Lens Distortion Correction

Lens Distortion Correction

Every lens has some amount of distortion. Wide angle lenses have barrel distortion where straight lines bulge outward. Telephoto lenses can have pincushion distortion where straight lines curve inward. Even expensive cinema lenses have distortion, though it is minimized. If you try to camera track footage without correcting for lens distortion, your solve will be inaccurate because the tracking software assumes straight lines are straight.

Lens distortion is most visible at the edges of the frame. In the center, everything looks fine. But the tracking points near the edges are displaced by the distortion, and the solver cannot reconcile their movement with the movement of points in the center. This creates systematic errors that result in a poor solve. The solution is to undistort the footage before tracking, do all your tracking and compositing in undistorted space, and then redistort the final composite.

Creating a lens distortion profile requires filming a calibration grid or chart. This is a pattern of evenly spaced dots or a checkerboard that you film with the exact lens and camera settings you will use for the shoot. The tracking software analyzes the grid and calculates the distortion parameters. These parameters, usually k1, k2, and k3 for radial distortion, describe how to correct the image. Some software can also estimate distortion from the footage itself if there are known straight lines.

Most professional camera tracking software like 3DEqualizer, PFTrack, and SynthEyes have built in lens distortion tools. You can import your distortion profile and the software will undistort the footage automatically. Nuke and Fusion also have distortion nodes. The workflow is to undistort, track, composite, and then redistort. This ensures that your final result matches the original footage's lens characteristics.

Skipping lens distortion correction is one of the most common beginner mistakes in camera tracking. The results look okay at first, but when you start placing CG objects near the edges of the frame, they will not match. The object might drift or slide because the camera solve is slightly wrong everywhere. Taking the time to properly undistort and redistort is a small extra step that makes a huge difference in the final quality.