I'm doing some calibration for micro photogrammetry close range with a reflex (macro 60mm + extension 36mm maximum zoom) but I obtain always a value of K2 very big, from -100 to -500, even if not any particular distortion is evident in the photos. Any idea of the reason? In the opencv guide there's the explanation of the formulas behind the process of calibration where radial distortion coefficients are multiplied for "r", the distance between the point and the center, is that distance in pixels or mm?
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