I understand that you're interested in calculating the focal length in world units (millimeters) from the intrinsic camera parameters “fx” and “fy”. During camera calibration, the focal length is estimated in pixels, independent of the camera dimensions. To compute the focal length in world units, the pixel size on the camera sensor needs to be known, which may be obtained from the manufacturer’s specifications to estimate “sx” and “sy”, where “sx” and “sy” are the number of pixels per world units in the x and y direction respectively.
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