My understanding is that OpenCV uses a variant of the Brown-Conrady lens distortion to model radial and tangential distortion. The radial displacement, however, is not just an even-order polynomial as in Brown-Conrady but a variant where a low-order polynomial coefficients (k_1,k_2,k_3) are related to the same even-order polynomial but with higher order coefficients (k_4,k_5,k_6) in a fraction (`CALIB_RATIONAL_MODEL`

flag in `calibrateCamera`

and `stereoCalibrate`

). As such, this seems to be a combination of Brown-Conrady and the division model and likely aims at modeling lens distortion of wide-angle lenses more accurately.

Is this correct? Are there any references for the OpenCV lens distortion model variant?