Camera Calibration Extrinsics: Recover Scale

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Hello,

I am trying to perform a calibration procedure on my usb camera. For this I am basing my solution on the open cv c++ samples using a checker board pattern: https://github.com/opencv/opencv/blob...

I seem to get good enough intrinsics and my rms is around 0.1 However my translation values have a huge range: For example in Z I get values from -1.92353e-26, to 5.04921e+24

For the size of my checkerboard square I use mm, i.e. 29. Thus I expected for the translation vector to be in mm, however the values are not plausible. The picture I take are maximally 1 meter distance from the calibration pattern, so I expected to get values maybe in the range of e+3.

Is there a step I am missing to reconstruct the extrinsics up to scale?

Best, Marc