Option --square for camera calibration with circle/acircle pattern

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Calibration patterns work because they have clear features which can be precisely located, the corner of a squares in the case of a checker board or the centre of circles in the case of the circular pattern. In both cases the spacing is the distance between these features.

In the case of the circular pattern you need to specify the distance between the centres of the circles. ROS uses OpenCV to perform the vision processing for this, which is described here.

Hope this helps.