To illustrate principal planes of a compound lens system.◙
Lamp, screen, and two-lens cylinder.◙
The cylinder in the photograph contains a 10 cm focal length convex lens and a 10 cm focal length concave lens separated by 5 cm, so the focal length of the system is 20 cm. Because the principal planes are both outside of the cylinder lens system and on the same side, the focal distances with the lens cylinder oriented in the two directions are considerably different. Form an image of the light on the screen with one end of the cylinder facing the light, then reverse the lens cylinder and move the cylinder to refocus.