GEOMETRIC CALIBRATION OF ZOOM LENSES FOR COMPUTER VISION METROLOGY

Zoom lenses are used extensively in computer vision to overcome the li mited resolution provided by the small focal planes of solid-state cam eras. Laboratory studies of zoom lenses, with a focal range of 12.5 to 75 mm, showed that geometric distortions could amount to several tens of pixels across the focal plane, and that there were significant cha nges in the distortion patterns at different focal settings. Changes i n position of the principal point amounting to as much as 90 pixels we re measured. These changes were found to be highly systematic, repeata ble, and stable over time. A mathematical model was developed to model the geometric distortions at a fixed focal setting with an RMS error better than +/- 0.1 pixel. A method was devised to model the changes i n the interior geometry of zoom lenses, with the resulting residual di stortions amounting to less than +/- 0.4 pixel (RMS).