commonly encountered and most severe. They cause an inward (barrel distortions) or outward (pincushion distortions) displacement of a given image point along the radial direction from its undistorted location (Fig. 4). A radial distortion can also be a combination of both barrel and pincushion distortions, which is called a mustache (or wave) distortion. In an image with a radial distortion, a straight line that runs through the image center (usually also being the center of distortion) remains straight. Since most radial distortions are circularly symmetric (i.e., rotationally symmetric with respect to any angle), or approximately so, arising from the circular symmetry of the optical imaging systems, a circle that is concentric with the image center remains a circle in its image, although its radius may be affected. Some complex distortions include both radial and tangential components, i.e., a given image point displaces along both radial (radial distortion) and tangential (tangential distortion) directions (Fig. 1). Such distortions, called radial-tangential distortions in this paper, include decentering distortions and thin prism distortions.2,3 Unless otherwise specified, distortions hereafter mentioned in this paper mean radial geometric distortions—the focus of this paper.