Recording Intrinsic Lens Calibrations

Intrinsic lens calibration trials are used to determine parameters associated with the camera lenses and are used to correct for distortion and other visual effects. Lens calibration trials are required for all camera systems that make use of a chessboard calibration method, except for Sony RX0-ii cameras. Users with wand calibration-capable systems (e.g. Qualisys Miqus, Vicon Vue or FLIR Blackfly S systems) are not required to complete lens calibration unless they are planning to use the chessboard calibration method.

Lens calibrations must be performed at least once per video resolution that will be used to record movement data, and any time the lenses or focal lengths change. If you intend on collecting data at 1080p, 720p, and 540p, you will need to record separate lens calibration trials at each of those resolutions.

Adjusting the aperture and focus of OptiTrack Prime Color cameras using the dials on their lenses does not necessitate new lens calibrations.

Recommendations:

• Record lens calibration trials at a relatively low frame rate (20-30 Hz) to reduce file size and processing time.

• Keep the chessboard as flat as possible during the calibration trial.

• Use a computer monitor facing the person performing the calibration to provide visual feedback during the calibration.

• Move slowly and deliberately to prevent chessboard blur.

• Lens calibrations can be performed for smaller groups or individual cameras and merged afterwards. If you are finding it challenging to calibrate all at once, try recording separate groups.

Recording Lens Calibrations:

1. Place all cameras side by side on a desk or table, facing the same direction and capturing as similar views as possible. It is generally easier to calibrate the lenses with the cameras in 'landscape' orientation.

2. Stand at a distance where the chessboard occupies approximately 1/4 of the camera views.

3. Begin the recording.

4. Slowly move the chessboard in a systematic grid pattern, covering the entire field of view for every camera. Ensure the chessboard goes slightly beyond every edge of every camera field of view, and every corner.

Example pattern of chessboard movement during lens calibration. Repeat or retrace this movement as required or maximize complete coverage of the camera view(s).

1. Take a step back, and repeat Step 4., covering the camera fields of view again.

2. While covering the field of view, angle the chessboard slightly in multiple directions, varying its orientation throughout.

3. When you are confident that the entire field of view has been covered for all cameras, end the recording.

To process intrinsic lens calibrations, see Lens Calibration.