lens_to_camera_mm in telephoto-lens

Hi, I am using mavlink_apriltags_landing_target example with telephoto-lens, but I don’t know the lens_to_camera_mm parameter.
Could you provide some information about that?

This is the distance from the top of the lens to the camera IC. It changes depending on what you are focusing on. Just get out a ruler and measure from the top of the lens down to about the bottom of the lens mount area.