Conversion of camera intrinsic parameters (pixels to meter)

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I need the camera parameters in metric value (meters or centimeters). Previously, i have obtained the parameters in pixel value and also in its normalized form, however from my understanding we need something called scale factor in order to convert this to metric. Is there any formula how to solve this?

I tried to specify the size of square when calibrating (i used 19mm) but i dont know how to get the metric parameters after this.

If you specify the size of squares, you obtain extrinsic parameters in metric mode. For intrinsics, the value in millimeters don't change anything? If you want to calculate the size of object in image, you only know the apparent size in image, but the scale factor doesn't come from the focal, but from the distance of object to the camera plane. If you can access to the focal parameter, you can infer the distance (see information write on photographic camera lenses where focal parameter is also write in meter).