How to get the focal length of stereocalibrate?

Hi every one.
I have two webcams that i calibrated to make depth estimation. when i make stereocalibrate, the intrinsic matrices of the cameras doesn’t change (even thogh the two cameras has not the same focal).
But when i make cv2.decomposeProjectionMatrix from the projection matrix of stereorectify, i get a different focal
can i get the new focal length directly or i’m i obliged to use cv2.decomposeProjectionMatrix ?