It changes the focal length of the lens, although it does not seem to affect the lens (blue ring) in the left-hand pane which seems like a bug.
There is a demonstration later in the article (below the 5th mention of "focal length") which does affect the lens (blue ring) in the left-hand pane. Rotate the scene so that you are viewing the camera side-on, and you will see the focal length of the lens changing when you move the slider.
It's changing the focal length of the lens (roughly the distance between focus points of the lens). If you rotate the scene, you see the lens is blue. Further in the article, they mention the focal length, using the same coloring.
It changes the focal length of the lens, although it does not seem to affect the lens (blue ring) in the left-hand pane which seems like a bug.
There is a demonstration later in the article (below the 5th mention of "focal length") which does affect the lens (blue ring) in the left-hand pane. Rotate the scene so that you are viewing the camera side-on, and you will see the focal length of the lens changing when you move the slider.
It's changing the focal length of the lens (roughly the distance between focus points of the lens). If you rotate the scene, you see the lens is blue. Further in the article, they mention the focal length, using the same coloring.