Camera calibration is the process of finding the intrinsics of a camera, i. e. the principle point H, the principle distance c, and distortion parameters (according to Brown). In a laboratory this can be done by using a setup of collimators. The collimators were mounted in a cross formation in the following way: the front node of the lens is made to coincide with the point of intersection of the collimators and the focal plane was set perpendicular to the central collimator. The central collimator sees its own cross hairs which is visible on the focal plane. This position is the principle point H. The angle α between the collimators are known. The position of any projected cross hairs of the collimators onto the image plane can be measured. One can use the tangens in order to compute the respective angle α´: tan( α´) = c / d. The difference between both angles (Δ = α - α´) is caused by distortion of the lens. But before we have to compute the principle distance c. Another fact is that radial-symmetric distortion is equal where the radius (around H) is constant. So wen can use four measurements (left, right, up, and down) in order to compute two unkowns. That gives us an overdetermined equation system. So usually after all measurements are done one performs an adjustment algorithm to compute the principle distance and the parameters of distortion according to Brown.
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