You have to place the work at the focal point of the laser lens, it will always be the same distance from the lens. The Bed should move up and down so depending on the thickness of the work piece, you can adjust the bed to get the correct focal position.
the 2nd image is wide lines because it is out of the focal point (to far away in this case)
Oh!! that’s poor design as this is a very important feature and almost necessary to be changed for every project that has a new material thickness. My machine has a powered Z axis and I’m always moving it up and down. hope you find a good solution to make this more simple