How to calculate Focal length in OpenCV?

Question

在其上选择图像像素和直线（从像素[x_1, y_1]到像素[x_2, y_2] ），如何在OpenCV中获得焦距（Python或C ++; 2.4或3.x）？

Answer 1

If you are need the

focal length   

of camera,I think you should make calibration and you will get a the

camera matrix 

which contains the focal length.

here is a good documentation to camera calibration

http://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_calib3d/py_calibration/py_calibration.html