fix focal length in calibrateCamera()

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Hi;

I am using OpenCV's calibrateCamera() to gt the intrinsic matrix of a camera. I know beforehand the precise focal length by another method (using projection geometry instrument), and would like to fix the calibrateCamera() process with that known focal length value (i.e. only the principle point and the distortion coeffs are needed by calibrateCamera()). I found that the focal length found by just using calibrateCamera() deviates a bit(about 1%) from the more precisely known focal length. Is there any way to pivot the calibrateCamera() with the known focal length value?

Fun Wey