How to calculate the focal length in mm by the result of camera calibration?

Question

The focal length in the camera-calibration result is in pixels,but i need the value in mm.How to calculate?

I used OpenCV to calibrate the camera and got the internal parameter matrix.I also get the camera sensor size in the logitech community.

the camera matrix is
[1432.379199223981, 0, 949.4032591833937;
 0, 1432.97806149025, 518.5291260500587;
 0, 0, 1]
and the size of sensor is 6.00mm, width of 4.80mm and height of 3.60mm (16:9 format).

I want to get the focal length in mm ,how to calculate?

Answer 1

It's just a proportion:

sensor_width_mm : sensor width_pixels = focal_length_mm : focal_length_pixels

There is one physical focal length (the distance between the exit pupil of the lens and the focus), but you can write two of these equations, one for the width and one for the eight, so you get two results. The difference is theoretically due to non-squareness of the pixels. If the spec sheet of the sensor says the pixels are square, and no disuniform image processing is performed, the difference itself may be illusory, depending on how accurate your calibration is. In this case, just average.