Lens Distortion

The way a real lens's image deviates from the ideal pinhole model, bending straight lines into curves.

The pinhole camera model assumes a straight line in space still photographs as a straight line, but real lenses deviate from this — that deviation is lens distortion. Radial distortion worsens toward the edges of the image: wide-angle and fisheye lenses commonly bow outward (barrel distortion), while telephoto lenses commonly bow inward (pincushion distortion); tangential distortion comes from the lens not being mounted perfectly parallel to the sensor. It's commonly described with the Brown-Conrady model, which OpenCV represents using three radial coefficients (k1, k2, k3) and two tangential coefficients (p1, p2); these are solved together with focal length and the principal point through checkerboard camera calibration, and an undistort function then straightens the image. Robots need to undistort before computing a 3D position from pixels, performing hand-eye calibration, or running visual SLAM, or the error near the edges of the frame grows significantly.

ExampleA wrist camera's wide-angle lens photographs the table's edge as a curved arc. After calibrating the distortion coefficients and calling OpenCV's cv.undistort, the table edge straightens back out, letting edge objects' pixel coordinates convert accurately into arm coordinates.

Also called

Radial Distortion, Tangential Distortion, Undistortion

Related

Camera Calibration · Camera Intrinsics · Pinhole Camera Model · Fisheye Camera · Calibration Board · OpenCV (Open Source Computer Vision Library)

Sources

OpenCV Tutorial: Camera Calibration
Distortion (optics) - Wikipedia