How do I get the focal length from a camera matrix?

Question

Given the camera matrix, can I find the focal length of this camera ?

Answer 1

Depends on what form the camera matrix is in.
Typically it is a 3x3 matrix that contains the focal lengths along the diagonal - with different Fx and Fy axis focal lengths to measure the astigmatism of the lens (the third value Fz isn't used)

See http://opencv.willowgarage.com/documentation/camera_calibration_and_3d_reconstruction.html